30 Years of the Digest ... founded August 21, 1981
The Telecom Digest for March 23, 2012
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Date: Thu, 22 Mar 2012 19:38:58 -0400 From: email@example.com (Jim Bennett) To: firstname.lastname@example.org. Subject: Re: What Hath Bell Labs Wrought? The Future Message-ID: <4F6BB812.email@example.com> On 2012-03-21 11:02, Eric Tappert wrote: > > The problem with these frequencies is that they do not toggle all the > bits in the encoding, thus testing at those frequencies is incomplete. > Accordingly, the test sets for digital circuits test at 1015 Hz, > wihich causes all the bits to toggle in the encoding. > > There is nothing "magic" about 1 kHz. It does sort of correspond to > the peak energy in voice (even though every voice is a bit different). > Note that any sampling rate would have the same problem with > frequencies that are, as Bill put it, "even sub-multiples" of the > sampling frequency. > > I doubt that most engineers would consider this a big screw up by Bell > Labs, and certainly there are lots of other cases where the impact was > greater. All in all, they did a pretty good job of designing the > telephone network... > > ET Every "milliwatt" test tone I have seen is set at 1004 hertz. I am not saying there is no such thing as a 1015 hertz source, just that this is the first I am hearing of it. Maybe I just need to get out more. Over the years I have heard at least three different explanations for why the 1000 hertz test tone was changed to 1004 hertz. It is certainly true that sampling a 1000 hertz tone at 8000 samples per second will result in the exact same eight patterns, repeating ad nauseum. [assuming that the test tone is stable]. This is one of the explanations I have heard, i.e., that it is not a rigorous enough test for the A/D converters and/or the actual line drivers. I have always found this somewhat dubious, as my experience with these things is that they either work or they don't. Timing slips and jitter were the primary problems in the days of true DS-1 signaling over copper, and testing often involved sending "all ones." Another explanation I have heard is that 1000 hertz was never a problem until ESF came along. Extended Super Frame finds the framing bit by looking for the one bit out of 193 that consistently alternates between a "one" and a "zero" with each frame. Apparently the 1000 hertz tone would result in actual time slot bits doing the same thing, and bumping it up to 1004 solved the problem. I am not saying this is fact, just relating what I have heard over the years. Jim ================================================== Speaking from a secure undisclosed location.
Date: 22 Mar 2012 23:38:30 +0100 From: "John R. Levine" <firstname.lastname@example.org> To: email@example.com. Subject: FCC sues AT&T for Nigerian TDD scams Message-ID: <alpine.BSF.firstname.lastname@example.org> AT&T provides a TDD relay service for which the government reimburses the costs. According to the WSJ, the Justice Department is suing AT&T for fraudulent reimbursement claims. TDD relay has a dreadful problem with Nigerians calling and defrauding people. The FCC required that operators register users and verify that they're in the US. The suit claims that AT&T deliberately uses a registration scheme that doesn't keep out crooks, and as a result 95% (NINETY FIVE PERCENT) of their TDD calls are Nigerian fraud. http://online.wsj.com/article/SB10001424052702304636404577297454154404874.html Regards, John Levine, email@example.com, Primary Perpetrator of "The Internet for Dummies", Please consider the environment before reading this e-mail. http://jl.ly
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