Page 118 - DCAP104_EXPOSURE_TO_COMPUTER_DISCPLINES

Unit 6: Data Communication

When the user is finished, the escape sequence, “+++” followed by a pause of about a second, Notes
may be sent to the modem to return it to command mode, then a command (e.g. “ATH”) to hang
up the phone is sent. Note that on many modem controllers it is possible to issue commands to
disable the escape sequence so that it is not possible for data being exchanged to trigger the mode
change inadvertently.
The commands themselves are typically from the Hayes command set, although that term is
somewhat misleading. The original Hayes commands were useful for 300 bit/s operation only, and
then extended for their 1,200 bit/s modems. Faster speeds required new commands, leading to a
proliferation of command sets in the early 1990s. Things became considerably more standardized
in the second half of the 1990s, when most modems were built from one of a very small number
of chipsets. We call this the Hayes command set even today, although it has three or four times
the numbers of commands as the actual standard.

6.3.1.1 Increasing Speeds (V.21, V.22, V.22bis)
The 300 bit/s modems used audio frequency-shift keying to send data. In this system the stream
of 1s and 0s in computer data is translated into sounds which can be easily sent on the phone
lines. In the Bell 103 system the originating modem sends 0s by playing a 1,070 Hz tone, and 1s
at 1,270 Hz, with the answering modem putting its 0s on 2,025 Hz and 1s on 2,225 Hz. These
frequencies were chosen carefully, they are in the range that suffer minimum distortion on the
phone system, and also are not harmonics of each other.
In the 1,200 bit/s and faster systems, phase-shift keying was used. In this system the two tones
for any one side of the connection are sent at the similar frequencies as in the 300 bit/s systems,
but slightly out of phase. By comparing the phase of the two signals, 1s and 0s could be pulled
back out, for instance if the signals were 90 degrees out of phase, this represented two digits, 1,
0, at 180 degrees it was 1, 1. In this way each cycle of the signal represents two digits instead of
one. 1,200 bit/s modems were, in effect, 600 symbols per second modems (600 baud modems)
with 2 bits per symbol.

Figure 6.5: A 2,400 bit/s Modem for a Laptop

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