Path: utzoo!attcan!uunet!mailrus!accuvax.nwu.edu!nucsrl!telecom-request From: rpw3%rigden.wpd@sgi.com (Rob Warnock) Newsgroups: comp.dcom.telecom Subject: Re: Data Access Lines Message-ID: <8393@accuvax.nwu.edu> Date: 30 May 90 05:22:18 GMT Sender: news@accuvax.nwu.edu Reply-To: Rob Warnock Organization: Silicon Graphics Inc., Mountain View, CA Lines: 55 Approved: Telecom@eecs.nwu.edu X-Submissions-To: telecom@eecs.nwu.edu X-Administrivia-To: telecom-request@eecs.nwu.edu X-Telecom-Digest: Volume 10, Issue 395, Message 1 of 9 In article <8371@accuvax.nwu.edu> John Higdon writes: | Mike Riddle writes: | > My understanding is that a 9600 bps modem actually operates at 2400 baud, | > with 4 levels, creating a 9600 bps signal... | I don't have the reference in front of my and can't give a detailed | explanation of PEP (Packetized Ensemble Protocol), but it is somewhat | more complex than that. PEP (I don't know anything at all about the | theory of v.32) tries for as many as 512 separate carriers (each | operating very slowly) over the line... Excerpts (scraps, really, the original is almost 300 lines) from a document posted to comp.dcom.modems 6 Mar 90 by Mike Ballard & Cerifin Castillo of Telebit (write to for more info): Telebit Corporation Revision 1.01 01 DECEMBER 1989 A BRIEF TECHNICAL OVERVIEW OF TELEBIT MODEMS ... This technique (DAMQAM) divides the voice bandwidth into 511 individual channels each capable of passing 2, 4, or 6 bits per baud based on the measured characteristics of the individual frequencies associated with each channel. On a typical phone connection, the modem uses a subset of about 400 of those channels. Each time the modem connects to a circuit established on the dialup Public Switched Telephone Network (PSTN), the TELEBIT modem measures the quality of the connection, and determines the usable subset of the 511 carriers. The aggregate sum of bits modulated on this subset of carriers multiplied times the baud rate yields a bit per second rate that on a local telephone connection (i.e. round trip through your local telco) is 18031 bps. This 18031 bps is then reduced by about 20% to allow for the CRC overhead, to about 14400 bps of data throughput. ... The modem operates at 7.35 and 88.26 baud, transparently changing baud rates to accomodate the pace and quantity of data traffic. When in "interactive mode" the modem sends data using 11 msec packets (which run at 88.26 baud). Each packet contains 15 bytes of data. In "file transfer mode" the modem uses 136 msec packets (that transfer at 7.35 baud) that contain 256 bytes of data. The TrailBlazer decides which packet size to use on an ongoing dynamic basis. No intervention from the user is required. So the rate never exceeds 88.26 baud. Your local telco ought to be able to do *that* at least... ;-} ;-} Rob Warnock, MS-9U/510 rpw3@sgi.com rpw3@pei.com Silicon Graphics, Inc. (415)335-1673 Protocol Engines, Inc. 2011 N. Shoreline Blvd. Mountain View, CA 94039-7311