Path: utzoo!utgpu!news-server.csri.toronto.edu!mailrus!accuvax.nwu.edu!nucsrl!telecom-request From: Nickolas Landsberg Newsgroups: comp.dcom.telecom Subject: Re: Reverse-voltage Phone Line Test Message-ID: <6199@accuvax.nwu.edu> Date: 9 Apr 90 19:00:52 GMT Sender: news@accuvax.nwu.edu Organization: TELECOM Digest Lines: 33 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 242, Message 8 of 9 Hi Patrick, A couple of articles have appeared where people question why sometimes their phones ring in the middle of the night, apparently on some scheduled basis. While I'm not sure that this is THE explanation for that behavior, it is a possible explanation. Most switches can be programmed to perform a "line insulation test" on subscriber lines. In the days of #1XB and #5XB this involved actually placing a special circuit on the line which measured the line resistance, and there was a special "frame" installed in the switch to do it. (I had the pleasure of seeing one of these in operation about 10 years ago. Still used vacuum tubes and a mechanical timer for when to start the test.) While I'm not familiar with the internals of how the digital switches do it, I presume the general concept is the same. Since measuring the resistance of the line requires placing some voltage across it (no, I don't know how much), it is conceivable that this voltage may be enough to trip the ringer on some newer phones. This test was/is also used to determine patterns of trouble, such as wet insulation in a cable. If a number of lines in the same cable all show low resistance, the chances are the cable has sprung a leak, and particularly in old cable plant, the paper insulation is breaking down. The test is run at night because there is typically low traffic volume and the testing doesn't have to complete with the dial-tone provisioning which is first priority and, at least in theory, any moisture in the cable will "condense" in the evening as the temperature drops. Nick Landsberg