Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!rutgers!mit-eddie!bbn!apple!sun-barr!texsun!texbell!vector!telecom-gateway From: chip@vector.dallas.tx.us (Chip Rosenthal) Newsgroups: comp.dcom.telecom Subject: Re: Modems and LD Carriers Message-ID: Date: 21 May 89 04:45:33 GMT Sender: news@vector.Dallas.TX.US Reply-To: chip@vector.dallas.tx.us Organization: Dallas Semiconductor Lines: 34 Approved: telecom-request@vector.dallas.tx.us X-Submissions-To: telecom@eecs.nwu.edu X-Administrivia-To: telecom-request@vector.dallas.tx.us X-TELECOM-Digest: volume 9, issue 172, message 3 of 6 Jim Gottlieb writes: >X-TELECOM-Digest: volume 9, issue 169, message 5 of 8 >Certain resellers use compression to squeeze more voice circuits onto a >channel (i.e. DS1 or DS3). This tends to send data throughput down the drain. The only tariffed service using compression is M44, which uses the 32K ADPCM speech compression algorithm. It is so named because the 24 8-bit channel bandwidth of a T1 line is allocated as 44 4-bit voice channels and 2 8-bit bundled signalling channels. The ADPCM algorithm has been engineered specifically to support modems. The Europeans had a G.721 ADPCM algorithm months before the ANSI T1Y1 committee approved theirs. What was the holdup? They wanted to get modem traffic right! Turns out that the original algorithm was found to have problems with V.22bis and some patches were made to account for this. The CCITT has since gone back and updated their standard with this same algorithm. There is no compression done at the DS3 level. In practice, DS3 is used in the public network only to multiplex several DS1's. I don't believe that ADPCM is commonly used by carriers for their normal public lines. Even if it were, most modem traffic would be unaffected. (I'm not sure about PEP or V.32.) There are lots of reasons for modem problems, but I don't think this is one of them. The assertion that compression is a problem is possibly qualified by mentioning "resellers". True, there is stuff out there which can make anybody sound like an old 78-rpm record which went through a car wash. But I would think you would have to go out of your way to get a super-cheap economy service to get this kind of stuff. -- Chip Rosenthal / chip@vector.Dallas.TX.US / Dallas Semiconductor / 214-450-5337