| ==Phrack Inc.== | |
| Volume One, Issue Five, Phile #9 of 12 | |
| Mobile Telephone Communications | |
| By Phantom Phreaker | |
| Presented by The Alliance | |
| (618)667-3825 | |
| Mobile telephone communications is not the same thing as Cellular. Mobile | |
| telephone service is not as advanced as Cellular, and not as efficient. Mobile | |
| telephone service limits the number of customers sharply, while Cellular is | |
| designed to solve the problems of Mobile telephone service. | |
| The signals for Mobile communications are sent by high-power transmitters and | |
| antennas that provide an area of approx. 20-30 miles with service. | |
| A base unit of a Mobile communications system transmits and receives on | |
| different frequencies at the same time. Typical power for the radio base | |
| station transmitter is 200-250 watts. | |
| Mobile telephone facilities tie in with the normal fixed-position telephone | |
| system, however base units can be owned by a Radio Common Carrier (RCC). RCCs | |
| running mobile telephone systems are charged by the telephone company for use | |
| of the normal phone system. | |
| DIAGRAM: | |
| -------- | |
| ^-Base antenna Mobile unit | |
| | | | |
| |- - - - - - - - - /-- --\ | |
| | ^Signal^ | (Car) | | |
| --------------------------------------- | |
| ^-------<-20-30 Miles->---| | |
| ^ |-From | |
| /===========\ | mobile | |
| |Receiver/ | | antenna | |
| |Transmitter| ============= | |
| /===========\ |Receiver/ | |-| | |
| |Control | |Transmitter|-|*| | |
| |Terminal | ============= |-| | |
| \===========/ ^ | |
| | Handset | |
| | <-Telephone | |
| | <-Land line | |
| | | |
| ======= ======= | |
| | C.O.| | C.O.|---[-*-] | |
| ======= ======= |*| | |
| | | ----- | |
| | | Fixed | |
| ========= ========= Phone | |
| |Switch |--------------|Switch | | |
| |Network| Transmission |Network| | |
| ========= Link ========= | |
| --------------------------------------- | |
| (Above diagram from 'Understanding Telephone Electronics' chapter 10.) | |
| As you can see from the above diagram, calls placed from the Fixed position | |
| telephone are routed through a Central Office as normal, through a Switch | |
| Network, to another Switch network, and to another CO. From the second CO | |
| (nearest to the Mobile unit), the signals are sent on a telephone line to the | |
| control terminal, to the receiver, then to the base unit (antenna). From the | |
| base unit, the radio signals are sent to the site of the mobile telephone. | |
| Calls from the mobile telephone operate in the same manner. An idle radio | |
| channel is selected (like seizure of a trunk for a LD call) and the signals are | |
| sent over the mobile network. | |
| If no channel is available for use, then a busy indication is triggered | |
| (similar to a re-order). If a channel is available, the customer will be | |
| prompted with a dial tone, similar to normal fixed-position telephone service. | |
| The area that this would work in is called the subscriber's home area. When a | |
| mobile telephone service subscriber leaves the service area, he is then | |
| referred to as a Roamer. Since the mobile unit is out of the service area, | |
| special preparations have to be made to continue communications to/from that | |
| mobile unit. | |
| SIGNALLING | |
| ---------- | |
| Mobile signalling tones are selected (like touch tones) to avoid possible | |
| reproduction of the signalling tone on the voice link, to cause a signalling | |
| mistake. The IMTS (Improved Mobile Telephone Service) uses in band signalling | |
| of tones from 1300Hz-2200Hz. Another method of signalling is the MTS (Mobile | |
| Telephone System). MTS is older than IMTS, and MTS uses in band signalling of | |
| tones from 600Hz-1500Hz, and some use 2805 Hz in manual operation. | |
| CALL COMPLETION | |
| --------------- | |
| In this instance, let's say a call is being placed from a normal telephone to | |
| a Mobile unit. First off, the base station selects one idle channel and places | |
| a 2000Hz idle tone on it. All on hook Mobile units active in that service area | |
| find and lock onto the channel that carries the 2000Hz idle tone. Now each | |
| Mobile unit listens for it's specific number on that channel. When an idle | |
| channel becomes busy, a new channel is selected for use, and the process is | |
| repeated. | |
| Now the caller's call is sent through the telephone network the same way as a | |
| normal telephone call. When this call reaches the control terminal, the | |
| terminal seizes the already marked idle channel (with every on-hook mobile unit | |
| listening to it) and applies a 1800Hz seize tone. This tone keeps other mobile | |
| units from using it to complete other calls. The called number is outpulsed | |
| over the base station transmitter at ten pulses per second, with idle tone | |
| represented as a mark, and a seize-tone represented as a space. | |
| Since every idle mobile unit is waiting on that channel, they compare the | |
| number being outpulsed with their own number. If the first digit of the called | |
| mobile unit is three, and a specific mobile unit 'listening' on the channel has | |
| a first digit of four, it stops listening to that channel, and moves to the | |
| next channel with 2000Hz applied. | |
| When the mobile unit receives the correct destination number, all other | |
| mobile units are no longer listening on that particular channel. When the 7 | |
| digit number is received, the mobile supervisory unit turns on the mobile | |
| transmitter and sends an acknowledgement signal (2150Hz guard tone) back to the | |
| control terminal. If this signal isn't received in three seconds after | |
| outpulsing, the seize tone is removed from that channel, and the call is | |
| dropped. If the signal is received at the control terminal, then the mobile | |
| phone will ring (standard two seconds on, four seconds off). If the mobile unit | |
| being called doesn't answer in forty five seconds, the call is also dropped. | |
| When the person answers the mobile phone and takes it off hook, the mobile | |
| supervisory unit sends a connect tone of 1633Hz, for an answer signal. When | |
| this is received by the control terminal, the ringing stops, and a voice path | |
| between the two phones is established. When the mobile subscriber hangs up, a | |
| disconnect signal is sent which consists of alternating disconnect/guard tone | |
| (1336Hz and 2150Hz respectively) signals. Then the mobile unit begins searching | |
| for another idle channel, and readies itself for more calls. | |
| For an outgoing call placed by the Mobile subscriber, the mobile unit must | |
| already be locked on the idle channel. If the unit is not, a warning light will | |
| flash advising the user of the problem. This is similar to a re-order signal. | |
| If the unit is already on an idle channel, the calling number will be sent to | |
| the control terminal for billing purposes. | |
| CELLULAR TELEPHONES | |
| ------------------- | |
| To improve over the problems of mobile telephone service such as low amount of | |
| users, high price, etc. AT&T invented the Cellular Concept, or the AMPS | |
| (Advanced Mobile Phone System). This is the cellular phone concept that is used | |
| in major cities. Los Angeles, Ca. currently has the largest cellular | |
| communication system in the world. | |
| Calls sent to cellular telephones are sent through the MTSO (Mobile | |
| Telecommunications Switching Office). The MTSO handles all calls to and from | |
| cellular telephones, and handles billing. | |
| All incoming calls from the MTSO are sent to a Cell site in each cell, to the | |
| actual cellular telephone. The major difference between mobile and cellular is | |
| that cellular can use the same channel many more times than a mobile telephone | |
| system can, providing more customers and making the service less expensive. | |
| Once a vehicle goes out of range of one cell site, the signal is transferred | |
| immediately, with no signal loss, to another cell site, where the call is | |
| continued without interruption. This is called a Cellular hand-off. | |
| Cellular communications areas are divided up into several cells, like a | |
| honeycomb. | |
| DIAGRAM | |
| ------- | |
| /---\ /---\ /---\ | |
| / * \/ * \/ * \ | |
| ==== |Cell ||Cell ||Cell | | |
| |CO| | Site|| Site|| Site | | |
| ==== /---\\ /\ /\ / | |
| | / * \\---/ \---/ \---/ | |
| | |Cell | /---\ /---\ | |
| | | Site|/ * \/ * \ | |
| | \ /|Cell ||Cell | | |
| ====== \---/ | Site|| Site | | |
| |MTSO| \ /\ / | |
| ====== \---/ \---/ | |
| --------------------------------------- | |
| More cell sites are used for the area they are needed for. The signals are | |
| sent from the MTSO to the each cell site. So if you were travelling in the cell | |
| site to the far left, the signal from the MTSO would be sent to that cell. As | |
| you move, the signal is moved. | |
| Here is a quote from AT&T's Cellular Telephones pamphlet. | |
| 'AT&T cellular phone transmission sounds as good as your home and office | |
| phone. Basically it's a simple concept. Each metropolitan area is divided into | |
| sectors which form a honeycomb of cells. Each cell incorporates its own | |
| transmitter and receiver which connects to the local phone network. | |
| As you drive from cell to cell, sophisticated electronic equipment transfers | |
| or 'hands off' the call to another cell site. This automatic sequence maintains | |
| service quality throughout the conversation without interruption.' | |
| I hope this file has been of some assistance to anyone who is curious about | |
| the more technical aspects of the telephone system. | |
| References | |
| ---------- | |
| Understanding Telephone Electronics-by Texas Instruments 1983 | |
| TELE Magazine issues three and four | |
| AT&T Mobile communications pamphlet | |
| AT&T Cellular concept pamphlet | |
| -End of file- | |
| 4/14/86 | |