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T1 Tip/Ring Polarity

I've been trying to determine the significance of maintaining correct polarity for T1 circuits. Based on the little bit of reading I've done on this, I have a theory about it and a corollary. Is this basically correct?

For bipolar signals, such at DS-1, correct polarity is requred to exactly reproduce the incoming pulse train but failure to maintain correct polarity has no practical consequences.

For unipolar signals, incorrect polarity may cause the receiver to fail to properly interpret the signal. This depends on the receiver circuit's characteristics.

Reply to
Bob Simon
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That is correct, but I would expect any receiver worth its salt to be able to detect the polarity inversion and correct for it -- phone technicians know what polarity is, but only so they can sneer at it.

AFAIK the only signaling over T1 _is_ bipolar (I assume this is the DS-1 that you're talking about); it was done because the lines are often transformer coupled. I suppose that other schemes that maintain zero average voltage would work -- do you know of any?

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Reply to
Tim Wescott

Receivers are indifferent to the line (tip and ring) physical polarity.

The DS1 line signal (Line Code) is bipolar, Alternate Mark (binary 1) Inversion (AMI), and it may be modified by Binary 8 Zero Suppression (B8ZS) to satisfy the AMI ones density requirement. More plainly, the Nyquist frequency for the DS1 rate is 772 kHz and each half cycle may be a pulse, denoting a binary 1, or no-pulse, denoting a binary zero. At receivers, the AMI signal is converted to mono-polar, examined for the B8ZS codeword which if found, is replaced by an all-zero octet, and then processed by the framer, etc.

Don

Reply to
Don Bowey

I'm having a bit of trouble relating to the 772kHz freq you stated.

I know that the phone company developed T1 equipment to economically transport multiple (24) analog voice channels in the digital domain. Since those voice channels carry a 4kHz signal, an 8k sample rate is required. (Nyquist)

24 channels * 8 bits/sample + 1 framing bit = 193 bits/frame
  • 8k samples/sec = 1.544 Mbps

I'd like to hear how I can use the fact that the Nyquist frequency for a DS1 is 772kHz. Does this have some physical significance?

Reply to
Bob Simon

Worst case bit pattern of a AMI coded signal is all ones which results in a waveform with fundamental frequency of 772kHz if the bit rate is 1.544Mbps.

Regards, Allan

Reply to
Allan Herriman

The answer is in what I already said. Each cycle of 360 degrees conveys 2 bits of data; the first 180 degrees is one pulse position, and the second

180 degrees is another pulse position. If I send an all-1s signal, the energy is at 772 kHz, and the bit rate is 1.544 Mbit/s.

Don

Reply to
Don Bowey

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