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 ------- ----------------------------------------------------------------- gopher://quux.org/ conversion by John Goerzen of http://communication.ucsd.edu/A-News/ This Usenet Oldnews Archive article may be copied and distributed freely, provided: 1. There is no money collected for the text(s) of the articles. 2. The following notice remains appended to each copy: The Usenet Oldnews Archive: Compilation Copyright (C) 1981, 1996 Bruce Jones, Henry Spencer, David Wiseman.