Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!rutgers!netsys!vector!telecom-gateway From: clements@bbn.com Newsgroups: comp.dcom.telecom Subject: Re: those {{{{{{{{{{{{ 's Message-ID: Date: 25 Jun 89 19:56:49 GMT Sender: news@vector.Dallas.TX.US Lines: 41 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 213, message 2 of 5 >Could someone give me a semi-technical explanation for those {{{{{{{{ >which appear as interference at 1200 baud? I am going round and round >with USWestComm about this. > [Saga, involving new phone exchange in the area, deleted.] Assuming that the errors happen at a quite steady rate, this is the problem known as "clock slip" or "frame slip" on a digital inter-office trunk. The basic concept is that all the digital phone exchanges and trunks in the USA are supposed to be run from one master clock! That is, the 1.544 MHz frequency on all T1 lines (and related ones on other lines) and the internal clocks of the digital exchanges which pass samples through them are all supposed to be locked together. If they are not, one end will supply bits slightly faster or slower than the other end will consume them. When this happens, and the error builds up to the length of one sample on the trunk, all channels on the trunk drop or gain one digital sample, causing a phase error of 1/8000 second, typically causing the data error you are seeing. So the new exchange, or some of the trunks through it, are not properly phase-locked to the rest of the system. Fixing it is PROBABLY just a matter of flipping a configuration switch (in hardware or software) to establish master clock distribution correctly through the system. The hard part is finding one of the few people in your local TELCO who understands the issue. Keep asking for technical supervisors until you find someone who sounds like they recognize the concepts of master clocks and phase locking and frame slip on a digital trunk. You may have to find a name at the "engineering" department of the TELCO, who normally don't deal directly with customer complaints, and get them to work back from their side. They probably specified the system and would be annoyed that it wasn't implemented correctly. >thanks >hal > >haleden@boulder.colorado.edu Bob Clements, K1BC, clements@bbn.com