hash stringlengths 32 32 | doc_id stringlengths 7 13 | section stringlengths 3 121 | content stringlengths 0 2.2M |
|---|---|---|---|
5a4d9711ae9929bfa143700e1f8d2372 | 101 288 | 13.4.2 Page-type definitions | To ensure uniformity of approach by both broadcasters and decoder manufacturers, the expected content of the TV-related Teletext pages that can be indicated by the MIP are defined in this clause. With one exception (Subtitle Page) the format of each page, i.e. normal or subtitle style, is at the discretion of the editor. There is sufficient information in the page header packet of a Teletext page for a decoder to choose the most appropriate method of display. |
5a4d9711ae9929bfa143700e1f8d2372 | 101 288 | 13.4.2.1 TV index page | A single index page, or multiple sub-pages, carrying the page numbers of the most important TV-related pages in the service. ETSI ETSI TR 101 288 V1.3.1 (2002-12) 47 |
5a4d9711ae9929bfa143700e1f8d2372 | 101 288 | 13.4.2.2 Current TV programme information page | This page contains details of the current TV programme on that channel. This may include warning information as to the programme's content, or recommend a minimum viewing age. Ideally it contains sufficient supplementary information regarding start and finish times, date and channel (i.e. PDC or VPS codes) to enable a suitably equipped video recorder to be programmed to record the event. The page, or sub-pages, should remain in the transmission throughout the duration of the programme. |
5a4d9711ae9929bfa143700e1f8d2372 | 101 288 | 13.4.2.3 Current TV programme warning page | The primary function of this page is to convey warning information as to the current programme's content, or to recommend a minimum viewing age. It should remain in the transmission throughout the duration of the programme. |
5a4d9711ae9929bfa143700e1f8d2372 | 101 288 | 13.4.2.4 "Now and Next" TV programme | This page contains simple details (perhaps only start/finish times and programme title) about the programmes showing now on one or more channels. It may also include details of the following programme items. The page should be updated at every programme junction. |
5a4d9711ae9929bfa143700e1f8d2372 | 101 288 | 13.4.2.5 TV schedule page | This type of page includes TV programme listings for one or more channels. Ideally it contains sufficient supplementary information regarding start and finish times, date and channel (i.e. PDC or VPS codes) to enable a suitably equipped video recorder to be programmed to record each event, especially where the information is not for the current day or for the current channel. It is unlikely that a page will be updated to remove a single programme entry when that programme finishes. This type of page is very likely to occur as a multi-page set. |
5a4d9711ae9929bfa143700e1f8d2372 | 101 288 | 13.4.2.6 Subtitle page | A page providing subtitles for the current programme. The C6 bit in the page header can be assumed to be set to "1". The 3 LSBs of the actual Subtitle code value inserted in the MIP should match the value for the C12 - C14 control bits in the page header. These bits are used for selecting a set of national option characters. In the event of subtitle in different languages being transmitted on different pages, a decoder could offer the viewer a choice of subtitle by language rather than page number. Ideally the relevant code(s) would only appear in the MIP while genuine subtitles were being broadcast and not while a "placeholder" or apology message was present. |
5a4d9711ae9929bfa143700e1f8d2372 | 101 288 | 13.4.2.7 NexTView Transport Page | If a Magazine Inventory Page is transmitted which does not correctly indicate the NexTView Transport Page; certain receivers will fail to receive NexTView even if it is transmitted on the default page number of 1DF. |
5a4d9711ae9929bfa143700e1f8d2372 | 101 288 | 13.4.2.8 NexTView Network Identification | Broadcasters should set the Network Identification field in the Application Information structure to the actual value transmitted by the TV channel being referenced. In the event of a TV channel transmitting more than one code (i.e. transmitting two or more of Packet 8/30 Format 2 PDC, VPS or Packet 8/30 Format 1 Network Identification) the priority of the codes used should be; first that from the Packet 8/30 Format 2 (CNI) then VPS service (shortened CNI) or if VPS is not transmitted the code in the Packet 8/30 Format 1 (NI). In the case there is no Network Identification code defined for a TV channel EN 300 707 [1] specifies the use of the value 0 to indicate an unknown TV channel. There are TV sets in the market that will not work correctly with the value 0. In this case the EPG service provider should use an identification code that is currently not allocated for any TV channel: Preferred are codes in the range 0xFFF0..0xFFFF. For each channel for which such a code is used to indicate that there is no identification defined a different value should be used. ETSI ETSI TR 101 288 V1.3.1 (2002-12) 48 |
5a4d9711ae9929bfa143700e1f8d2372 | 101 288 | 14 Encoding and decoding transparent strings | |
5a4d9711ae9929bfa143700e1f8d2372 | 101 288 | 14.1 General | This clause provides guidelines on how to implement the transparent string component of EPGs. The aim is to bring together all the relevant information, some of this appears in EN 300 707 [1] and the present document. Other items have been noted during first attempts at implementing the specification. Consideration is given to the required performance of decoders with lower display capabilities when presented with enhanced strings (i.e. those coded to Teletext display Levels 2.5 and 3.5). |
5a4d9711ae9929bfa143700e1f8d2372 | 101 288 | 14.2 Transparent strings | Transparent strings are sequences of data defining text, graphics and attributes for display. They occur in their generic form in many EPG data structures. All strings consist of sequences of 7-bit data (the MSB of each byte has no meaning as it is used as an odd parity indicator when the data is encoded for transmission via Teletext). The data values represent spacing attributes (0x00 to 0x1F) and G0 characters or G1 graphics (0x20 to 0x7F). The strings in some data structures can be accompanied by an optional escape sequence. Each escape command within an escape sequence comprises a 10-bit (insert) position value, a 6-bit escape_mode value and an 8-bit escape_data value. Such commands can select alternative characters to those defined at certain positions within the string. In addition, a Carriage Return command (effectively a combined carriage return AND line feed command) is available to alter the display format of the string. In EPG services with enhanced displays, a wider range is available to select further alternative characters and graphics, non-spacing attributes and objects. |
5a4d9711ae9929bfa143700e1f8d2372 | 101 288 | 14.3 Code of practice | 1) A length value is an element of the definition for all forms of transparent string. Values greater than 0 define the number of bytes in the string. A value of 0 indicates that a string is empty, i.e. no characters or attributes are defined. There cannot be an escape sequence where a string is empty. 2) Transparent strings are processed in groups of up to 40 characters or spacing attributes, starting with the first item in the string. Each group forms one display row and is mapped onto the display from left to right. Each subsequent group is displayed on the row immediately below the row most recently addressed, beginning at the same column start position as the row above. 3) The starting position of a string intended for display in the Header or Message Area is column 0 in the top most row of the target area. However, if the number of rows of text of a string to be displayed in the Message Area is less than the number of rows available, the manufacturer is free to reposition the text vertically within the area. The manufacturer has full control over the vertical position and spacing of programmes title strings and navigation strings from NI blocks in the Event Area. 4) If a Carriage Return escape mode command is encountered within an accompanying escape sequence before a group of 40 characters has been assembled, the current group is considered to have been terminated and the remainder of that row is to be displayed as if the character "space" had been transmitted for each column position. The string character, or valid escape sequence character, at the point where the Carriage Return attribute was inserted becomes the first character of a new group to be displayed on the following row at the same start column as the row above. 5) The last group may contain less than 40 characters in which case the remainder of the row is to be displayed as if the character "space" had been transmitted for each column position. Similarly, if string data is not defined for some or all of the display rows available within the target area, the rows not addressed should be displayed as if the character "space" had been transmitted for each column position. ETSI ETSI TR 101 288 V1.3.1 (2002-12) 49 6) When the number of characters in the processed string exceeds the number of display locations within the target area, the decoder is under no obligation to make the extra characters visible. 7) At the beginning of each display row all attributes adopt their default state unless the new row was started as a result of a Carriage Return escape mode command, see point 15. The default attribute settings in the absence of any spacing attributes are: alphanumeric mode, black background, white foreground, normal height, steady and contiguous graphics. 8) String codes 0x00 to 0x1F select spacing attributes according to table 26 in EN 300 706 [3]. The Teletext functions End Box (0x0A), Start Box (0x0B), Double Width (0x0E), Double Size (0x0F), Conceal (0x18) and ESC (0x1B) are not valid for use in transparent strings and their codes should not be transmitted. A decoder should interpret the remaining attributes in the same way as for Level 1 Teletext. Each will occupy a column position and will normally be displayed as a "space" unless hold graphics mode is in effect. However, a decoder may ignore spacing attributes within strings intend for display in the Event Area. 9) In alphanumeric mode, string codes 0x20 to 0x7F select characters from a G0 character set specified by the 4 MSBs of the default_alphabet value (defined in the AI block) for the network to which the string applies. The default_alphabet value defines an entry in table 32 of EN 300 706 [3]. In graphics mode, these codes select characters from the G1 character set. 10) In alphanumeric mode, string codes 0x23, 0x24, 0x40, 0x5B, 0x5C, 0x5D, 0x5E, 0x5F, 0x60, 0x7B, 0x7C, 0x7D and 0x7E select national option characters. For a given network, the set to be used is specified by the 3 LSBs of the default_alphabet value. In graphics mode, only codes 0x40, 0x5B, 0x5C, 0x5D, 0x5E, 0x5F have this function. 11) An element of an escape command is the insert_pos value to indicate the position to be addressed within the string. This is a pointer to the string data. As it is not a reference to a screen position its meaning is not affected or modified by the Carriage Return command. A value of 0 refers to the first character in the string. Escape commands shall be coded in ascending order of insert_pos. 12) A service limited to Level 1.5 display features should only transmit the escape_mode values shown below. A decoder with Level 1.5 display capabilities should respond to these codes and no others when receiving display enhanced transmissions. Apart from the Carriage Return command there are direct equivalents in the Level 1.5 extensions of Teletext as defined in EN 300 706 [3]. They allow diacritical marks to be added to G0 characters and the display of some characters from the G2 supplementary character set. A character inserted in this manner should only be used to overwrite a G0 character within the string and not spacing attributes or G1 graphics characters (i.e. codes 0x20 to 0x3F and 0x60 to 0x7F). A service provider should try to include a suitable fallback character in the transparent string in case a decoder is unable to display the character specified in the escape command (see table 6). 13) Escape sequences which insert characters at locations where serial attributes already exist could cause display differences between Level 1.5 and Level 2.5 decoders. If a serial attribute has to be overwritten by a character, for the benefit of a Level 2.5 decoder the attribute should be restored by transmitting an escape sequence to insert it as parallel attribute. Thus a Level 1.5 decoder will give priority to the text rather than display enhancement. 14) Where there are multiple escape commands at a particular string position, the order of coding will determine the final display. No more than one character carried by an escape sequence is allowed for each position In the string. If Present a Carriage Return should always be coded first. 15) The Carriage Return escape mode command can be used to enhance transmission efficiency but only if the command is used before column 35. It should not be used in the Title string of a PI block. 16) Some early decoders and all those employing Level 1.5 display generators do not fulfil the EN 300 707 [1] requirement that states that the attributes current at the point where the Carriage Return was inserted should be preserved at the first position on the following row. Therefore, it is recommended to use this command only when all the attributes are at their default state to ensure consistency of display by all decoder types. Repeating the spacing attributes required to give the desire display effect at the start of the new row may be sufficient under some circumstances but cannot be guaranteed for all. If consistent displays cannot be achieved the Carriage Return command should not be used and sufficient instances of the character "space" should be inserted in the string to complete a 40 character group. ETSI ETSI TR 101 288 V1.3.1 (2002-12) 50 17) When a double height spacing attribute (0x0D) is used, any Carriage Return shall be preceded by a normal height attribute or the transparent string shall be padded to complete a 40 character group before data for the following row is defined. It is only permitted to terminate the group early with a Carriage Return command if all the attributes have their default state. Any characters in the following group will not be displayed by Level 1.5 decoders as the displayed row will show the bottom half of the double height text. It is permitted to inserted one Carriage Return command in place of a full group for the lower row. 18) The maximum recommended sizes for specific strings in a PI block are as follows (see clause 8.3.1): Title: 40 characters; Short Info: 255 characters; Long Info: 1 000 characters. 19) Double Width (0x0E) and Double Size (0x0F) spacing attribute can be inserted but may only be interpreted correctly by Level 2.5/3.5 decoders. Other decoders may ignore both commands or interpret Double Size as Double Height only. The rules applying to Double Height also apply to Double Size. Once horizontal expansion has been enabled, each character that is intended to be displayed at twice its normal width should be followed by a space character to ensure compatibility with decoders which ignore these commands. 20) In a Full EPG transmission, up to 15 individual strings can be defined an NI block for display in the Event Area. Each string should be a maximum of 40 displayable characters to ensure it fits on one display row. However, some decoders may still have to truncate such strings. Table 6 Escape_mode Function Action Valid range for escape_data 0x09 G0 alphanumeric character Overwrite string character 0x20 - 0x7F (notes 1 and 2) 0x0A Carriage Return Retain string character Reserved for future use 0x0F G2 alphanumeric character Overwrite string character 0x20 - 0x7F (notes 1 and 2) 0x10 - 0x1F (notes 1 and 3) G0 character with diacritical mark Overwrite string character 0x20 - 0x7F (notes 1 and 2) NOTE 1: The values used should fall within the limits specified. However, local alphabet and language requirements, and to some extent decoder capabilities, will determine the subset of these values that will be used in practice. NOTE 2: If a decoder cannot display the character specified, it should display the fallback character from the transparent string where possible. NOTE 3: This command adds a diacritical mark to a character from the G0 set. The diacritical mark is defined by the 4 LSBs of escape_mode referencing an entry in column 4 of the G2 set. The G0 character is defined by escape_data. This function is only valid for the Latin alphabet. ETSI ETSI TR 101 288 V1.3.1 (2002-12) 51 Annex A: Commercial name for EPG services The term "NexTView" is to be adopted as the commercial name for EPG services and decoding products compliant with EN 300 707 [1], EN 300 708 [2] and the present document. ETSI ETSI TR 101 288 V1.3.1 (2002-12) 52 Annex B: List of programme attributes The following attributes and other parameters may be defined for each programme event. They can all, in theory, be used within a suitable decoder, either singularly or in combination, to sort the database. Channel (network operator) Date Start-time Stop-time Editorial rating Parental rating Theme; pre-defined categories (see annex C) Theme; service provider defined categories (Full EPG service only) Mono/2 channel sound/Stereo/Surround sound Widescreen format PALplus Digital Encrypted Live programme Repeat programme Teletext subtitles Sound track language Language of in-vision subtitles ETSI ETSI TR 101 288 V1.3.1 (2002-12) 53 Annex C: Pre-defined programme theme categories The programme categories defined in EN 300 707 [1] for use in EPG services are listed in table C.1. Table C.1: Pre-defined programme theme categories Code Description 0x00 ... 0x0F undefined content Drama and Films 0x10 movie (general) 0x11 detective/thriller 0x12 adventure/western/war 0x13 science fiction/fantasy/horror 0x14 comedy 0x15 soap/melodrama/folklore 0x16 romance 0x17 serious/classical/religious/historical drama 0x18 adult movie 0x19 ... 0x1E reserved for future use 0x1F user defined News/Current Affairs/Social 0x20 news/current affairs (general) 0x21 news/weather report 0x22 news magazine 0x23 documentary 0x24 discussion/interview/debate 0x25 social/political issues/economics (general) 0x26 magazines/reports/documentary 0x27 economics/social advisory 0x28 remarkable people 0x29 - 0x2E reserved for future use 0x2F user defined Show/Game Show/Leisure hobbies 0x30 show/game show (general) 0x31 game/show/quiz/contest 0x32 variety show 0x33 talk show 0x34 leisure hobbies (general) 0x35 tourism/travel 0x36 handicraft 0x37 motoring 0x38 fitness and health 0x39 cooking 0x3A advertisement/shopping 0x3B ... 0x3E reserved for future use 0x3F user defined Sports 0x40 sports (general) 0x41 special events (e.g. Olympic games, World Cup etc.) 0x42 sports magazines 0x43 football/soccer 0x44 tennis/squash 0x45 team sports/excluding football 0x46 athletics 0x47 motor sports ETSI ETSI TR 101 288 V1.3.1 (2002-12) 54 Code Description Sports (continued) 0x48 water sports 0x49 winter sports 0x4A equestrian 0x4B martial arts 0x4C local sports 0x4D ... 0x4E reserved for future use 0x4F user defined Children/Youth/Education/Science 0x50 children's youth programmes (general) 0x51 pre-school children's programmes 0x52 entertainment programmes for 6 to 14 0x53 entertainment programmes for 10 to 16 0x54 informational/educational/school 0x55 cartoons/puppets 0x56 educational/science/factual topics (general) 0x57 nature/animals/environment 0x58 technology/natural sciences 0x59 medicine/physiology/psychology 0x5A foreign countries/expeditions 0x5B social/spiritual sciences 0x5C further education 0x5D languages 0x5E reserved for future use 0x5F user defined Music/Ballet/Dance 0x60 music/ballet/dance (general) 0x61 rock/pop 0x62 serious music/classical music 0x63 folk/traditional music 0x64 jazz 0x65 musical/opera 0x66 ballet 0x67 ... 0x6E reserved for future use 0x6F user defined Arts/Culture (without music) 0x70 Arts/Culture (without music, general) 0x71 performing arts 0x72 fine arts 0x73 religion 0x74 popular culture/traditional arts 0x75 literature 0x76 film/cinema 0x77 experimental film/video 0x78 broadcasting/press 0x79 new media 0x7A arts/culture magazines 0x7B fashion 0x7C ... 0x7E reserved for future use 0x7F user defined 0x80 ... 0xFE series codes 0xFF reserved ETSI ETSI TR 101 288 V1.3.1 (2002-12) 55 Annex D: Editorial committee The present document was compiled on behalf of the EBU and EACEM by the following: Alexander Kulpok (Chairman) ARD/ZDF-Videotext/Berlin Frans Collignon NOS Teletekst/Hilversum Gerhard Eitz IRT/Munich Norman Green ITC/London Sandor Gyarmati Thomson/Villingen Rolleiv Solhom NRK TEKST-TV/Oslo David Tarrant Philips Semiconductors/Southampton Peter Weitzel BBC/London Uwe Welz ARD/ZDF-Videotext/Berlin ETSI ETSI TR 101 288 V1.3.1 (2002-12) 56 History Document history Edition 1 October 1996 Publication as ETR 288 V1.2.1 December 1997 Publication V1.3.1 December 2002 Publication |
fd725f06335eff4cc5ea94e4aedbf58e | 101 279 | 1 Scope | The present document objectively documents the experimental use of various tools for Computer Aided Test Generation from the point of view of testing methodology. It is not intended that this document imply any comparison of the tools, nor is it intended that this document be used as a basis for ETSI recommending the use of one tool or another. |
fd725f06335eff4cc5ea94e4aedbf58e | 101 279 | 2 References | The following documents contain provisions which, through reference in this text, constitute provisions of the present document. • References are either specific (identified by date of publication, edition number, version number, etc.) or non-specific. • For a specific reference, subsequent revisions do not apply. • For a non-specific reference, subsequent revisions do apply. • A non-specific reference to an ETS shall also be taken to refer to later versions published as an EN with the same number. [1] ETS 300 771-1 (1997): "Broadband Integrated Services Digital Network (B-ISDN); Digital Subscriber Signalling System No. two (DSS2) protocol; B-ISDN user-network interface layer 3 specification for point-to-multipoint call/bearer control; Part 1: Protocol specification; [ITU-T Recommendation Q.2971, modified]". [2] ETS 300 443-1 (1996): "Broadband Integrated Services Digital Network (B-ISDN); Digital Subscriber Signalling System No. two (DSS2) protocol; B-ISDN user-network interface layer 3 specification for basic call/bearer control; Part 1: Protocol specification; [ITU-T Recommendation Q.2931 (1995), modified]". [3] ITU-T Recommendation Z.100 (1994): "CCITT Specification and Description Language (SDL)". [4] ISO/IEC 9646-3 (1992): "Information technology - Open systems interconnection - Conformance testing methodology and framework - Part 3: The Tree and Tabular Combined Notation (TTCN)" [5] ITU-T Recommendation Z.120 (1993): "Message Sequence Chart". [6] ETS 300 771-5 (1998): "Broadband Integrated Services Digital Network (B-ISDN); Digital Subscriber Signalling System No. two (DSS2) protocol; B-ISDN user-network interface layer 3 specification for point-to-multipoint call/bearer control; Part 5: Test Suite Structure and test purposes [TSS&TP] specification for the network". [7] ETS 300 771-6 (1998): "Broadband Integrated Services Digital Network (B-ISDN); Digital Subscriber Signalling System No. two (DSS2) protocol; B-ISDN user-network interface layer 3 specification for point-to-multipoint call/bearer control; Part 6: Abstract Test Suite (ATS) and PIXIT proforma specification for the network". [8] EG 201 022: "Broadband integrated Services Digital Network (B-ISDN); Digital Suscriber Signalling system No. two (DSS2) protocol; B-ISDN user-network interface layer 3 specification for point-to-multipoint call/bearer control; service Description Language (SDL) validation model". ETSI TR 101 279 V1.1.1 (1998-07) 5 |
fd725f06335eff4cc5ea94e4aedbf58e | 101 279 | 3 Abbreviations | For the purposes of this TR, the following abbreviations apply: ASN.1 Abstract Syntax Notation one B-ISDN Broadband ISDN CATG Computer-Aided Test Case Generation DSS2 Digital Subscriber Signalling System two INAP Intelligent Networks Application Protocol ISDN Integrated Services Digital Network IUT Implementation Under Test MSC Message Sequence Chart SDL Specification and Description Language TP Test Purpose TTCN Tree and Tabular Combined Notation UNI User Network Interface |
fd725f06335eff4cc5ea94e4aedbf58e | 101 279 | 4 Use of tools for TP development and validation | |
fd725f06335eff4cc5ea94e4aedbf58e | 101 279 | 4.1 Test Purpose development | None of the tools studied offered a real alternative to the intellectual processes that are applied when producing test purposes manually. The tools supported either: - semi-automated techniques which rely on user interaction with a simulator to generate MSCs as defined in Z.120 [5] which express test purposes; or - fully-automated techniques which systematically base test purposes on single state transitions. While MSCs can be a useful complement to documenting test purposes they should not be regarded as a complete substitute for textual test purposes, which will often contain additional and necessary information not easily expressible in the MSC format (e.g. verdict assignment). Also, MSCs cannot cope with dynamic alternatives, as in this example from N-ISDN, User Side: Ensure that the IUT in Null call state U00, on receipt of a valid SETUP message with the sending complete information element, sends any of a CALL PROCEEDING, ALERTING or CONNECT message and enters the relevant call state Call Proceeding U09, Call Received U07 or Connect Request U08. With fully-automated techniques there are two difficulties. In the first case, a single-state transition is not always an adequate expression of purpose where we may wish to express test purposes in terms of requirements not necessarily restricted to a single state transition (i.e., the level of granularity is too restrictive). In the second case there is a potential for generating very many test purposes. Although the number of test purposes generated can limited by applying sets of criteria, these criteria are often arbitrary (e.g., maximum depth) and do not always bear relation to what a test engineer would normally call a ‘good’ test purpose. Finally, even if test purposes are generated automatically they must still be subject to a time-consuming manual review if we are to have full confidence in them. Conclusion: Using SDL/MSC based tools as aids to the development and documentation of test purposes is useful and produces high-quality documentation of test purposes. The informal expression of TPs in textual format (using templates as is current ETSI practice) accompanied by the corresponding MSCs is especially effective. Tools are not suitable for automatic generation of TPs. |
fd725f06335eff4cc5ea94e4aedbf58e | 101 279 | 4.2 Test Purpose validation | Using tools to validate test purposes by simulation proved to be more successful. This was done by checking through simulation the behaviour described by each TP. The work was made easier by the implementation of a simple graphical user interface. ETSI TR 101 279 V1.1.1 (1998-07) 6 NOTE: EG 201 022 [8] describes the use of SDL simulation techniques to develop and validate TPs. Once the system was setup it took about 8 man-days to validate 830 test purposes taken from ETS 300 771-5 [6]. The following table summarises the results of this process (this does not include the time needed to develop the SDL model (about 2 man-months), or the time needed to build the graphical user interface(about 2 man-weeks)): Table 1: Errors found by the TP validation process Errors in the test purposes (missing or too many parameters, incorrect messages etc.) 51 Errors found in the Q.2931 and Q.2971 19 Errors (bugs) found in the SDL model 20 It is worth noting that this process has the effect of not only validating the test purposes (an error rate of 8% was noted) but also the SDL model and, by implication, the standard itself. Due to the fact that all three components in the validation process (the standard, the SDL model and the test purposes) were produced by different parties we have confidence that the exercise was more than an academic. Conclusion: The use of SDL simulation models can be very useful in the development and validation of test purposes. At this level they can also have the useful side-effect of validating the base standards. |
fd725f06335eff4cc5ea94e4aedbf58e | 101 279 | 5 Use of tools for Test Suite validation | The SDT/ITEX simulators were used to execute a manually written TTCN test suite for the network B-ISDN DSS2 against an enhanced SDL model. The basis for the protocol simulation was the SDL model for test purpose validation. However, the test purpose validation model had only limited support of protocol data and could not cope with some special protocol situations. The following additions were necessary: - detailed protocol data (messages, information elements etc.) descriptions in ASN.1; - provision of detailed protocol data checking functions in SDL; - provision of functions which generate appropriate signals with detailed data contents in reaction to protocol activities; - provision of an encoding/decoding of protocol data; - provision of a full functional call processing to simulate switch behaviour, define a mechanism how to provide user and network side simulations in one SDL model with minimal maintenance effort; - the user side simulation (i.e., both network and user). These tasks were completed in approx. 3 man-months. NOTE - Much of the SDL work was done on a voluntary basis by Deutsche Telekom ETSI TR 101 279 V1.1.1 (1998-07) 7 Using the Telelogic ITEX tool, C-code was generated from the test suite. This code could not interwork with the SDT simulation mainly because the data format was not unique. In Tau version 3.11 it is not possible to interchange signals which contain ASN.1 sets with optional fields because there is no tag to distinguish these fields. Therefore the data coding defined in ETS 300 771-1 [1] and ETS 300 443-1 [2] had to be implemented and used as the interchange format between ITEX simulation and SDT simulation. It was necessary to write code for the encoding and decoding of protocol data on the ITEX side. Other tasks needed to create an executable simulation were: - provide PIXIT values; - provide user defined test operations; - provide protocol data encoding/decoding functions and other tool fixes. The ITEX fixes that needed doing can be grouped as follows: - data access errors (CHOICE values, SET fields, wrong type settings); and - simulation run time errors (logging and scheduling). The first kind of errors could be fixed in the generated code. The second kind of errors were fixed through a library update provided by Telelogic. These tasks were completed in 1 man-month. Through the provided encoding/decoding, the ITEX and SDT simulation could interwork. All test cases of the network side test suite were executed. Due to time constraints only a few user side test cases were executed. This task took approximately 1 ½ man-months. Through simulation errors in the TTCN specification as well as in the SDL specification were identified. Table 2: Errors found in the TTCN test suite Parameter order errors (parameters where not in the right order (these errors could have been detected by a better TTCN checker than ITEX) 33 Procedural errors (race conditions could have invalidated the test case) 12 Parameter value errors (parameters had wrong value) 42 Ignoring the parameter order errors the parallel simulation detected approximately 50 test cases which were erroneous in the 800+ test case test suite (6% error ratio). It is very unlikely that these errors would have been found by a manual review of the test suite. Conclusion: by using the TTCN/SDL simulation techniques the quality of the TTCN test suite was significantly improved. However, it should be noted that building the SDL model and writing the encoder/decoder interface is probably not economically sensible unless the SDL model is to be used in other contexts (e.g., CATG, implementation etc.). ETSI TR 101 279 V1.1.1 (1998-07) 8 |
fd725f06335eff4cc5ea94e4aedbf58e | 101 279 | 6 Use of tools for CATG | This section summarises the experiences of evaluating the following CATG tools: - ITEX Link (Telelogic); - ITEX Autolink (Telelogic); - TTCgeN (Verilog); TTCN Maker (INTOOLs project). Table 3: Effort allocated to using each tool Tool Installation/Education Test Total (man-days) ITEX Link 1 3 4 ITEX Autolink 2 4 6 TTCgen 1 3 4 TTCN Maker 2 3 5 19 In general the major limitations of the tools were either - that the tools could not handle complex SDL specifications of the kind that would be typically produced by ETSI; and/or - that the tool worked but was cumbersome to use and did not offer greatly improved efficiency to an experienced test writer; and/or - that the additional effort required to manually transform the raw TTCN output from the tool to a level of detail expected in an ETSI standard cancelled out the original benefit of using the tool in the first place. Another drawback is that tools cannot generate tests for invalid behaviour (unless such behaviour is explicitly programmed into the SDL). In the B-ISDN DSS2 tests for the network side (point-to-multipoint) for example, out of a total of 800 test cases about 600 are for invalid behaviour. Thus, even if CATG were used it would only generate 200 of the necessary 800 test cases. Conclusion: While CATG tools indicate future possibilities it was evident that for the purposes of B-ISDN testing they did not offer a faster alternative to manual development. |
fd725f06335eff4cc5ea94e4aedbf58e | 101 279 | 7 The ITEX link tool | ITEX Link is a semi-automatic CATG tool that allows the user to interactively build a test suite from an ITEX editor that is connected to a simulateble SDL model of the system under test. The tool transforms the SDL data to equivalent TTCN data automatically. TTCN SEND events are entered by the user, who must also specify the appropriate constraint. The tool automatically responds with the correct RECEIVE event (or set of events if there is the possibility of more than one) together with the correct constraint(s). In this manner the dynamic behaviour and corresponding PDU constraints are built-up to form the whole test case. Link provides no support for the generation of test purposes. The manually produced test purposes are interpreted by the human user to guide the interactive development of the corresponding Test Cases. Link generates correct TTCN test cases, but in the case of Q.2971 as modified by ETS 300 771-1 [1] the dynamic behaviour is reasonably simple, with a limited number of ‘generic’ behaviours. Most tests involve complex variations of message parameters over these generic behaviours. Because of the limited data modelling in the SDL many of these data variations cannot be generated by the tool. It was therefore decided that the tests would be produced more efficiently on a manual basis. In the case where one has more complex behaviour (for example, in INAP) and where even the experienced test writer cannot anticipate all dynamic outcomes, a tool such as TTCN Link could provide valuable support. ETSI TR 101 279 V1.1.1 (1998-07) 9 Table 4 shows the extent of the TTCN generation offered by Link. The following items from this list are considered to be major deficiencies: - link always generates completely flat constraints even if the ASPs (or PDUs) definitions are structured i.e., all structuring information is lost. This is particularly confusing if complex data structures are used; - derived constraints cannot be used in synchronised mode; - matching symbols and Test Suite Parameters cannot be used in synchronised mode, i.e., only explicit values may appear in constraints; - Test Steps and Constraints cannot be parameterised in synchronised mode; - concurrent TTCN is not supported. NOTE: Table 4 also gives a general guide to what constitutes a resynchronisable edit (R) and one that is not resynchronisable (NR). The classification Limited Resynchronisation (LR) refers to the cases (e.g., addition of Test Case Variables) where these items may be declared in the ATS without affecting synchronisation but may not used in all circumstances (e.g., as values in constraints or the dynamic part). Link is a well-integrated tool and is reasonably user-friendly. Facilities such as Show_SDL (which animates the path a particular Test Case dynamic behaviour takes through the SDL) and Show_MSC (which draws an MSC for any given Test Case behaviour sequence) were found to be quite useful. The ability (not provided) to name the generated send constraints on the fly would be useful. Telelogic are aware of this request. Link appears not to place restrictions on the scope of SDL used (i.e., SDL ‘92 is supported). |
fd725f06335eff4cc5ea94e4aedbf58e | 101 279 | 8 The ITEX Autolink tool | In this tool, test purposes must be developed in the form of MSCs using the SDT simulation and MSC tools. These test purposes are then input to Autolink which generates ‘raw’ TTCN from them. The raw TTCN must be postprocessed in order to make it suitable for standardisation. This post-processing is similar to that needed with the other tools: - Major tasks: - splitting of the non-concurrent (i.e. interleaved) test cases produced by the tool into parallel TTCN behaviour trees and the addition of Configuration tables; - addition of missing information in the Constraints (certain parameters are not modelled in the SDL specification); - addition of several pre-ambles (the AAL was not fully modelled in the SDL specification); - addition of Test Suite Parameters; - parameterization of constraints and test steps. - Minor tasks - Re-formatting and re-structuring of the ASN.1 constraints for readability; - Re-naming of automatically named objects (for readability); - Addition of test steps (not strictly necessary, but reduces the size of the test suite and helps readability); - Addition of comments. By using specially-written scripts to do some of the post-processing it is possible that the overhead could be reduced. There are no figures currently available to indicate what improvements might be expected. ETSI TR 101 279 V1.1.1 (1998-07) 10 The performance aspects of the Autolink were disappointing. A single test case of medium complexity took over 40 hours to generate. Simpler test cases took anything from ½ - 2 hrs. It is probable that with more powerful hardware (top of the range Sparc) that these times could be significantly reduced. The hardware used in this trial was a Sun Sparc 20 with 128 MB RAM and 256 MB swap. Autolink appears not to place restrictions on the scope of SDL used (i.e., SDL ‘92 is supported). Finally, it has not determined the quality of the CATG test cases with respect to execution in a real test system against a proper IUT. The lack of detail in some instances (due to lack of detail in the SDL model) may give cause for concern. The manually produced test suite compiled on a commercial ATM test system the first-time that it was tried. The ATS is also being evaluated in the field by several manufacturers of ATM equipment. Early feedback indicates that the test suite is executing well in a real environment. |
fd725f06335eff4cc5ea94e4aedbf58e | 101 279 | 9 The TTCgeN tool | TTCgeN generates test cases entirely automatically. Test Purposes are represented by MSCs. Tools are used to generate complete MSCs from partial MSCs, which have been created manually. These complete MSCs are then used as input then to derive complete MSCs. However the actual effort (e.g., finding suitable starting (partial) MSCs and then setting the boundary conditions to restrict the output of the MSC generator to reflect real situations TTCgeN supports the test suite generation process as follows: - for each test purpose MSC, the user binds it with the SDL model to generate a pair (model, observer) that defines the test system. An observer here is the tool-internal representation of the MSC; - the tool then automatically runs this test system and generates TTCN test cases. TTCgeN is a batch command that takes as input the pair (model, observer), executes the co-simulation which may be seen as an exhaustive exploration of the behaviour of the SDL model strictly constrained by the test purpose observer, and derives the test cases from this reduced state graph; - the result comes out as a 'testcase.mp' file that can be visualized using a TTCgeN facility, or that can be processed with a TTCN editor such as ITEX. Table 5 shows the extent of the TTCN generation offered by TTCgeN. The following items from this list are considered to be major deficiencies: - only the top-level of structured PDU (or ASP) definitions are translated. All sub-structure definitions must be created manually; - only the top-level of structured PDU (or ASP) constraints are translated. All sub-structure constraints must be created manually; - derived constraints cannot be generated; - matching symbols and the use of Test Suite Parameters in constraints may not be generated, i.e., only explicit values appear in constraints; - parameterised Test Steps and Constraints cannot be generated; - timers are handled incorrectly; - concurrent TTCN is not supported. NOTE: Where Same as Link is used in table 5 all references to resynchronisation in table 1 should be ignored. The concept of synchronisation with the SDL is not relevant to TTCgeN. It is clear that TTCgeN is a prototype and will need some tuning if it is to be useful product. The tool is not integrated with ObjectGEODE and the TTCgeN output must be input to a TTCN editor such as ITEX in order to continue with manual editing. This makes TTCgeN less easy to work with. TTCgeN does not support SDL ‘92 and does not always handle signal parameters and abstract data types correctly, e.g., the type Charstring is ignored. ETSI TR 101 279 V1.1.1 (1998-07) 11 No system problems were encountered during this study. |
fd725f06335eff4cc5ea94e4aedbf58e | 101 279 | 10 The TTCN Maker tool | This tool generates both the test purposes and the test cases automatically. The generation is based on a single state transition. However, this is not always an adequate expression of purpose where we may wish to express test purposes in terms of requirements not necessarily restricted to a single state transition (i.e., the level of granularity is too restricted). Also, there is a potential for generating very many test purposes. The number of test purposes generated is limited by applying a set of criteria. These criteria are often arbitrary (e.g., maximum depth) and do not always bear relation to what a test engineer would normally call a ‘good’ test purpose. Finally, even if test purposes are generated automatically they must still be subject to a time-consuming manual review if one is to have full confidence in them. Table 6 shows the extent of the TTCN generation offered by Link. The following items from this list are considered to be major deficiencies: - it does not support many of the SDL ’92 constructs that are extensively used in the SDL model; and - even if the tool performs well as a test generator it can only generate tests for one process at a time. In the SDL for Q.2971 as modified by ETS 300 771-1 [1], for even a simple testing situation of say two parties, at least nine concurrently executing processes are created. It is simply not feasible to generate separate pieces of TTCN for each of these processes and then merge them into a single test suite. Neither is it feasible to re-write the SDL as a single process as it is exactly the concurrent properties that are wishes to model. The tool has good performance characteristics and appears robust. ETSI TR 101 279 V1.1.1 (1998-07) 12 |
fd725f06335eff4cc5ea94e4aedbf58e | 101 279 | 11 CATG tool summaries | Table 4: TTCN Link summary 1 Test Purposes Not generated Test purposes must be produced manually before using the tool. They are interpreted by the human user as he/she interactively generates the Test Cases. - 2 Test Structure Test Groups Not generated Structuring of the Test Suite into Test Groups must be done manually. The Test Suite indexes can be automatically generated by ITEX. R 3 Concurrent TTCN Not generated Parallel test component declarations and the configuration declarations must be defined manually. Where an SDL specification defines behaviour at more than one PCO the events occurring at the different PCOs are interleaved in a TTCN Link Test Case. This means that that all behaviour trees must manually be split into their component trees, in our example this would mean two PTCs, one for A and one for B. Note that it should possible to write a script that converts the interleaved TTCN.MP to Concurrent TTCN. MTC activity such as CREATE and the specification of co-ordination between the PTCs has to be implemented manually. LR NR NR 4 Type Definitions Generated Newtypes defined in the SDL (including struct types) are translated to corresponding ASN.1 types in TTCN. LR 5 User Defined Ops Not generated LR Test Suite Params Not generated All Test Suite Parameters must be declared manually. Even when declared these parameters have no semantic connection to the SDL, i.e. TTCN Link does not recognise them. Test Suite Parameters should not be used as values in constraints if synchronisation with TTCN Link is required. LR 6 Test Suite Constants Test Suite Variables Test Case Variables Not generated Same as for Test Suite Parameters. LR 7 PCOs Generated All channels to the environment are treated as PCOs. Traffic on internal channels is not explicitly seen in the Test Case. NR 8 Test Case Behaviour Generated Semi-automatic TTCN Link generates Test Cases semi-automatically. The user manually enters the desired send events (indicated in the Test Purpose) and the system automatically responds with the correct receive events. TTCN Link generates received alternatives according to the (reverse!) order in which the corresponding inputs appear in the SDL. This may be a problem in TTCN where the ordering of events in a single set of alternatives is significant. NR 9 Timers Generated Timer operations (such as START and CANCEL) may be manually added to event lines. Timeouts are automatically generated (where applicable). It is not allowed to have Timer Operations on separate event lines. LR 10 Assignments & Qualifiers Not generated Assignments and qualifiers (i.e. Boolean expressions) may be manually added to event lines but they do not have any semantic connection to the SDL, i.e. TTCN Link ignores them. It is not allowed to have Assignments or Qualifiers on separate event lines. LR ETSI TR 101 279 V1.1.1 (1998-07) 13 11 REPEAT GOTO Not generated There is no concept of repeated behaviour in TTCN Link. NR 12 Test Steps Not generated Test Steps may be created by manually copying behaviour from Test Cases into Test Steps. However, TTCN Link requires that the attached Test step appear on a separate line and may not be an alternative among other alternatives, e.g., the following is not allowed: A? PDU1 + TestStep1 R 13 Default Behaviour Generated (Limited) TTCN Link generates a single default behaviour for the whole test suite comprising ?OTHERWISE (for each PCO) and a general ?TIMEOUT. More complex defaults must be created manually. R 14 Parametrisation Not generated Test steps cannot be parameterised. Constraints cannot be parameterised. NR NR 15 ASP definitions Generated All SDL signals on channels to the environment are translated to ASN.1 ASPs (See also PDUs). 16 PDU definitions Generated If it is required to express the ATS in terms of PDUs rather than ASPs then the ASP definitions must be manually cut and pasted into the PDU definitions in ITEX. If it is required that the PDUs are carried in ASPs then the ASPs must be defined manually and all send/receive events updated accordingly, possibly through the use of aliases. Because signal parameters in SDL are not named TTCN Link uses dummy names (e.g., integer1, charstring2) to identify names of top-level ASP parameters. These must be changed manually in both the ASP/PDU definitions and constraints. R NR NR 17 Constraints Generated (Limited) Constraints are expressed as ASN.1 values. Unfortunately TTCN Link flattens all constraints. That is, any structuring of ASPs/PDUs is not reflected in the constraint. If it is required to have structured constraints the structuring must be re-created manually after the generation process. During generation of send events the system will prompt for the required input. This may either be a new constraint, which can be defined on-the-fly or an existing constraint. Received constraints are automatically generated by the system. They are given generated names which will usually mean that a manual re-naming must be done. NR R 18 Matching symbols Not generated Only explicit values may appear in constraints. TTCN matching mechanisms such as ranges and wildcards may not be used if synchronisation with TTCN Link is to be maintained. NR 19 Derived constraints Not generated Derived constraints are not generated. This must be done manually if required. Note that a bug in ITEX does not allow the use of multiple REPLACE or OMIT in an ASN.1 derived constraint(!). NR 20 Aliases Not generated Aliases are not generated. This must be done manually if required. NR 21 Verdicts Not generated The only verdicts assigned by TTCN Link are FAIL on the ?OTHERWISE and INCONC on the ?TIMEOUT in the default behaviour generated by TTCN Link. PASS and other verdicts (including preliminary) results are not generated, these must be added manually. R 22 Comments Not generated Comments in the SDL are not reflected in the TTCN. All comments must be added manually. R ETSI TR 101 279 V1.1.1 (1998-07) 14 Table 5: TTCgeN summary 1 Test Purposes Generated (Limited) Test purposes are in the form of system level MSCs. The MSCs were generated semi-automatically from incomplete MSCs using the ObjectGEODE simulator. 2 Test Structure Test Groups Not generated Same comment as for TTCN Link 3 Concurrent TTCN Not generated Same comment as for TTCN Link 4 Type Definitions Not generated Newtypes defined in the SDL (including struct types) are not translated. 5 User Defined Ops Not generated Same comment as for TTCN Link Test Suite Params Not generated Same comment as for TTCN Link 6 Test Suite Constants Test Suite Variables Test Case Variables Not generated Same comment as for TTCN Link 7 PCOs Not Generated PCOs are not generated. This needs to be fixed. 8 Test Case Behaviour Generated Once the Test Purposes as MSCs have been created with the simulator, TTCgeN runs fully automatically in the form of a batch command. The output is standard TTCN-IS MP format on a Test Case basis. These MP files can then be (manually) merged into a Test Suite and input to ITEX or TTCN export. 9 Timers Generated Timer operations (such as START and CANCEL) added later automatically. However, TTCgeN appears to generate a START Timer operation for all Send events and a corresponding CANCEL Timer for all Receive events. This needs to be fixed. 10 Assignments & Qualifiers Not generated Same comment as for TTCN Link 11 REPEAT GOTO Not generated Same comment as for TTCN Link 12 Test Steps Not generated Same comment as for TTCN Link 13 Default Behaviour Generated (Limited) Same comment as for TTCN Link 14 Parametrisation Not generated Same comment as for TTCN Link 15 ASP definitions Generated If it is required to express the ATS in terms of ASPs rather than PDUs then the PDU definitions must be manually cut and pasted into the ASP definitions in ITEX. If it is required that the PDUs are carried in ASPs then the ASPs must be defined manually and all send/receive events updated accordingly, possibly through the use of aliases. 16 PDU definitions Generated All SDL signals on channels to the environment are translated to TTCN PDUs (See also ASPs). 17 Constraints Generated (Limited) Constraints are expressed in tabular (not ASN.1) format. Because sub-types are not translated, only the top-level constraints are generated. 18 Matching symbols Not generated Same comment as for TTCN Link 19 Derived constraints Not generated Same comment as for TTCN Link 20 Aliases Not generated Same comment as for TTCN Link 21 Verdicts Generated (Limited) TTCgeN assigns the following verdicts: PASS for the matching SUT behaviours, FAIL for the impossible SUT behaviours, INCONC for the unexpected SUT behaviours Preliminary verdict results are not generated and must be added manually 22 Comments Not generated Same comment as for TTCN Link ETSI TR 101 279 V1.1.1 (1998-07) 15 Table 6: TTCN Maker summary 1 Test Purposes Generated Test purposes are based on single state transitions. Directives such as hiding of certain signals can be used to limit the test purposes. 2 Test Structure Test Groups Generated Directives can be applied to generate test group paths. Like the test purposes these are state and transition oriented. The Test Suite indexes can be automatically generated by ITEX. 3 Concurrent TTCN Not generated Same comment as for TTCN Link 4 Type Definitions Not generated Same comment as for TTCN Link 5 User Defined Ops Not generated Same comment as for TTCN Link Test Suite Params Not generated Same comment as for TTCN Link 6 Test Suite Constants Test Suite Variables Test Case Variables Generated These are derived from constants and variables that appear in the SDL. Directives can be used to indicate whether or not an SDL constant/variable shall be reflected in the TTCN. 7 PCOs Generated All system channels are treated as potential PCOs. A directive can be used to hide PCOs, if wished. 8 Test Case Behaviour Generated Once the directives have been set the tool generates TTCN.MP V8.3. The tool is very fast. 9 Timers Generated NOAC Timer operations (START and CANCEL) are added automatically. 10 Assignments & Qualifiers Not generated Same comment as for TTCN Link 11 REPEAT GOTO Not generated Same comment as for TTCN Link 12 Test Steps Not generated The contents of test steps for preambles and check-state sequences are not generated. These must be added manually. 13 Default Behaviour Generated (Limited) Same comment as for TTCN Link 14 Parametrisation Generated Constraints are parameterised. 15 ASP definitions Not generated If it is required to express the ATS in terms of ASPs rather than PDUs then the PDU definitions must be manually cut and pasted into the ASP definitions in ITEX. If it is required that the PDUs are carried in ASPs then the ASPs must be defined manually and all send/receive events updated accordingly, possibly through the use of aliases. 16 PDU definitions Generated All SDL signals on channels to the environment are translated to TTCN PDUs (See also ASPs). The tool also generates SDL Timeouts as PDUs. As these are not considered to be signals in a ‘real’ protocol these PDUs need to be deleted. 17 Constraints Generated The tool generates a base constraint for each PDU. Individual constraints are built using the TTCN REPLACE mechanism. 18 Matching symbols Not generated Same comment as for TTCN Link 19 Derived constraints Generated See 17 20 Aliases Not generated Same comment as for TTCN Link 21 Verdicts Generated TTCN Maker assigns PASS verdicts in the body. FAIL is assigned in the default behaviour and INCONC is associated with the NOAC timeouts. 22 Comments Not generated Same comment as for TTCN Link ETSI TR 101 279 V1.1.1 (1998-07) 16 |
fd725f06335eff4cc5ea94e4aedbf58e | 101 279 | 12 Errors/ambiguities found in Q.2931 and Q.2971 | One of the beneficial side-effects of this work number discovered a number of errors/ambiguities in Q.2931 as modified by ETS 300 443-1 [2] and Q.2971 as modified by ETS 300 771-1 [1]. A list of these errors was submitted to WG SPS 5 for further submission to ITU-T. ETSI TR 101 279 V1.1.1 (1998-07) 17 History Document history V1.1.1 July 1998 Publication ISBN 2-7437-2347-5 Dépôt légal : Juillet 1998 |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 1 Scope | The present document describes the Digital Video Broadcasting Terrestrial (DVB-T) specification for digital terrestrial TV broadcasting. It tries to draw attention to the technical questions that need to be answered in setting up a DVB-T network and offers some guidance in finding answers to them. It does not cover issues linked to the content of the broadcasts such as Service Information (SI), Electronic Programme Guides (EPG) and Access Control (CA). Guidelines for implementation of MPEG-2 and Service Information (SI) can be found in ETR 154 [i.1] and ETR 211 [i.2]. |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 2 References | References are either specific (identified by date of publication and/or edition number or version number) or non-specific. For specific references, only the cited version applies. For non-specific references, the latest version of the reference document (including any amendments) applies. Referenced documents which are not found to be publicly available in the expected location might be found at http://docbox.etsi.org/Reference. NOTE: While any hyperlinks included in this clause were valid at the time of publication ETSI cannot guarantee their long term validity. |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 2.1 Normative references | The following referenced documents are necessary for the application of the present document. Not applicable. |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 2.2 Informative references | The following referenced documents are not necessary for the application of the present document but they assist the user with regard to a particular subject area. [i.1] ETSI ETR 154: "Digital Video Broadcasting (DVB); Implementation guidelines for the use of MPEG-2 Systems, Video and Audio in satellite, cable and terrestrial broadcasting applications". [i.2] ETSI ETR 211: "Digital Video Broadcasting (DVB); Guidelines on implementation and usage of Service Information (SI)". [i.3] ETSI EN 300 421: "Digital Video Broadcasting (DVB); Framing structure, channel coding and modulation for 11/12 GHz satellite services". [i.4] ETSI EN 300 429: "Digital Video Broadcasting (DVB); Framing structure, channel coding and modulation for cable systems". [i.5] ETSI EN 300 744: "Digital Video Broadcasting (DVB); Framing structure, channel coding and modulation for digital terrestrial television". [i.6] ETSI ETS 300 813: "Digital Video Broadcasting (DVB); DVB interfaces to Plesiochronous Digital Hierarchy (PDH) networks". [i.7] ETSI ETS 300 814: "Digital Video Broadcasting (DVB); DVB interfaces to Synchronous Digital Hierarchy (SDH) networks". [i.8] CENELEC EN 50083-9: "Cable networks for television signals, sound signals and interactive services - Part 9: Interfaces for CATV/SMATV headends and similar professional equipment for DVB/MPEG-2 transport streams". ETSI ETSI TR 101 190 V1.3.2 (2011-05) 10 [i.9] ISO/IEC 13818-1: "Information technology - Generic coding of moving pictures and associated audio information: Systems". [i.10] ITU-R Recommendation BT.1306: "Error-correction, data framing, modulation and emission methods for digital terrestrial television broadcasting". [i.11] Void. [i.12] ITU-T Recommendation G.703: "Physical/electrical characteristics of hierarchical digital interfaces". [i.13] ETSI TS 101 191: "Digital Video Broadcasting (DVB); DVB mega-frame for Single Frequency Network (SFN) synchronization". [i.14] EBU BPN 005: "Terrestrial digital television planning and implementation considerations. Third issue, Spring 2001". [i.15] CEPT Chester 97: "The Chester 1997 Multilateral Coordination Agreement relating to Technical Criteria, Coordination Principles and Procedures for the introduction of Terrestrial Digital Video Broadcasting (DVB-T)". [i.16] ITU-R Recommendation BT.419: "Directivity and polarization discrimination of antennas in the reception of television broadcasting". [i.17] ITU-R Recommendation P.370: "VHF and UHF propagation curves for the frequency range from 30 MHz to 1 000 MHz. Broadcasting services". [i.18] ITU-R Recommendation BS.1203-1: "Digital sound broadcasting to vehicular, portable and fixed receivers using terrestrial transmitters in the UHF/VHF bands". [i.19] ITU-R Recommendation BT.1368-4: "Planning criteria for digital terrestrial television services in the VHF/UHF bands". [i.20] ETSI EN 301 192: "Digital Video Broadcasting (DVB); DVB specification for data broadcasting". |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 3 Symbols and abbreviations | |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 3.1 Symbols | For the purposes of the present document, the following symbols apply: α constellation ratio for hierarchical modulation Δf OFDM carrier spacing ΔF Frequency difference ΔT guard interval duration Ω electrical impedance (ohm) φmin minimum power flux density at receiving place (dBW/m2) φmed minimum median power flux density, planning value (dBW/m2) Aa effective Antenna Aperture (dBm2) b number of bits per carrier B receiver noise bandwidth BO bandwidth (in MHz) in which the two DVB-T signals are overlapping BW bandwidth (in MHz) of the wanted signal Cl location correction factor (dB) CRI inner code rate CRRS Reed-Solomon code rate (188/204) Emin equivalent minimum field strength at receiving place (dBμV/m) Emed minimum median equivalent field strength, planning value (dBμV/m) F receiver noise figure ETSI ETSI TR 101 190 V1.3.2 (2011-05) 11 FA actual frequency being considered fk RF position of the kth carrier FR reference frequency k Boltzman's constant Lb building penetration loss (dB) Lf feeder loss (dB) Lh height loss (10 m a.g.l. to 1,5 m. a.g.l.) (dB) M megaframe index Pmmn allowance for man made noise (dB) Pn receiver noise input power PR protection ratio PR(CCI) co-channel protection ratio PS min minimum receiver signal input power RU useful bitrate RS symbol rate T0 absolute temperature TU time duration of the useful (orthogonal) part of a symbol, without the guard interval TS time duration of an OFDM symbol US min minimum equivalent receiver input voltage into Zi Zi receiver input impedance |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 3.2 Abbreviations | For the purposes of the present document, the following abbreviations apply: a.g.l. above ground level AAL ATM Adaptation Layers ACI Adjacent-Channel Interference ACTS Advanced Communications Technologies and Services NOTE: Research programme supported by the European Commission. API Application Programming Interface ATM Asynchronous Transfer Mode AWGN Additive White Gaussian Noise BBC British Broadcasting Corporation (UK) BER Bit Error Ratio BPN EBU numbering system for documents C/N Carrier to Noise ratio CA Access Control CATV Community Antenna TeleVision CCI Co-Channel Interference COFDM Coded Orthogonal Frequency Division Multiplex CW Carrier Wave DAB Digital Audio Broadcasting DC Direct Current DCF77 high precision standard frequency transmitter (77,5 MHz) in Germany dTTb digital Terrestrial Television broadcasting DVB Digital Video Broadcasting DVB-C DVB Cable DVBIRD DVB Integrated Receiver Decoder NOTE: ACTS project AC 108. DVB-PI DVB Professional Interface DVB-S DVB Satellite DVB-T DVB Terrestrial EBU European Broadcasting Union ETSI ETSI TR 101 190 V1.3.2 (2011-05) 12 END Equivalent Noise Degradation EPG Electronic Programme Guides ERP Effective Radiated Power FFT Fast Fourier Transform FM Frequency Modulation GPS Global Positioning System HP High Priority IDFT Inverse Discrete Fourier Transform IF Intermediate Frequency IFFT Inverse Fast Fourier Transform LP Low Priority MFN Multi-Frequency Network MIP Mega-frame Initialization Packet MPEG Moving Picture Experts Group MSF high precision standard frequency transmitter (60 kHz) in England MUX MUltipleX NICAM Near-Instantaneous Companded Audio Multiplex OFDM Orthogonal Frequency Division Multiplex PAL Phase Alternation Line (Colour TV-System) PAL B carriers location variant of PAL for VHF band and 7 MHz channel bandwidth PAL D carriers location variant of PAL for VHF band and 7 MHz channel bandwidth PAL G carriers location variant of PAL for UHF band and 8 MHz channel bandwidth PAL I carriers location variant of PAL for UHF band and 8 MHz channel bandwidth PAL K carriers location variant of PAL for UHF band and 8 MHz channel bandwidth PCR Programme Clock Reference PDH Plesiochronous Digital Hierarchy PES Packetized Elementary Stream PID Packet IDentifier PR Protection Ratios PRBS Pseudo-Random Bit Sequence PTS Presentation Time-Stamp QAM Quadrature Amplitude Modulation QEF Quasi Error Free QPSK Quadrature Phase Shift Keying (4-PSK) RCPC Rate Compatible Punctured Convolutional RF Radio Frequency RMS Root Mean Square (value) SAW Surface Acoustic Wave SDH Synchronous Digital Hierarchy SECAM SequentiellE Couleur Avec Memoire (French Colour-TV System) SECAM D variant of SECAM used in Eastern Europe SECAM K variant of SECAM ised in French dependencies SECAM L variant of SECAM used in France SFN Single Frequency Network SHF Super High Frequency (3 GHz to 30 GHz) SI Service Information STM Synchronous Transport Module STS Synchronization Time Stamp TPS Transmission Signalling Parameters TS Transport Stream TV TeleVision UHF Ultra-High Frequency (300 MHz to 3 000 MHz) VALIDATE Verification And Launch of Integrated Digital Advanced Television in Europe NOTE: ACTS project AC106. VHF Very High Frequency (30 MHz to 300 MHz) ETSI ETSI TR 101 190 V1.3.2 (2011-05) 13 |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 4 DVB-T system - outline | The DVB-T system addresses the terrestrial broadcasting of MPEG-2 coded TV signals. Therefore an appropriate adaptation of the digital coded transport stream to the different terrestrial channel characteristics is necessary. These requirements result in a flexible transmission system that uses a multi-carrier modulation, the so called Orthogonal Frequency Division Multiplex (OFDM) technique, combined with a powerful concatenated error correction coding (Coded Orthogonal Frequency Division Multiplex, COFDM). The aim of the following clauses is to give a general idea of the parameters of the DVB-T system. To achieve a maximum spectrum efficiency when used within the UHF bands, the OFDM technique with two options in the number of carriers, three modulation schemes and different guard intervals allows the operation of small and large Single Frequency Networks (SFN). In a specified range, the reception of identical programmes from a number of transmitters on the same frequency is beneficial. As far as bandwidth requirements are concerned the preferred channel spacing is 8 MHz, but if desired, 7 MHz or 6 MHz spacing is also possible by scaling down all system parameters (see ITU-R Recommendation BT.1306 [i.10]). The concatenated error correction can be separated in two blocks: the outer coding and outer interleaving are common to the Satellite and Cable Baseline Specifications and the inner coding is common to Satellite Baseline specification. The use of inner interleaving is specific to the DVB-T system. To accommodate different transmission rates, in addition to five code rates, three types of non-differential modulation schemes can be selected: QPSK, 16-QAM and 64-QAM. The 16-QAM and 64-QAM can also be used in combination with uniform or non-uniform mapping rules and thus input data streams can be separated in a low and a high priority data stream with different error protection for hierarchical transmission purposes. This feature allows the simulcast broadcasting of different programmes with different error protection and coverage areas. For reasons of receiver economy hierarchical transmission, is supported by the DVB-T system whilst hierarchical coding is not. The characteristics of this very highly flexible transmission system are described in more detail within the following clauses. |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 4.1 Modes of operation | To avoid disturbances by interference from echoes or from the signals from adjacent transmitters in SFNs, a guard interval is inserted between consecutive OFDM symbols. The guard interval precedes every OFDM symbol. Echoes of the previous symbol should abate within the guard interval. Otherwise the echoes would disturb the following OFDM symbol and increase the Bit Error Ratio (BER). Therefore, the required length of the guard interval depends on the application to be covered. Considering an SFN, the distance between two adjacent transmitter stations determines the necessary length of the guard interval. Simulations have shown that a guard interval of at least 200 µs is necessary for large area SFN. A longer guard interval could compensate longer echoes: • lengthening the guard interval without changing the absolute duration of the useful interval would accordingly decrease the channel capacity, thus reducing the deliverable bit rate; • alternatively, lengthening both the guard interval and the useful interval would not bring any penalty to the channel capacity, but would make the signal processing more difficult because of the higher number of carriers that would result from the larger symbol duration. ETSI ETSI TR 101 190 V1.3.2 (2011-05) 14 Table 1 summarizes the possible lengths of the guard interval specified in the DVB-T (see EN 300 744 [i.5]) specification depending on the chosen FFT length. Table 1: Specified lengths of the guard interval Proportion to the length of the useful interval Length of the guard interval 8k-mode 2k-mode 1/4 224 µs 56 µs 1/8 112 µs 28 µs 1/16 56 µs 14 µs 1/32 28 µs 7 µs The longer guard intervals are suitable for networks with longer distances between the particular transmitter station, as for example with national single frequency networks. The shorter intervals are suitable for regional or local broadcast transmissions. According to table 1, there are two different modes regarding to the number of carriers. The length of the useful interval is 896 µs for the 8k-mode and 224 µs for the 2k-mode. Due to the orthogonality of the system, this corresponds to a carrier distance of 1 116 Hz and 4 464 Hz, respectively. One basic requirement for the DVB-T system was the bandwidth constraint in order to match an 8 MHz channel spacing. From this requirement one can derive the number of possible carriers. 6 817 carriers per OFDM symbol for the 8k-mode (6 048 useful, the others for synchronization and signalling) and 1 705 carriers per OFDM symbol for the 2k-mode (1 512 useful carriers) are specified in the DVB-T system. The OFDM symbols can be calculated by the Inverse Discrete Fourier Transform (IDFT). Virtual carriers are inserted in such a way that the total number of carriers becomes a power of two, so that the faster algorithm of the Inverse Fast Fourier Transform (IFFT) can be used. At the receiving side, the corresponding signals can be easily recovered using the respective 2k-FFT or 8k-FFT. In order to ensure robust transmission of the OFDM signal, an error protection code is applied. In addition to the fixed algorithm of energy dispersal, block coding, outer and inner interleaving, a Rate Compatible Punctured Convolutional (RCPC) code has been defined as in the DVB Satellite standard. The mother code has a constraint length of 7 bits and works with a code rate of 1/2. The two generator polynomials of the convolutional encoder are 171 and 133 in octal notation. To adapt the error protection to the actual transmitting conditions, several code rates can be chosen. The following code rates are specified in the DVB-T (see EN 300 744 [i.5]) (and DVB Satellite (DVB-S) (see EN 300 421 [i.3])) system: 1/2, 2/3, 3/4, 5/6, 7/8 The code rate 1/2 has the highest redundancy, but the highest transmission safety. This mode should be applied to strongly disturbed channels. On the other hand a code rate of 7/8 has a low redundancy but a very weak error protection. Therefore, it should be used for channels with only low interference. As mentioned above, every carrier is modulated by a modulation symbol. QPSK, 16-QAM and 64-QAM are used as modulation methods, e.g. 2, 4 or 6 bits per modulation symbol. The bits are assigned to the particular points in the phase space according to the so called Gray-code mapping. The advantage of this mapping is the fact that closest constellation points differ only in one bit. The constellation diagrams for each modulation method are illustrated in figure 1. ETSI ETSI TR 101 190 V1.3.2 (2011-05) 15 10 00 11 01 1000 1010 0010 0000 1001 1011 0011 0001 1101 1111 0111 0101 1100 1110 0110 0100 100000 100010 101010 101000 001000 001010 000010 000000 100001 100011 101011 101001 001001 001011 000011 000001 100101 100111 101111 101101 001101 001111 000111 000101 100100 100110 101110 101100 001100 001110 000110 000100 110100 110110 111110 111100 011100 011110 010110 010100 110101 110111 111111 111101 011101 011111 010111 010101 110001 110011 111011 111001 011001 011011 010011 010001 110000 110010 111010 111000 011000 011010 010010 010000 QPSK 16-QAM 64-QAM Q I Q Q I I Figure 1: Constellation diagram for the modulation methods specified for DVB-T A further feature defined in the DVB-T (see EN 300 744 [i.5]) specification is hierarchical modulation. While the audio and video quality of the analogue TV decreases gradually, digital transmission techniques preserve their reception quality up to a certain point but then suddenly show total signal disruption, as the transmission conditions become progressively poorer. To overcome this problem, the data to be transmitted can be split into two parts. The first part provides the basic TV service with a relatively low data rate and a high error protection. The second part could be used for additional services with higher data rates and weaker error protection. In general, there are two possibilities for using this second data part. On the one hand, additional programmes can be transmitted, on the other the higher data rate can be used to increase the quality of the basic service. The level of error protection can be adjusted by choosing different code rates of the inner convolutional encoder. Both data streams are modulated simultaneously. Each carrier is modulated by two data symbols with different error protection. The symbol with the higher protection is modulated using the more resilient modulation method. It carries the information about the quadrant of the constellation point in the phase space. The other symbol gives the information about the location of that constellation point within each quadrant. The need to completely separate signal processing of each data stream is a disadvantage of the method described above. Figure 2 illustrates the constellation diagrams for the hierarchical 16-QAM and 64-QAM. ETSI ETSI TR 101 190 V1.3.2 (2011-05) 16 1000 1010 0010 0000 1001 1011 0011 0001 1101 1111 0111 0101 1100 1110 0110 0100 100000 100010 101010 101000 001000 001010 000010 000000 100001 100011 101011 101001 001001 001011 000011 000001 100101 100111 101111 101101 001101 001111 000111 000101 100100 100110 101110 101100 001100 001110 000110 000100 110100 110110 111110 111100 011100 011110 010110 010100 110101 110111 111111 111101 011101 011111 010111 010101 110001 110011 111011 111001 011001 011011 010011 010001 110000 110010 111010 111000 011000 011010 010010 010000 16-QAM 64-QAM Q Q I I Figure 2: Constellation diagram for the hierarchical modulation (α = 2) The distance between the constellation points is determined by the modulation parameter α. Here, the parameter α is defined as the relation of the distance between two neighbouring constellation points of two quadrants and the distance between two neighbouring constellation points within one quadrant. In the DVB-T specification (see EN 300 744 [i.5]), three values for this parameter are defined: • α = 1 (uniform modulation); • α = 2 and α = 4. ETSI ETSI TR 101 190 V1.3.2 (2011-05) 17 In summary, the following parameters can be chosen in the DVB-T system: • code rate of inner error protection(1/2, 2/3, 3/4, 5/6, 7/8); • carrier modulation (QPSK ⇒ 2 bit per carrier; 16-QAM ⇒ 4 bit; 64-QAM ⇒ 6 bit); • guard interval length (1/4, 1/8, 1/16, 1/32); • modulation parameter α (1 ⇒ non-hierarchical; 2, 4 ⇒ hierarchical); • FFT length; number of carriers (2k ⇒ 1 705 carriers; 8k ⇒ 6 817 carriers). As noted above the net deliverable data rate depends on the code. Redundancy is added by the inner coding (dependent on the code rate) and by the outer code (204 bytes instead of 188 bytes). The net bit rate depends on the code rate of the inner error correction, the method of the carrier modulation and the chosen guard interval length. Table 2 and figure 3 summarize all possible net data rates in the DVB-T system. The net date rates are calculated from the following formula: RU = RS × b × CRI × CRRS × (TU/TS); where: RU: the useful net data rate (Mbit/s); RS: the symbol rate, 6,75 Msymbols/s; b: bits per carrier; CRI: inner code rate; CRRS: Reed Solomon code rate, 188/204; TU: duration of (useful) symbol part; TS: symbol duration, including guard interval; TU/TS: 4/5, 8/9, 16/17 or 32/33 depending on guard interval. Table 2: Net data rates in the DVB-T system (in Mbit/s) Modulation Bits per Inner code Guard interval sub-carrier rate 1/4 1/8 1/16 1/32 QPSK 2 1/2 4,98 5,53 5,85 6,03 2 2/3 6,64 7,37 7,81 8,04 2 3/4 7,46 8,29 8,78 9,05 2 5/6 8,29 9,22 9,76 10,05 2 7/8 8,71 9,68 10,25 10,56 16-QAM 4 1/2 9,95 11,06 11,71 12,06 4 2/3 13,27 14,75 15,61 16,09 4 3/4 14,93 16,59 17,56 18,10 4 5/6 16,59 18,43 19,52 20,11 4 7/8 17,42 19,35 20,49 21,11 64-QAM 6 1/2 14,93 16,59 17,56 18,10 6 2/3 19,91 22,12 23,42 24,13 6 3/4 22,39 24,88 26,35 27,14 6 5/6 24,88 27,65 29,27 30,16 6 7/8 26,13 29,03 30,74 31,67 ETSI ETSI TR 101 190 V1.3.2 (2011-05) 18 1/2 2 2/3 2 3/4 2 5/6 2 7/8 2 1/2 4 2/3 4 3/4 4 5/6 4 7/8 4 1/2 6 2/3 6 3/4 6 5/6 6 7/8 6 1/4 1/8 1/16 1/32 0 5 10 15 20 25 30 35 net data rates [Mbps] code rate of inner error protection / bits per sub-carrier guard interval DVB-T net data rates QPSK 16QAM 64QAM Figure 3: Net data rates in the DVB-T system 64-QAM 16-QAM QPSK ETSI ETSI TR 101 190 V1.3.2 (2011-05) 19 Considering the diagram in figure 4, the net bit-rates increase with higher code rates of the inner error protection, shorter guard intervals and higher stages of carrier modulation. That means a higher data rate can only be achieved by decreasing the amount of the error protection. Therefore, the lowest specified data rate (4,98 Mbit/s) corresponds to the best protected transmission (guard interval = 1/4; inner code rate = 1/2; QPSK modulation). This is illustrated by the column in the left front corner of the diagram in figure 3. At the other extreme, the column in the right back corner corresponds to a data transmission with the highest specified data rate (31,67 Mbit/s), but with the weakest error protection (guard interval = 1/32; inner code rate = 7/8; 64-QAM modulation). In practice, it is necessary to find a compromise between the deliverable data rate and the error protection for every application. Figure 4 shows another overview of all DVB-T modes, that gives both Carrier to Noise ratio (C/N) and net bit-rate values, as a function of the constellation, code rate, guard interval length and the different channel profiles referred to in the DVB-T specification (see EN 300 744 [i.5]). ETSI ETSI TR 101 190 V1.3.2 (2011-05) 20 64QAM 7/8 net bitrate (Mbit/s) in an 8 MHz channel 0 required C/N for quasi error-free DVB-T reception C/N (dB) (assuming a perfect receiver) D=1/32 D=1/16 D=1/8 D=1/4 Rice profile Rayleigh profile 64QAM 5/6 64QAM 3/4 64QAM 2/3 16QAM 7/8 16QAM 5/6 64QAM 1/2 16QAM 3/4 16QAM 2/3 16QAM 1/2 4QAM 7/8 4QAM 5/6 4QAM 3/4 4QAM 2/3 4QAM 1/2 26,13 24,88 22,39 19,91 17,42 16,59 14,93 13,27 9,95 8,71 8,29 7,46 6,64 4,98 29,03 27,65 24,88 22,12 19,35 18,43 16,59 14,75 11,06 9,68 9,22 8,29 7,37 5,53 30,74 29,27 26,35 23,42 20,49 19,52 17,56 15,61 11,71 10,25 9,76 8,78 7,81 5,85 31,67 30,16 27,14 24,13 21,11 20,11 18,10 16,09 12,06 10,56 10,05 9,05 8,04 6,03 4 8 12 16 20 24 28 2 6 10 14 18 22 26 Figure 4: C/N and net bit-rate as a function of the constellation, code rate, guard interval length and channel profile for all DVB-T modes ETSI ETSI TR 101 190 V1.3.2 (2011-05) 21 |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 4.1.1 Choice of modulation scheme and inner coding | As described above, three different modulation schemes (signal constellations) are available in the DVB-T (see EN 300 744 [i.5]) specification: QPSK, 16-QAM and 64-QAM. Any of these signal constellations can be combined with any of five different code rates: 1/2, 2/3, 3/4, 5/6, 7/8. The performance of a specific transmission mode depends on the combined effect of code rate and modulation scheme; from a performance point of view it is not therefore possible to treat the choice of signal constellation separately from the choice of inner code rate. Compared with QPSK modulation and for a given code rate, the data capacity for 16-QAM is doubled and for 64-QAM tripled. The corresponding required C/N values required for good reception are approximately 6 dB and 12 dB higher respectively. Similarly, both the data capacity available and the required C/N increase with higher code rates. Simulations of a Ricean channel (typical of good reception with a roof top antenna) show that the code rate of 7/8 requires approximately 6 dB higher C/N compared with a code rate of 1/2, for a given signal constellation, while the data capacity increases by a factor of 7/4. These values of required C/N are based on simulations and it is expected that the difference in a practical consumer receiver will be larger, due to a greater implementation loss for rate 7/8 compared with code rate 1/2. This is especially true when the signal constellation is 64-QAM. The C/N required at a receiver has a direct consequence on the required Effective Radiated Power (ERP) of a transmitter, which has to be increased correspondingly, for a given coverage in many cases however the maximum transmitted ERP will be restricted due to potential interference to existing analogue TV services. The choice of modulation scheme and code rate depends on the nature of the impairments expected in the channel. Figure 4 shows that the difference between the required C/N for roof-top reception (Rice profile) and for reception on an indoor portable (Rayleigh profile) is quite small for a code rate of 1/2, but for a code rate of 7/8 the difference in C/N is of the order of 8 dB. This is because the coding used in the DVB-T (see EN 300 744 [i.5]) specification is particularly robust in an OFDM system against frequency-selective interference that does not change greatly from one OFDM symbol to the next, such as stationary delayed signals or interference from analogue TV transmissions. So if such echoes or interference are expected to be the main limitation on reception, then a lower code rate will offer significantly better performance. A comparison between the two modes 64-QAM R = 1/2 and 16-QAM R = 3/4 illustrates the impact of code rate. The two modes provide the same bit rate (14,93 Mbit/s to 18,1 Mbit/s, depending on guard interval), but the performance depends on the channel: according to simulations, in Gaussian and Ricean channels (corresponding to stationary roof-top reception) the 16-QAM R = 3/4 mode is the better whereas in a highly selective channel, such as a Rayleigh channel (corresponding to portable reception), 64-QAM R = 1/2 is the preferred choice. The choice of signal constellation therefore always has to be made in conjunction with code rate and the nature of channel impairments. Reception on portable receivers is one obvious case where echoes and interference are expected to be the main limitation on reception. But even for reception with rooftop antennas the coverage area for those DVB-T transmitters that share frequency bands with analogue TV networks can be limited by interference from analogue TV transmitters. And where SFN techniques are used, delayed signals from adjacent transmitters will be common. Since robustness against interference from analogue TV signals and from delayed signals is more strongly related to the code rate than to the constellation, it will generally be better to choose a mode with a lower code rate. |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 4.1.2 Choice of number of carriers | The length of the guard interval is defined as a proportion of the useful interval Tu. The maximum length of guard interval for the 8k-mode is 224 μs compared with 56 μs for the 2k-mode. The guard interval is used to protect the signal from natural and artificial (SFN) echoes. The smallest 2k guard interval (7 μs) is usually sufficient to protect the signal from natural echoes; only in some cases, such as mountainous areas, are natural echoes longer than 7 μs. The main parameters for the choice of guard interval length are station separation distances and the size of the SFN. The choice of number of carriers mainly depends on the question whether the network will be some kind of SFN or not. If no SFN transmitters are to be included the available guard interval lengths of the 2k-mode are usually sufficient for the system to be rugged against natural echoes, although if very long echoes are expected a higher bit rate can be achieved with the 8k-mode. ETSI ETSI TR 101 190 V1.3.2 (2011-05) 22 There are in principle 4 kinds of SFN: • large area SFN (with many high power transmitters and large transmitter spacing); • regional SFN (with few high power transmitters and large transmitter spacing); • Multi Frequency Network (MFN) with a local dense SFN around each MFN transmitter (one existing site plus a number of medium power SFN transmitters and medium transmitter spacing); • SFN gap fillers (low power transmitters to fill in a small gaps in the coverage area of an MFN). The 8k-mode can cope with all of these SFN situations. The 2k-mode can cope with SFN gap fillers. It may also cope with dense MFN/SFNs if the transmitter spacing is small enough (four times more close than the corresponding 8k transmitter spacing). The maximum possible transmitter spacing depends not only on the absolute length of the guard interval, but importantly on other factors such as the length of the useful interval Tu (significantly better coverage with 8k than 2k with the same absolute guard interval length, e.g. 56 μs), signal constellation, code rate and receiver implementation. For a given length of guard interval therefore the 8k-mode provides a higher net bit-rate. The choice between the two modes depends on the need for SFN operation in the overall network and the availability and cost of receivers. Receivers built for the 2k-mode (only) cannot receive 8k transmissions. Dual mode 2k/8k receivers will however be able to receive both 2k and 8k transmissions. |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 4.1.3 Choice between hierarchical and non-hierarchical mode | The DVB-T (see EN 300 744 [i.5]) specification makes it possible to choose between a hierarchical and a non-hierarchical transmission mode. This possibility is reflected in figure 5 showing the functional block diagram of such a system, and indicating the signal processing in the transmitter stage. MUX adaptation Energy dispersal Outer coder Outer interleaver Inner coder MUX adaptation Energy dispersal Outer coder Outer interleaver Inner coder Inner interleaver Mapper Frame adaptation OFDM Guard interval insertion D/A Front end Pilots and TPS signals Transport MUXes MPEG-2 source coding and multiplexing TERRESTRIAL CHANNEL ADAPTER To aerial Encoder Encoder Encoder Encoder Figure 5: Functional block diagram of the system ETSI ETSI TR 101 190 V1.3.2 (2011-05) 23 For hierarchical transmission, the functional block diagram of the system has to be expanded to include the modules shown dashed in figure 5. Two entirely separate MPEG transport streams, referred to as the high priority stream and the low priority stream, are processed before being combined onto the signal constellation by the mapper and modulator which have to provide an appropriate number of inputs. As far as hierarchy is concerned the DVB-T system restricts itself to hierarchical modulation and channel coding. Within the system, there are no means for hierarchical source coding. This enables the receiver to be designed very economically. A programme service could be broadcast as a low bit-rate, rugged version together with another version of higher bit- rate and less ruggedness. This mode is referred to as the "simulcast mode". Alternatively, entirely different programmes could be transmitted on separate streams with different ruggedness. In each case, the receiver requires only one set of inverse elements: inner de-interleaver, inner decoder, outer de-interleaver, outer decoder and multiplex adaptation. The only additional requirement of the receiver is the ability for the demodulator/de-mapper to produce one stream selected from those at the sending end. The basic features as well as preferred applications for both of these modes will be explained in the following clauses. |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 4.1.3.1 Non-hierarchical mode | Referring to figure 5, the non-hierarchical mode requires only the solid signal processing path. As the splitter is no longer necessary for that application, all MPEG transport packets will undergo the same interleaving and channel coding procedure and will then be mapped onto the appropriate constellation pattern. This means that all MPEG transport packets will be equally treated by the modulator and will thus be equally rugged while being transmitted. As the packet payload will be scrambled due to the interleaver modules, there is no predetermined relationship between a particular bit of the packet payload and the position of that bit in the constellation diagram. In other words, the channel encoding procedure does not allow for particular bits of the MPEG packets to be mapped onto specific positions in the constellation diagram. Thus, it is of no benefit to use non-uniform modulation parameters for the modulator, so an uniform modulation factor (α = 1) is mandatory for non-hierarchical transmission mode. The non-hierarchical transmission mode does not necessarily imply that only one programme can be broadcast at a time. It is likely that several programmes will be transmitted within one OFDM signal, i.e. in one RF channel (multi-programme mode); depending on the MPEG transport multiplex, several programmes can be transmitted as long as their capacity requirements do not exceed the available bit-rate of the chosen transmission mode. In the non-hierarchical transmission mode, all MPEG transport packets are processed and encoded in the same way leading to an equal grade of ruggedness for all programmes within that stream. To receive one complete programme of the received stream, the receiver has to select the desired programme by identifying the appropriate MPEG transport packets after demodulation. This is performed by the demultiplexer which is incorporated in the receiver to ensure exactly this capability. Typical applications of non-hierarchical modes can generally be divided into multi- and single-programme transmissions. Single programme modes are mainly dedicated for applications where the transmission constellation requires the full bandwidth for one transmitted programme, e.g. to achieve high quality or a large coverage area. For multi-programme transmission, on the other hand, the channel capacity is shared by more than one programme. A typical example would be a multiplex of four different programmes. It is the network provider who chooses the appropriate modulation and channel code for the multiplex. ETSI ETSI TR 101 190 V1.3.2 (2011-05) 24 |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 4.1.3.2 Hierarchical mode | As noted above, the DVB-T system enables the possibility of a hierarchical transmission mode which can be considered as an opportunity to transmit a service multiplex in two independent channels which can thus be protected differently in order to optimally match the channel or coverage requirements. Two different modes are feasible for this mode, which are referred to as "simulcast" and "multi-programme" broadcast. Simulcast transmission principally carries one or more programmes which are identically covered in two complete separate MPEG transport streams, a low bit-rate stream and a high bit-rate stream. The low bit-rate stream will usually be encoded with a high grade of redundancy, i.e. low code rates (for example 1/2 or 2/3) and will be mapped onto those non-uniform constellation points which show utmost robustness among all other positions. Preferable positions for that purpose are the four quadrants in the case of a QPSK modulation in combination with α > 1. These two provisions together will enable high robustness during transmission. For that reason, the associated low bit-rate stream is referred to as the High Priority (HP) stream. It carries data, which should be received even under poor or difficult channel conditions such as portable reception or reception at the border of coverage area. Conversely, the other bit-stream carries the same programme content with a higher bit-rate, which most likely has been derived by a different MPEG encoding process. The recovery of this Low Priority (LP) stream at the receiver will of course lead to a better quality on the display, but will require better reception condition for error-free decoding. Depending on the antenna installation and the reception conditions, the receiver is able to decode the most convenient bit-stream, either the low or the high priority one. An example of the system performance for both different streams is given in figure 6, which shows the bit error rate versus the carrier to noise ratio for the low and the high priority stream. 1,00E-05 1,00E-04 1,00E-03 1,00E-02 1,00E-01 1,00E+00 0,00 2,00 4,00 6,00 8,00 10,00 12,00 14,00 16,00 18,00 C/N BER HP LP Figure 6: System performance for low and high priority bit-stream in a hierarchical transmission scenario (parameters: α = 2; HP: QPSK, r = 2/3, LP: 16-QAM, r = 3/4) It can be seen from figure 6 that the LP stream needs a better carrier to noise ratio in order to obtain the same bit error rate as the HP stream. Parameters to be modified are the modulation factor α, which, if increased, will make the HP stream a little bit more robust but will shift the LP curve to higher carrier to noise ratios. A second parameter is the code rate which will control the ruggedness against available bit-rate. ETSI ETSI TR 101 190 V1.3.2 (2011-05) 25 The shape of the curves shown in figure 6 would not change very much, but their position in the diagram depends on the parameter setting. That is the reason, why only one outstanding point is necessary in order to indicated the total performance of one curve. In the DVB-T system specification, this particular point has been chosen as the necessary carrier to noise ratio in order to provide a BER of 2 × 10-4 after Viterbi decoding. This is the threshold for a proper operation of the succeeding Reed-Solomon decoder. A table comprising all values for all possible parameter setting is provided on the DVB-T specification (see EN 300 744 [i.5]). Given the fixed shape of the performance curves themselves and knowing the threshold value for the carrier to noise ratio, one can deduce all possible performance curves which can be derived from the variation of suitable system parameters. It will be up to the broadcasters and/or network provider to choose an appropriate parameter constellation. It has to be pointed out that this simulcast mode should not be seen as a graceful degradation approach, since the price for the receiver economy is that reception can not switch from one stream to the other (e.g. to select the more rugged stream in the event of reception becoming degraded) while continuously decoding and presenting pictures and sound. A pause is necessary (e.g. video freeze-frame for approximately 0,5 s and audio interruption for approximately 0,2 s) while the inner decoder and the various source decoders are suitably reconfigured and re-acquire lock. As a result, the simulcast mode is dedicated to decode either the low priority or the high priority stream. A favourable application is that one programme can be decoded by portable receivers with reduced quality while fixed antenna receivers are capable to recover the same programme content with enhanced sound and display quality. Applications for hierarchical transmission are not only restricted to simulcast operations. As shown in the functional block diagram of figure 5, the low priority bit-stream does not need to contain the same programme, but may carry one or more totally different programmes. The behaviour of the DVB-T system regarding performance of multi-programme operation is similar to that of the simulcast system, i.e. performance figures can be derived from the appropriate modulation scheme being either the QPSK modulation for the HP stream or the QPSK or 16-QAM modulation for the LP stream. One attractive transmission scenario for multi-programme operation is the robust transmission of a programme that can be decoded by a portable receiver. In addition to that, a fixed receiver with a directive antenna would be able to also decode the LP stream providing additional programmes with nearly the same quality compared to that of the HP one. Following the general conclusion of the simulcast mode, it will be up to the broadcaster and/or network provider to choose the suitable transmission parameter setting for the trade-off between available bit-rates and robustness. The parameter of robustness of course is directly convertible in an appropriate coverage area. Again, by means of applying a specific set of transmission parameters for both the HP and the LP stream, the broadcaster is able to serve an appropriate coverage area. |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 4.1.4 TPS explanation (use of TPS) | The Transmission Parameter Signalling (TPS) information is mapped onto specific carriers within the OFDM frame. This information is transmitted for the benefit of the receiver and is used for signalling parameters related to the transmission scheme. These parameters are: • frame number in a superframe; • modulation scheme; • hierarchy information; • inner code rates; • guard interval length; • transmission mode. The information listed above is conveyed in a block of TPS pilots. The number of TPS pilots in one OFDM frame depends on the transmission mode: • 17 TPS blocks are transmitted in parallel for the 2k-mode; and • 68 TPS blocks are transmitted in parallel for the 8k-mode. Figure 6 shows an OFDM frame with TPS pilots at the black marked positions. A column of the black marked positions builds one TPS block. ETSI ETSI TR 101 190 V1.3.2 (2011-05) 26 The number of information bits transmitted in a TPS block is equal to the number of TPS pilots in a TPS block because each TPS pilot is coded by one information bit. For the security that a TPS block will be received and decoded correctly a TPS block is transmitted in parallel on different carrier positions as shown in figure 7. That is why each OFDM symbol (located in a row in figure 7) conveys only one TPS information bit but more than one TPS pilot. ⇒ TPS pilots in the same OFDM symbol convey the same information. The TPS information bits for modulation scheme, hierarchy information, inner code rate, guard interval and transmission mode transmitted in the actual superframe m (a superframe is a set of four frames) always apply to the following superframe M + 1. All other bits refer to the actual transmitted superframe m. 0 0 5 10 62 67 34 50 209 346 413 596 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . T P S p i l o t s data, continual pilots and scattered pilots carrier indices O F D M symbol number carrier indices for TPS pilots one TPS block (transmitted in parallel for 17 times in 2k mode or for 68 times in 8k mode) Figure 7: Location of TPS pilots in a frame containing data signals and pilots |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 4.2 Transmitter input signal | The transmitter input signal is specified as an MPEG-2-TS multiplex (see ISO/IEC 13818-1 [i.9]), which may contain several TV programmes and also possibly some sound/data only programmes. This clause gives some guidance on the bit-rates needed for services and explains some of the terms relevant to the MPEG-2 multiplex, it also explains how the DVB-T (see EN 300 744 [i.5]) specification relates to the Cable (DVB-C EN 300 429 [i.4]) and Satellite (DVB-S EN 300 421 [i.3]) specifications. Clause 8 covers the synchronization requirements that need to be imposed in SFN operation. ETSI ETSI TR 101 190 V1.3.2 (2011-05) 27 |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 4.2.1 MPEG-2-TS multiplex signal | |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 4.2.1.1 Services and bit-rates | The DVB-T (see EN 300 744 [i.5]) specification offers a range of deliverable bit-rates from 4,98 Mbit/s to 31,67 Mbit/s. In planning a service it is important to have an idea of the bit-rates needed for different kinds of service. Because the impairments of MPEG-2 video coding are very different from those of analogue coding and transmission it is difficult to specify bit-rates giving quality equivalent to today's television. As a very general guide, most non-critical programmes can be satisfactorily coded with 4 Mbit/s to 4,5 Mbit/s for the video component whilst for prestige broadcasts or critical material (e.g. sport) at least 6 Mbit/s may be necessary. Improvements in MPEG coding might reduce these figures by 10 % over the next few years, but such improvements are more likely to improve the small proportion of scenes where present coders fail dramatically rather than to make a significant difference to the quality of average scenes. Further improvements are likely to require new coding techniques that would not be compatible with today's MPEG decoders. Stereo audio may be coded in as little as 192 kbit/s (although pseudo-surround-sound systems such as Pro Logic will require more). Standards for multi-channel surround sound are currently being debated; bit-rates between about 400 kbit/s and about 900 kbit/s have been suggested, depending on the coding technique used and whether it is backwards compatible with stereo-only decoders (if this compatibility is required, the bit-rate will be towards the higher end of this range). Viewers are used to selecting TV programmes by selecting an RF channel. Since digital broadcasting offers several programmes in a single channel, some kind of Electronic Programme Guide (EPG) is essential to help viewers to navigate between the programmes offered. The bit-rate needed for an EPG depends on the Applications Programming Interface (API) chosen for the receiver, but could be as little as 0,25 Mbit/s to 0,5 Mbit/s. Although it may be convenient to allocate a constant bit-rate to each service, it is not essential to do so. Some existing MPEG coding and multiplexing equipment allows dynamic control of the bit-rate for each service; however, implementing such dynamic multiplexing may impose some technical constraints - for example it would probably be necessary for the MPEG-2 coders and the multiplexer to be physically close to each other and controlled by the same computer. Dynamic multiplexing will also cause additional difficulties with downstream drop-and-insert multiplexing such as would be needed in a regional network. |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 4.2.1.2 Some technical background | The MPEG-2 Transport Stream (TS) contains one or several Programmes. A programme, in MPEG terms, is a single broadcast service such as BBC-2 or RAI Uno, not an individual TV programme as broadcasters normally use the word. A programme comprises one or more Packetized Elementary Streams (PES), each containing a single digitally coded component of the programme, for example coded video or coded stereo audio; it will also contain time stamps to ensure that specified elementary streams are replayed in synchronism at a decoder. In the TS there are also tables of Service Information (SI) giving details of the multiplex and the nature of the various elementary streams, access control information and Private Data channels whose content is not specified by MPEG such as for Teletext or broadcasters' internal communication and control channels. The TS was devised for multi-programme applications in error-prone channels such as broadcasting. It comprises a succession of packets, each 188 octets long, called Transport Packets. Each transport packet carries data relating to one elementary stream only. No error protection is specified by MPEG, but appropriate protection such as a Reed-Solomon code and packet interleaving can easily be applied to the TS to suit the expected error characteristics of the transport medium. The bit-rate of the TS is determined by the application. An MPEG multiplexer inserts null packets to adapt the sum of the bit-rates of its inputs to the required output bit-rate. The physical layer (serial/parallel, signal levels, connectors, etc.) is not specified by MPEG. In DVB however, TS interfaces for broadcasting applications are standardized (see EN 50083-9 [i.8]). ETSI ETSI TR 101 190 V1.3.2 (2011-05) 28 |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 4.2.2 Relation to DVB-C and DVB-S signals | The DVB-T (see EN 300 744 [i.5]) specification is one of a family of specifications including DVB-C (see EN 300 429 [i.4]) for cable systems and DVB-S (see EN 300 421 [i.3]) for satellite broadcasting. All use MPEG-2 coding for video and audio and MPEG syntax for multiplexing. They all use the Reed-Solomon RS(204,188, t = 8) code which allows correction of up to 8 random erroneous bytes in a received word of 204 bytes. However, the satellite specification adds an inner error correction code and for the terrestrial specification the error correction coding of the satellite specification, frequency and bit interleaving are added. The methods of modulation are also different, being adapted to the characteristics of the channels. However, from the operational point of view, the main difference is that the satellite and cable specifications allow higher bit-rates (35 Mbit/s to 40 Mbit/s); thus conversion between either satellite or cable and terrestrial specifications will involve an MPEG remultiplexing operation, dropping services if converting to the terrestrial specification and adding services (or null packets) if converting from the terrestrial specification. The SI data will have to be revised at the remultiplexer to reflect the changed content of the multiplex and the change of delivery medium. |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 4.3 Transmitter output signal | |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 4.3.1 Power definition as RMS value | The output signal of a DVB-T transmitter consists of thousands of carriers modulated in phase and amplitude. Therefore it resembles a Gaussian noise signal. It should be noted, however, that very high peaks of the sum signal are limited due to effects in the process of generating and amplifying the signal. The only simple way to define the power of a COFDM signal like DVB-T is an RMS definition. It is also closely linked to the theoretical system analysis. As the number of carriers of a given DVB-T system (either 2k or 8k) is constant and all carriers have defined power, the total power of a DVB signal is the sum of all carrier power values. In practice only the total power can be measured. In principle one symbol is insufficient for assessing the power. With thermal power meters the integration time constant is much larger than a symbol period allowing valid measurements. |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 4.3.2 Spectrum mask to limit adjacent channel interference | The nominal bandwidth of a DVB-T signal is approximately given by the product of the number of carriers and the intercarrier distance. Adjacent to the signal within the nominal bandwidth the spectral density does not completely vanish but exists at a level which is dependent on the prefiltering after signal generation, the non-linear distortion of the power amplifier and the filtering after this amplifier. The side lobes of the DVB signal extend into the adjacent channels and consequently interfere with signals in this channels. Different network configurations and different systems which suffer out-of-band interference require specific attenuation of those side lobes. EN 300 744 [i.5] on DVB-T provides examples for co-sited analogue TV-transmitters. It should be noted that the spectrum masks are expressed as the attenuation of a 4 kHz portion compared to the total symbol power at a given frequency outside the nominal bandwidth in order to comply with the general usage of interference considerations. ETSI ETSI TR 101 190 V1.3.2 (2011-05) 29 |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 4.3.3 Characterization of behaviour for planning by protection ratios | In planning terrestrial transmitter networks or even single transmitters an important aspect is the mutual interaction of different transmissions, either of the same system or of different types. To simplify the matter a single technical term is used: the protection ratio. It is the ratio of the wanted signal power to the interfering signal power for a given degree of subjective or objective degradation of the wanted signal. The chosen degradation is often the limit of allowed degradation for a reception point to be considered as belonging to the service area. There are different levels of acceptability for the degradation depending on its duration, the "continuous" protection ratios apply for 50 % of time, whereas the "tropospheric" protection ratios apply for only 1 % of time. The protection ratios have to be determined for all relevant combination of signals. The measurement is normally done in a well defined laboratory environment. While the so-called co-channel-protection ratios are mainly system dependent, the so-called adjacent-channel protection ratios are dependent on the out-of-band parts of the signal (see clause 4.3.2) and the spectrum filtering in the receiver. During the system implementation the properties of receivers may change and consequently protection ratio measurements should be undertaken from time to time to follow the technological development. |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 5 Basic aspects of DVB-T networks | |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 5.1 MFN or conventionally planned networks | |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 5.1.1 Principle of MFN | Conventionally planned DVB-T networks consist of transmitters with independent programme signals and with individual radio frequencies. Therefore they are also referred to as Multi Frequency Networks (MFN). Whether a number of transmitters is considered to belong to a specific network is an administrative matter rather than a technical one. In order to cover large areas with one DVB-T signal a certain number of radio-frequency channels is needed. The number of channels depends on the robustness of the transmission, i. e. the type of modulation associated with the applied channel code rate and on the objective of planning, (full area coverage or coverage of densely populated areas only). |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 5.1.2 Frequency resources needed for MFN | As the robustness of a broadcasting system is generally expressed in terms of protection ratios, one might expect that the number of channels needed for DVB-T is significantly lower than for analogue broadcasting as the protection ratios are generally lower in the digital case. However, due to the "brick-wall behaviour" of digital signals the direct application of the planning rules for analogue transmission is not appropriate without an extra allowance of the order of 10 dB to 20 dB (to be verified by field tests) for the local variation of the signal strength. Therefore the number of radio-frequency channels needed for conventionally planned DVB-T networks tends to be in the same order as with analogue TV systems. The frequency resource expressed as the number of channels needed to provide one signal at any location is far higher with MFN than with Single Frequency Networks (SFN). Depending on how intensively the frequency bands for analogue TV are used, some DVB-T transmitters may be added without significant impact on the existing services. Accepting this prerequisite, only local services with restricted service areas may be possible in a given country. However, this may be considered as a starting scenario, which can be extended to achieve wider coverage later on when analogue services are to be faded out gradually. The allocation of radio frequency and radio power for each transmitter needs thorough calculation of the mutual interference of all transmitters inside and outside the network according to internationally agreed rules. Each time a non-covered area is to be included in the service a new process of finding and co-ordinating a frequency for this area is necessary (see clause 9 on network planning). ETSI ETSI TR 101 190 V1.3.2 (2011-05) 30 |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 5.1.3 Non-synchronous operation | The transmitters in an MFN have not to obey rules of synchronous emissions. Therefore no co-ordination between transmitter operators is absolutely necessary. The installation of local or regional services is easy with the MFN concept compared to the SFN concept. In an SFN it is not possible to provide an extra service for only a part of the common service area. Regional services however, can also make use of the SFN concept employing only few transmitters. |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 5.1.4 Excess power | Due to terrestrial propagation effects, the received power at a given distance from the transmitter varies significantly with location and, to a much smaller extent, with time. As digital transmission does not degrade gracefully as power level reduces, but suddenly breaks down, increased transmitter power is needed to compensate for these variations, especially at the edge of the service area. Possible values for this increase in power are of the same order as are considered for the increment of the protection ratios (see clause 5.1.2), i.e. 10 dB to 20 dB. If full area coverage is achieved by overlapping the service areas of adjacent transmitters, the location variations of signal strength from different transmitters may not be strongly correlated, so that not all transmitter signals will suffer the same attenuation at a given location in the overlap area. Thus the receiver may choose the strongest signal and excess power is not needed to the extent mentioned above. Location variation may also be reduced by the use of SFN techniques as described in clause 5.3. |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 5.2 Single Frequency Networks (SFN) | |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 5.2.1 Principle | In a SFN, all transmitters are synchronously modulated with the same signal and radiate on the same frequency. Due to the multi-path capability of the multi-carrier transmission system (COFDM) signals from several transmitters arriving at a receiving antenna may contribute constructively to the total wanted signal. However, the limiting effect of the SFN technique is the so-called self-interference of the network. If signals from far distant transmitters are delayed more than allowed by the guard interval they behave as noise-like interfering signals rather than as wanted signals. The strength of such signals depends on the propagation conditions, which will vary with time. The self-interference of an SFN for a given transmitter spacing is reduced by selecting a large guard interval. It should be noted that the impact of delayed signals outside the guard interval may depend on receiver design. As an empirical rule, to successfully reduce self-interference to an acceptable value the guard interval time should allow a radio signal to propagate over the distance between two transmitters of the network. In order to keep the redundancy due to the guard interval down to a reasonably low value (25 %), the useful symbol length has also to be large given the transmitter spacing in most European countries. Thus the 8k-mode was introduced. On the other hand a smaller guard interval would lead to a higher number of transmitters. |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 5.2.2 Frequency efficiency | With the SFN technique large areas can be served with a common multiplex at a common radio centre frequency. Therefore the frequency efficiency of SFNs appears to be very high compared to MFNs . However, taking into account the presence of similar networks offering other programme multiplexes in adjacent areas, further radio frequency channels are required. The number of channels needed for international co-ordination is 4 at minimum, in practice 5 or 6 are realistic (see also clause 9 on network planning). Gaps in the coverage area of an SFN are easily filled by adding a new transmitter without the need for additional frequencies. ETSI ETSI TR 101 190 V1.3.2 (2011-05) 31 |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 5.2.3 Power efficiency | The SFN technique is not only frequency efficient but also power efficient. This can be explained by considering the strong local variations of field strength of any given transmitter. In conventionally planned networks and particularly in single transmitter situations, a common way to achieve service continuity at a high percentage of locations is to include a relatively large fade margin in the link budget and thus to increase the transmitter power significantly. However with omnidirectional reception in SFNs, where the wanted signal consists of several signal components from different transmitters the variations of which are only weakly correlated, fades in the field strength of one transmitter may be filled by another transmitter. This averaging effect results in smaller variations of the total field strength. Accordingly SFNs can use lower powered transmitters. This power efficiency of an SFN is important in the fringe area of a given transmitter and is often called "network gain". The benefit occurs only for reception on low-gain, omnidirectional antennas as are often associated with portable reception. Conventionally planned networks offer a corresponding benefit only if the receiver is tuned to the frequency of the strongest signal after each change of location. |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 5.2.4 Synchronous operation | A price to pay for frequency and power efficiency is the synchronous operation of all transmitters in a given network. Achieving synchronism of all transmitters needs specific provisions (see clause 8 on SFN operation). In networks for large area coverage with 8k-mode and guard interval of 1/4 (i.e. 224 µs), tolerances of ±5 µs should not cause performance degradation. The requirement of synchronous transmitter operation has significant impact on the distribution of the programme multiplex signal to the transmitters (see clause 7.1 on primary distribution). In irregularly spaced networks the self-interference may be minimized by a specific time offset of certain transmitters (see clause 9 on network planning). The synchronous operation of all transmitters in an SFN does not preclude altering any part of the modulation signal at any transmitter within the SFN, e.g. to install a local service inside the network. The difference in the modulation signal causes the transmitter in question to turn to an interferer affecting the surrounding transmitters for the duration of signal difference. |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 5.3 MFN with local dense SFN around each MFN transmitter | Also in an MFN, based on an existing transmitter infrastructure, a system mode capable of SFN operation may be of great importance, since it allows for a future gradual improvement in coverage in general and portable coverage in particular without new frequency assignments being necessary. By introducing additional medium to high power SFN transmitters (with separate feeding) around a main transmitter a local dense SFN is achieved. In general 8k operation is needed for this kind of application, unless the transmitter spacing is in the order of 15 km, where a 2k-mode with 56 μs guard interval is conceivable. |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 5.4 Gap-filler | If gaps exist in a service area, as may be encountered in deep valleys, tunnels, subterranean locations or inside houses, the multipath capability of DVB-T enables these gaps to be filled in a very efficient way. It is also possible to extend the service area by such re-transmitters (see clause 8.2.1 and figure 11) without additional costs for primary distribution and modulators. The principle is as follows: outside the gap or the uncovered sub-area the DVB-T signal is picked up by a directional antenna. After filtering and amplification the signal is retransmitted (at the same frequency) into the uncovered area. The most important precondition for application of a gap-filler is a sufficient isolation between the antennas. To prevent the re-transmitter from oscillating, the gain of the re-transmitter has to be less than the feedback. ETSI ETSI TR 101 190 V1.3.2 (2011-05) 32 Feedback Transmitter gain From the receiving antenna To the transmitting antenna Figure 8: Principle of a re-transmitter |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 5.4.1 Professional gap fillers | A professional gap-filler should have sufficient power to provide coverage for an otherwise uncovered area. The maximum possible radiated power depends on both the isolation between the reception antenna and the transmitting antenna and the performance of the power amplifier of the repeater. The antenna isolation depends on: • the height and dimension of the tower or building where the repeater is located; • the position of the antennas on the tower or building; • the radiation diagram of the antennas; • the location of the area which should be covered in relation to the direction to the main transmitter; • the environment around the repeater (buildings or other objects which could cause multipath). In addition to the general problem of isolation explained above, even if the feedback is lower than the amplifier gain, a decrease in the system performance has to be expected. Among all reflections there will be one dominating path coming either from the limited isolation between the antennas and/or the feedback from reflectors around the repeater station. In general, there is a time delay between the input and the output of a gap-filler, mainly due to the SAW-filter within the device. This will cause frequency selective attenuation of the retransmitted signal similar to the characteristic of a two-path or multipath reception, resulting in a degradation of system performance. Practical tests show however that this effect is negligible if the frequency selective attenuation does not exceed 10 dB. As mentioned before, the isolation depends on the overall design of the place where the repeater is installed. Experiments have shown that a sufficient isolation can be reached if a large radio tower made by concrete is used as a repeater station. Isolation values about 80 dB are realistic. If there are more levels (like platforms where antennas may fixed) it is helpful to install the antennas at different levels. |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 5.4.2 Domestic gap fillers | The domestic gap-filler is a device to amplify the signal from a domestic roof-top antenna and re-transmit it inside the house, thereby overcoming the building penetration and height-gain losses. This allows portable reception inside houses in areas with low field strength. As the field strength in rooms has to be no higher than outside the building there should be no problem with EMC. However limits for human exposure to electromagnetic radiation has to be respected. For a domestic gap-filler two implementations are being considered, the first one is a broad-band amplifier, the second one is a filtered version. The possibility to convert to Intermediate Frequency (IF), filtering and up-conversion has been evaluated to be too costly for consumer use and would have the disadvantage of being suitable for one channel only. Early tests have indicated that the concept of a domestic gap-filler is practically feasible. ETSI ETSI TR 101 190 V1.3.2 (2011-05) 33 The main aspect to be considered is the isolation between the receiving and transmitting antennas. In cases of MATV reception the isolation does not seem to be a problem. Tests in houses with individual antenna reception have shown that good isolation can be achieved in the case of a roof-top receiving antenna, but may be more difficult with the receiving antenna in the loft. Another potential problem is that in regions where the digital signals are interleaved with the analogue channels, the analogue signals could be amplified as well and could cause problems for portable analogue reception. In any case it is clear that there is a need to carry out more tests taking into account not only the reception conditions (MATV network, individual roof-top, loft or even indoor antenna) but also the various penetration losses presented by the different materials (concrete, wood, etc.) used in European houses. |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 6 Setting up DVB-T transmitters | |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 6.1 RF issues, existing sites/sharing with analogue | The digital TV transmitters will, in general, re-use the same sites as existing analogue TV transmitters. It is important to introduce digital terrestrial TV with technical and economical constraints as low as possible taking into account the current situation of the existing analogue network. Wherever possible, channels for digital broadcasting from a particular site are selected close to the analogue channels. In many cases this should allow viewers to re-use their existing receiving antenna system. During the introduction of digital services, it is important not to place unnecessary difficulties in front of potential viewers. As it is envisaged using the adjacent channels of analogue transmitters for digital TV broadcasting, the knowledge of the level of spurious emissions on these channels is of major interest. TV transmitters and especially high power TV transmitters produce out-of-channel emissions (spurious emissions). Great care is generally taken in the design of the low power stages of the transmitters in order to avoid such emissions, but the non-linearity of the power amplifiers generates emissions outside the nominal channel and particularly in the adjacent channels. In the adjacent channels, spurious emission of analogue transmitters will be seen by the digital receivers as co-channel interference. Several studies on this topic remain to be carried out. In order to minimize such spurious emissions, filters have to be used either at transmitter output or using RF selective combiners. If the existing antenna can be used two different implementations can be encountered: • the first is to use RF combining for both high power and secondary sites; • the second is an alternative solution dedicated to secondary sites which consists of using multi-channel amplification. |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 6.1.1 Use of existing antenna | In such a case, the channels chosen for digital terrestrial TV have to be inside or close to the bandwidth for which the analogue antenna has been matched. Hence, the use of the same antenna for analogue and digital channels can bring a similar coverage area for both services. More of all, most of the existing reception antennas should be suitable. On the other hand, possible ERP restrictions necessary to protect existing analogue channels broadcast from neighbouring sites cannot be satisfied. Existing antenna and feeders have to support the total multiplex power including all the peak power of digital channels. The cascading architecture leading to the multiplex of both analogue and digital channels can be problematic especially concerning adaptation losses. In this case, it will be necessary to define the relevant Equivalent Noise Degradation (END). ETSI ETSI TR 101 190 V1.3.2 (2011-05) 34 |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 6.1.1.1 RF combining | In many cases it will be useful to allocate the digital TV broadcasting band in adjacent channels of the existing analogue TV broadcasting channels. Under such conditions, considering the useful bandwidth of the DVB-T signal (7,61 MHz) to be included in a ITU-R channel (8 MHz in UHF), the selectivity of the combiner becomes a critical point. As shown in the figure 9, the combiner comprises two 3 dB couplers, two identical band pass filters and a dummy load. It has a selective input called "narrow band" and a "broadband input". The band pass filters are tuned to the narrow band input channel. narrow band input broad band input output 3 dB coupler 3 dB coupler 8-cavity filters Figure 9: RF DVB-T combiner In this case, a digital signal is connected to the narrow band input and analogue channels are connected to the broadband input. The digital signal is split two ways by a 3 dB coupler and passes through the two identical band pass filters. The two halves of the signal are then recombined by the second 3 dB coupler and passed on to the antenna. Any reflections from the filters, or any analogue signals leaking through are dissipated in the load. Similarly, the analogue signal (PAL or SECAM) is split two ways by a 3 dB coupler. This time, however, the two halves of the signal are reflected from the filters and recombined by the same 3 dB coupler before passing on to the antenna. The filters are required to pass the digital signal, yet block the channels of the broadband and especially the adjacent channel which is the more critical. Nevertheless, the use of filters leads to a group delay variation which is source of signal distortion. This distortion is directly related to the filter selectivity. In order to avoid such a problem, a baseband pre-corrector has to be used. |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 6.1.1.2 Multi-channel amplification | Secondary networks are intended to be sets of low power transmitters and repeaters used to complete the main sites coverage. Introduction of digital channels on secondary sites will lead to similar issues as those encountered on main sites. The two classical options envisaged are the installation of new antennas, dedicated to digital channels, or the implementation of RF combiners. Besides these classical issues, we focused on an alternative technique, called multi-channel amplification, experimented to ease digital channels introduction. Its main concept is to combine digital or analogue channels before amplification, which can be done with a low cost and non selective couplers. ETSI ETSI TR 101 190 V1.3.2 (2011-05) 35 As illustrated in figure 10, the treatment of the TV channels is achieved using the following procedure: • reception of the channels by means of antennas; • filtering and conversion of input channel in Intermediate Frequency (IF); • intermediate frequency treatment (use of adequate SAW filter for the adjacent channel treatment); • IF/RF conversion; • low power channel coupling; • multiplex power amplification. RF / IF converter IF / RF converter RF / IF converter IF / RF converter RF / IF converter IF / RF converter RF / IF converter IF / RF converter RF / IF converter IF / RF converter RF / IF converter IF / RF converter combiner to the antenna common amplifier C'1 C'2 C'3 C'4 C'5 C'6 C1 C2 C3 C4 C5 C6 splitter Figure 10: Typical structure of the multi-channel repeater site The treatment of the channels before power amplification is then achieved by means of typical cable network equipment. Multiplexing of the channels is simple, versatile and perfectly adapted to the addition of digital channels. The multi-channel amplification can really ease digital channels introduction. Further studies remain to be carried out in order to assess the economical advantages of such an architecture, compared to the current analogue existing one. ETSI ETSI TR 101 190 V1.3.2 (2011-05) 36 |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 6.1.2 New antenna dedicated to digital terrestrial TV | Firstly it is necessary to find an available location for a new antenna on the existing mast structure. In most cases the available aperture on existing structures is not convenient for ideal UHF use because of a more important cross-section. With these large cross-sections designing a wideband antenna is extremely difficult. The main advantage of such a situation is the absence of high power RF combiner but the drawback is the spurious emissions which are not filtered. Therefore it could be necessary to use specific filters at the transmitters output. For the new antenna diagram leads thus to a different coverage area from the analogue one. The bandwidth and matching of the antenna are specifically adapted to the broadcast of digital channels. ERP restrictions may be required in order to protect the existing analogue TV services. Because of all these reasons, the cost of a new antenna can be high. |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 7 Setting up DVB-T distribution networks | |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 7.1 Basic aspects of primary distribution | The primary distribution network (sometimes also called the transport network) carries the digital TV signal from the TV production premises to the transmitter sites (the broadcasting or secondary distribution network). This clause reviews the possible methods of primary distribution; practical networks may use a mixture of several of the possibilities described. |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 7.1.1 Centralized generation of the COFDM signal | The COFDM modulator may be at a central point and the modulated COFDM signal distributed to the transmitters using terrestrial analogue SHF links. A standard Frequency Modulation (FM) SHF link as used for analogue PAL or SECAM signals can be used for distribution of COFDM signals with reasonable performance over distances up to about 20 km. The performance could be improved by modifying the link equipment to remove the pre/de-emphasis and other circuitry not necessary for COFDM transmission and by improving local oscillators to reduce phase noise. Analogue satellite distribution of the COFDM modulated signal is also technically possible. |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 7.1.2 Decentralized generation of the COFDM signal | The MPEG-2-TS has to be distributed to all the COFDM modulators in the network. The distribution network may use fixed terrestrial or satellite links and may include further levels of MPEG-2 multiplexing, for example to provide regional programme variations. The synchronization requirements outlined in clause 7.2 have to be kept in mind when designing the distribution network. |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 7.1.2.1 Unequipped optical fibre ("Dark fibre") | Access to unequipped optical fibre (sometimes known as "dark fibre") is available in some countries depending on the regulatory regime and the network operators. If available it offers a convenient and economical method of distribution over distances up to about 100 km. The fibre is supplied without an optical source or receiver, or any other terminal or monitoring equipment; the user supplies suitable terminal equipment and makes a direct connection to the fibre, which is usually fitted with de-mountable connectors. For safety reasons, a maximum launch power is specified. Beyond this, the only restrictions on signal transmission - unless otherwise agreed - are determined by the user's terminal equipment and the optical properties of the fibre itself. ETSI ETSI TR 101 190 V1.3.2 (2011-05) 37 The use of bi-phase-mark channel code, as specified in DVB Professional Interface (DVB-PI) (see EN 50083-9 [i.8]), provides good transmission characteristics (no Direct Current (DC) component and frequent data transitions), although doubling the apparent bit rate. Short distances up to about 3 km may use multi-mode fibre with a light-emitting diode or laser transmitter; longer distances have to use single-mode fibre with a laser transmitter. Laser transmitters are available at two wavelengths: 1 300 nm and 1 500 nm. At 1 300 nm the limit of transmission distance is set by the attenuation of the fibre; at 1 500 nm, where fibre attenuation is at its minimum, the limit is set by dispersion and will therefore depend on the spectral purity of the laser transmitter. |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 7.1.2.2 PDH networks | Plesiochronous Digital Hierarchy (PDH) was designed for digitized signals based on 64 kbit/s. ITU-T Recommendation G.703 [i.12] specifies interfaces at various hierarchical levels; the interface at 34,368 Mbit/s is suitable for the TS. An interface between DVB Transport Streams and PDH networks has been specified in ETS 300 813 [i.6]. |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 7.1.2.3 SDH networks | Synchronous Digital Hierarchy (SDH) is a newer alternative to PDH using a simplified multiplexing and demultiplexing technique and offering improved network management capabilities. In Europe the network interface is at the STM-1 level of 155,520 Mbit/s. Equipment for adaptation of DVB MPEG-2-TS to SDH networks has been specified by DVB in ETS 300 814 [i.7]. |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 7.1.2.4 ATM networks | In the future, networks using the Asynchronous Transfer Mode (ATM) may be offered for primary distribution. ATM uses a cell-based multiplexing technique and may be carried over different kinds of transport networks including PDH (see ETS 300 813 [i.6]) and SDH (see ETS 300 814 [i.7]). ATM cells consist of a 5-octet header followed by 48 payload octets. Five different ATM Adaptation Layers (AALs) have been specified for adapting different types of signal to ATM networks. AAL1 or AAL5 may be used for the transmission of an MPEG-2-TS; the main difference is that AAL1 specifies error detection and correction techniques, whereas AAL5 does not. The network adapter specified by DVB for adaptation to PDH (see ETS 300 813 [i.6]) and SDH (see ETS 300 814 [i.7]) networks (see clauses 7.1.2.2 and 7.1.2.3) is based on adaptation of the MPEG-2-TS into ATM cells using AAL1 and then adapts the ATM cells to PDH or SDH framing. Thus these specifications for interfacing to PDH (see ETS 300 813 [i.6]) and SDH (see ETS 300 814 [i.7]) networks can be used for adaptation to an ATM network. |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 7.1.2.5 Satellite distribution | The TS can be distributed by satellite using the DVB-S (see EN 300 421 [i.3]) specification (see clause 4.2.2). However, a re-multiplexing operation will be required at each transmitter site to change the SI data to reflect the change of delivery medium. |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 7.1.3 Distribution network considerations for hierarchical modulation | Figure 5 in clause 4.1.3 shows the two transport streams required for hierarchical modulation being carried on two separate circuits. In cases where the two streams are generated at the same site, it is likely to be commercially desirable to carry them on a single circuit, to minimize circuit costs. The use of a conventional MPEG multiplexer to combine the two transport streams should be avoided, as this will inevitably lead to complex management of PID conflicts, etc. Indeed, for the case of an SFN, MPEG multiplexing may not be used, as it is not deterministic and cannot guarantee bit-identical streams at all transmit locations in the SFN. Carriage of the two transport streams may be achieved by the use of two virtual circuits in an ATM network, for example. However, there may be cases where it is desirable to carry the two transport streams (HP and LP) over a single ASI interface, such as on a satellite link using a single carrier. In this case, it would be possible to achieve the required deterministic combining and splitting by tunnelling the two transport streams as private data within a third transport stream, which acts as a container for the other two. EN 301 192 [i.20] describes methods of carrying private data. ETSI ETSI TR 101 190 V1.3.2 (2011-05) 38 |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 7.2 Synchronization | |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 7.2.1 MPEG timing aspects | The MPEG-2 decoder in the receiver has to regenerate the programme clock. It usually does this from samples of the programme clock (Programme Clock Reference - PCR) inserted in some of the MPEG packets - the specification requires a maximum interval between successive PCRs for each programme of 0,1 s. Any process that alters the original separation in time of successive PCRs without correcting them will cause jitter in the receiver's clock. A multiplexing or re-multiplexing operation will insert a varying number of packets between the packets of any given service, thereby potentially altering the separation of the PCRs. Therefore in general any multiplexer has to restamp the PCRs at its output; this should ensure that decoder clock jitter remains within specification. The splitting of a Transport Stream into high priority and lower priority streams for hierarchical modulation has to be regarded as a re-multiplexing operation and so will generally require re-stamping of the PCRs. In addition, each video service carries Presentation Time Stamps (PTS). These indicate the time at which a coded picture should be removed from the decoder buffer, decoded and displayed. This PTS is offset from the current time as indicated by the PCR by an amount depending on factors such as the size of the coder and decoder buffers and the bit-rate of the elementary stream. In a long chain of multiplexers, unless suitable steps are taken, it is possible for the packet to arrive after it should have been decoded - that is, the decoder buffer has underflowed. There is a mechanism in MPEG for regulating this problem. There is provision for a "multiplex buffer utilization descriptor" that indicates what timing tolerance is available for buffering the signal. Each multiplexer could use this descriptor to decide which packets should have highest priority and would re-stamp it by subtracting the tolerance that had been used in that multiplexer's buffer. However, it is not certain whether present MPEG equipments implement this descriptor. Another possibility is for the broadcaster to constrain the buffer occupancy of the coder to allow sufficient timing tolerance for the cumulative maximum expected delay through multiplexer buffers in the network. This will be difficult in practice because broadcasters may not control all the coders producing their signals, nor all the multiplexers in their distribution chains. In a regional network, drop-and-insert operations are required at regional centres to allow regional variations. Such operations can be made more easily if all MPEG coders and multiplexers are locked to a common frequency standard such as the 10 MHz reference from a frequency standard locked to GPS. In this case the same frequency standard may be used to lock the COFDM modulator as described in clause 7.2.2, third list item. However, some MPEG-2 multiplexers cannot be locked to an external clock; in such cases one of the other methods of synchronization described in clause 7.2.2 have to be used. |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 7.2.2 Synchronization of MPEG multiplexer and modulator | The bit-rate of the input MPEG-2-TS has to be constrained to that of the transmission mode chosen in the COFDM modulator. Four possible methods for synchronizing the MPEG multiplexer and the modulator have been implemented and demonstrated in practical trials. The choice should take into account the characteristics of the actual network: 1) COFDM modulator is the master: In the simplest case, where the MPEG multiplexer producing the TS and the modulator are co-sited, synchronization can be assured by a demand clock from the modulator to the multiplexer. However, the multiplexer will normally be remote from the modulator; in this case there are three further possibilities for synchronization that can also be used to overcome any problems due to wander and jitter of primary distribution network clocks. 2) Both COFDM modulator and the multiplexer are masters: The modulator has a simple remultiplexer before its input which adapts the input TS bit-rate to the bit-rate available by inserting null packets; this operation would require restamping of the Programme Clock References (PCR) for each service as appropriate and as described in clause 7.2.1. The usable input TS bit-rate has to be constrained to be less than the bit-rate deliverable by the modulator, but there is no need for synchronization between multiplexer and modulator. This method is completely flexible but is somewhat wasteful of bit-rate, more so if there are a number of remultiplexing nodes in the network upstream of the modulator. If the modulator is part of an SFN there has to be a unique stuffing and restamping unit serving all modulators of the SFN. ETSI ETSI TR 101 190 V1.3.2 (2011-05) 39 3) The multiplexer is master: The modulator has a large buffer memory at its input, the output clock of which is controlled by the degree of fullness of the buffer by a phase locked loop. 4) External master clock: Alternatively both modulator and multiplexer are synchronized to a universally available stable external clock such as the 10 MHz reference from a frequency locked to GPS, High precision standard frequency transmitter (77,5 kHz) in Germany (DCF77) or MSF. It has been shown experimentally to give perfect synchronization of a primary distribution network including a terrestrial ATM link concatenated with a satellite link with several remultiplexing operations. The use of SFN techniques will impose more stringent requirements for synchronization in the primary distribution network (see clause 8). |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 7.3 Network control and monitoring | Because of the large number of options in the DVB-T (see EN 300 744 [i.5]) specification each piece of equipment in the programme chain will have a control input to change modes, bit-rates etc. All sites will therefore need to be linked by a control and monitoring network. Although programme interfaces have been standardized by DVB, control interfaces are not standardized and are therefore proprietary to each manufacturer. Integrating equipment from different manufacturers will therefore cause difficulty in interfacing to a single control and monitoring network. |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 8 SFN operation | |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 8.1 Short recall on SFN exclusive features | • Gap-filling possibilities. • Smaller frequency reuse distance. • Spectrum efficiency. • The right power at the right place = power efficiency. • Smoother coverage. • Possibilities of tailoring/increasing the coverage area. |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 8.2 Different implementation possibilities | |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 8.2.1 Re-amplification of the signal at the RF level | In this case, a unique COFDM encoder feeds the sole main transmitter that is responsible for transposing the signal to the right RF frequency. Each gap-filler then re-amplifies - on the same RF frequency - the on-air signal received from the main transmitter. ETSI ETSI TR 101 190 V1.3.2 (2011-05) 40 main transmitter gapfillers Figure 11: Gapfillers fed from main transmitter |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 8.2.2 Analogue distribution of the COFDM signal | In this case, a unique COFDM encoder feeds all the transmitters; each transmitter is responsible for transposing the incoming signal to the right RF frequency. |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 8.2.3 Digital distribution of the MPEG stream | In this case, each transmitter is fed through a dedicated COFDM encoder and there is consequently a one-to-one correspondence between the set of transmitters and the set of encoders. This technique is the most complex one, but also the most powerful one. It can be used in combination with any of the two mentioned alternative techniques, or even with both of them. Only this digital technique for distributing the MPEG signal to the whole set of Single Frequency transmitters will be considered and further explained. |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 8.3 SFN constraints | As noted above for the Single Frequency approach, it is necessary that the signal received from any transmitter looks like an echo of the signal received from any other transmitter: as a consequence, all the related broadcast signals have to be frequency, time and "bit" synchronized. |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 8.3.1 Frequency synchronization | The OFDM signal is made of a plurality of parallel carriers and each of these thousands of carriers have to be broadcast at the same RF frequency when it is broadcast by different transmitters working on an SFN basis. The needed frequency accuracy for this depends on the spacing of carriers, or in other words, on the frequency distance between two adjacent carriers, which is often referred to as the "carrier spacing" and noted Δf. If fk denotes the ideal RF position of the kth carrier, then each transmitter should broadcast this kth carrier at fk ± (Δf/1 000) - tolerance value to be confirmed by adequate field test. To achieve this requirement, all the cascaded oscillators within each transmitter (from the baseband sampling frequency to the RF transposer, via the different IF stages) have to have a tolerance appropriate to keep the transmitted signal to the required accuracy. One way of doing this is for each oscillator to be driven by a reference oscillator, preferably accessible to all the different transmitting sites. ETSI ETSI TR 101 190 V1.3.2 (2011-05) 41 |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 8.3.2 Time synchronization | In theory: • COFDM systems have been designed to take benefit from echoes, as long as they enter the guard interval. This condition requires time synchronization of the various transmitters, since the same symbol has to be emitted at the same instant from several places, whatever the time delay introduced by the distribution network. The needed time accuracy for this is not very high, because of the intrinsic tolerance brought by the guard interval duration, which is often noted ΔT. However, since the guard interval should be used to make up for the terrestrial channel time delay spread and not to compensate inaccurate network time synchronization, an accuracy of ±1 µs seems a good basis. In practice: When echoes exceed the guard interval duration, the performances rapidly decrease for two reasons: • The orthogonality principle gets violated because of intersymbol interference. This results in a BER increase, that will be more severe as the data rate is higher: 64-QAM modes for instance will suffer from this problem more rapidly than QPSK ones. • The channel estimation is not able to correctly assess echoes longer than about a fourth of the symbol useful duration TU. Although this might depend on each receiver design, it is worth remembering that the mode ΔT = TU/4 is consequently expected to be less rugged than others with respect to the issue of echoes exceeding the guard interval duration. As a consequence of echoes management within a COFDM receiver, the actual coverage area produced by a set of SFN transmitters strongly depends on the performance of the time synchronization subsystem. A deliberate time offset at a given node of the network may in some cases allow for a fine adjustment of the coverage area, or for a greater smoothness of the available Carrier to Noise Ratio (C/N). |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 8.3.3 Bit level synchronization | Emitting the same symbol at the same time demands that all carriers be identically modulated. Consequently, the same bits have to modulate the same kth carrier. The tolerance to this rule is zero. |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 8.3.4 Energy dispersal synchronization | In order to ensure adequate binary transitions, the data of the MPEG-2-TS are randomized as soon as they enter the modulator. This is done through the binary addition of the incoming stream with a standardized PRBS which is reset every eight MPEG-2 packet. For the randomized stream to be absolutely identical in all the modulators, each PRBS generator is reset by a deterministic mechanism. |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 8.4 Network constraints | |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 8.4.1 Cable/satellite/terrestrial commonalties | To ensure the reusability of those network adapters that have been previously developed to feed the cable and satellite head-ends, the signal issued from the MUX to feed the parallel transmitters has to be MPEG-2-TS/DVB-PI (see EN 50083-9 [i.8]) compliant. |
cae8b5bfbb508d547ab0ec09c6aba240 | 101 190 | 8.4.2 Maximum time spread in the network | The maximum time spread introduced by the distribution network is the difference between the time needed by the signal to go from the MUX through the network to the nearest transmission site on the one hand and the time needed by the same signal to go from the same MUX through other branches of the network to the furthest site on the other hand. It strongly depends on the chosen technology for this network; the longest time spread are probably reached in hybrid networks, in which some transmitters are fed using one technology (e.g. fibre optics) whilst other sites are fed using another technology (e.g. satellite links). However, it is very unlikely that this transit time difference would exceed one second. ETSI ETSI TR 101 190 V1.3.2 (2011-05) 42 |
Subsets and Splits
No community queries yet
The top public SQL queries from the community will appear here once available.