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Phrack Inc. Volume 02 Issue 21 File 05
==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
