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Bell 103 ans 300(*) full 2025 2225 1070 1270 2225
Bell 202 1200 half 2200 1200 2200 1200 2025


(*)any speed up to 300 baud, can also include 75 and 110 baud
services
(**)service can either be half-duplex at 1200 baud or asymmetrical
full duplex, with 75 baud originate and 1200 baud receive (commonly
used as viewdata user) or 1200 transmit and 75 receive (viewdata
host)





Higher Speeds

1200 baud is usually regarded as the fastest speed possible on an
ordinary voice-grade telephone line. Beyond this, noise on the line
due to the switching circuits at the various telephone exchanges,
poor cabling, etc. make accurate transmission difficult. Indeed, at
higher speeds it becomes increasingly important to use transmission
protocols that include error correction.
Error correction techniques usually consist of dividing the
transmission stream into a series of blocks which can be checked, one
at a time, by the receiving computer. The 'parity' system mentioned
above is one example, but obviously a crude one. The difficulty is
that the more secure an error-correction protocol becomes, the
greater becomes the overhead in terms of numbers of bits transmitted
to send just one character from one computer to another. Thus, in the
typical 300 bit situation, the actual letter is defined by 7 bits,
'start' and 'stop' account for another two, and the check takes a
further one--ten in all. After a while, what you gain in the speed
with which each actual bit is transmitted, you lose, because so many
bits have to be sent to ensure that a single character is accurately
received!
Although some people risk using 2400 baud on ordinary telephone
lines--the jargon is the PTSN (Public Telephone Switched
Network)--this means using expensive modems. Where higher speeds are
essential, leased circuits, not available via dial-up. become
essential. The leased circuit is paid for on a fixed charge, not a
charge based on time-connected. Such circuits can be conditioned',
for example by using special amplifiers, to support the higher data
rate.
For really high speed transmissions, however, pairs of copper
cable are inadequate. Medium speed is obtainable by the use of
coaxial cable (a little like that used for tv antenna hook-ups) which
have a very broad bandwidth. Imposing several different channels on
one cable-length is called multiplexing and, depending on the
application, the various channels can either carry several different