| ==Phrack Inc.== | |
| Volume Two, Issue 21, File 5 of 11 | |
| /\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\ | |
| \/ \/ | |
| /\ Satellite Communications /\ | |
| \/ ~~~~~~~~~~~~~~~~~~~~~~~~ \/ | |
| /\ By Scott Holiday /\ | |
| \/ July 11, 1988 \/ | |
| /\ /\ | |
| \/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/ | |
| Satellite communications systems employ microwave terminals on satellites and | |
| ground to earth stations for highly reliable and high-capacity communications | |
| circuits. The communication satellites are positioned in geosynchronous orbits | |
| about 22,000 miles above the earth. Thus the rotation of the satellite matches | |
| that of the earth, and the satellite appears motionless above earth stations. | |
| Three equally spaces satellites are required to cover the entire world. | |
| The satellite's microwave terminals receive signals from an earth station and | |
| retransmit those signals on another frequency to another earth station. | |
| Because of the long distances involved, the round-trip communications path | |
| takes about a half second. This is referred to as the propagation delay. The | |
| propagation delay on a regular terrestrial phone line is about 1 millisecond | |
| (ms) per 100 miles. | |
| Each microwave terminal on the satellite, designated as a repeater or | |
| transponder, includes a receiver for uplink transmissions and a transmitter for | |
| down-link transmissions. Separate bands of frequencies for up-link and | |
| down-link transmissions are designated in the 1.5-30 GHz frequency range (1.5 | |
| GHz is equal to 1,500,000,000 Hz, or 1.5 billion hertz). Typical frequencies | |
| for communications satellites are 4-6 GHz for INTELSAT 5 and 12-14 GHz for | |
| Anik-B, a Canadian satellite. | |
| Each satellite transponder typically has twelve 36-MHz channels which can be | |
| used for voice, data, or television signals. Early communications satellites | |
| had some 12 to 20 transponders, and the later satellites have up to 27 or more | |
| transponders. INTELSAT 5, for example, has a total of 27 or more transponders | |
| providing 24,500 data/voice channels, one transponder providing two 17.5-MHz TV | |
| channels, and one SPADE transponder with 800 channels. SPADE (Single carrier | |
| per channel, Pulse code modulation, multiple Access, Demand assignment) is a | |
| digital telephone service which reserves a pool of channels in the satellite | |
| for use on a demand-assignment basis. SPADE circuits can be activated on a | |
| demand basis between different countries and used for long or short periods of | |
| time as needed. | |
| Propagation Delay: | |
| The approximate quarter second one-way propagation delay in satellite | |
| communications affects both voice telephone and data communications. Users of | |
| voice communications via satellite links face two objectionable | |
| characteristics; delayed speech and return echoes. Echo suppressors are | |
| installed to reduce the return echoes to an acceptable level. Data | |
| communications operations face more serious problems caused by propagation | |
| delay. Line protocol and error detection/correction schemes are slowed down | |
| dramatically by the quarter second of delay. User response time requirements | |
| can be difficult to meet because of these cumulative effects. | |
| Satellite delay compensation units are available to ensure a connection and | |
| afford better operation for the terrestrial communications terminal that were | |
| never designed to deal with the propagation delay of communications satellites. | |
| One delay compensation unit is required at each final destination. The units | |
| reformat the data into larger effective transmission blocks so that | |
| retransmision requests are sent back less frequently. This reduces the number | |
| of line turnarounds, each of which requires about a quarter second to go from | |
| or return to the destination terminal or computer. One error detection and | |
| correction method used, called GO-BACK-N, requires that all blocks of data held | |
| in the transmitting buffer, back to the one with the error in it, must be | |
| retransmitted. A more efficient method is to retransmit only the block of data | |
| with the error, but this requires more logic in the equipment at each end. | |
| Link to Earth Stations: | |
| Most users cannot afford a satellite earth station, so a land line is needed | |
| for a connection to the nearest earth station (Which they tell me is 65,000 bps | |
| for a leased line). Because of the great distance the signal must travel in | |
| space, the relatively short distance between the two users on earth becomes | |
| insignificant and actually does not affect the operating cost. It is generally | |
| not economical. This is particularly true of high-capacity or broadband | |
| applications. Even though operating costs are insensitive to distance, | |
| satellite companies may still charge more for longer distances based on | |
| terrestrial line competition. | |
| Nonterrestrial Problems: | |
| The nonterrestrial portion of satellite communications bypasses the problems | |
| encountered with broken phone lines, etc., but it has its own unique set of | |
| problems. Since satellite communications employ high-frequency microwave | |
| radio transmission, careful planning is required to avoid interference between | |
| the satellite and other microwave systems. Eclipses of the sun, and even the | |
| moon, can cause trouble because they cut off the source of energy for the | |
| satellite's solar batteries. Backup batteries are used to resolve most of | |
| these difficulties, but the problem that is the most severe is when the sun | |
| gets directly behind the satellite and becomes a source of unacceptable noise. | |
| This occurs 10 times a year for about 10 min each time. In order to obtain | |
| uninterrupted service, an earth station must have a second dish antenna a short | |
| distance away or the single dish antenna must have access to another satellite. | |
| Accessing the Satellite: | |
| There are three methods by which multiple users (earth stations) can access the | |
| satellite. The first is frequency-division multiple access (FDMA), whereby the | |
| total bandwidth is divided into separate frequency channels assigned to the | |
| users. Each user has a channel, which could remain idle if that user had no | |
| traffic. Time-division multiple access (TDMA) provides each user with a | |
| particular time slot or multiple time slots. Here the channels are shared, but | |
| some time slots could be idle if a user has no traffic to offer. With | |
| code-division multiple access (CDMA) each user can utilize the full bandwidth | |
| at any time by employing a unique code to identify the user's traffic. There | |
| are, of course, trade-offs among the three methods; they involve error rate, | |
| block size, throughput, interference, and cost. | |
| Advantages: | |
| o Satellite lines are exceptionally well suited for broadband applications | |
| such as voice, television, and picture-phone, and the quality of | |
| transmission is high. | |
| o Satellite lines are generally less expensive for all voice and data | |
| types of transmission, whether it be dial-up or a leased line that is not | |
| short. This is particularly true of overseas transmissions, and there is | |
| no underwater cable to create maintenance problems. | |
| Disadvantages: | |
| o The propagation delay of about a quarter second way requires the | |
| participants of a voice conversation so slightly delay their responses to | |
| make sure no more conversation is still on the way. The propagation delay | |
| has more of a severe effect on the transmission of data, and the effect | |
| becomes more pronounced with high speeds, half duplex operation, smaller | |
| blocks of data, and polling. Satellite delay units, front end processors, | |
| multiplexers, and other devices have been designed to get around these | |
| problems, but there is no solution to the half second lost in total | |
| response time for interactive applications. | |
| o Some of the modems currently in use today have not been designed to handle | |
| the long delay of the initial connection via satellite, and the result can | |
| be a lost connection. This can be frustrating when the common carrier | |
| elects to use satellite lines for regular dial-up calls up to say, 55 | |
| percent of all calls out of a particular city during the busy traffic | |
| periods. | |
| Closing: | |
| Satellite communications is a very interesting topic to study. Perhaps even | |
| the present/and future satellite and Ham radio "Hackers" will one day be | |
| running a Bulletin Board off of a WESTSTAR satellite -- Who's to say there | |
| isn't one now? (Devious Snicker) | |
| --Scott Holiday | |