Aucbvax.6662
fa.info-terms
utzoo!decvax!ucbvax!info-terms
Fri Apr 9 12:17:31 1982
Re: VADIC 3400 series modem
>From PMARTIN@SRI-AI Fri Apr 9 12:14:45 1982
Charley's description of the situation with the Vadic is essentially
correct, except that the "practical speed limit" for FSK (the
frequency shift coding used for 300 baud modems) is about 1200 baud.
The 600 baud estimate would be the one-way speed if symetric full duplex
communication is the goal.
The Bell 202 uses FSK to send 1200 baud one way, and then switches
around to go the other way (it's half duplex). The Stanford split
speed protocol uses the Bell 202 frequencies to produce and asymetric
channel, providing full duplex with 1200 to the terminal screen, and
150 baud from the keyboard. This is satisfactory because 150 baud =
15 chars/sec or in terms of normal typing ratings, about 180 wpm.
Many people are not seriously slowed by this constraint on their
typing speed! UDS, Prentice, Product Associates, and a host of
individuals make the split speed modems; they are in common use only
within the Stanford community.
The dibit scheme for the 3400 (and the 212) manages 2400 baud TOTAL
bandwidth, shared symetrically between the two directions. I have
heard rumors of a new Prentice modem that supports 2400 one way,, and
switches around like the old 202. It could make home terminals twice
as fast as they are now, which seems like a step in the right
direction...
Paul
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