|A T-1 circuit transmits 1.54 Mbps, which in a perfect world with zero |channel noise requires 1.54 MHz bandwidth to transmit. In reality it's |happy with a bit more bandwidth.
I would have thought that as the noise approaches zero (and the S/N ratio approaches infinity) the required bandwidth approaches zero. 1.54Mb/s shouldn't require anything like 1.54MHz of bandwidth until the S/N ratio approaches unity, and that's an incredibly noisy channel.
Dan Lanciani ddl@danlan.*com
The Los Angeles area had a variety of telephone companies. Two which come to mind, besides the Bell company, were the Sunjand-Tujunga Telephone Company and the California Water and Telephone Company. There were so mamy companies that it was common to see want ads seeking such things as an experienced telephone switchman or an experioenced installer or repairman.
The entire area was SxS into the 1930s and mostly in the 1940s.
They developed senders to put in front of the switches to do translations and other functions since the limitations of SxS surpassed the ability of SxS to route calls in the complex and growing area.
London, England, was all step with senders, too, even later than Los Angeles.
The L.A. area had operator toll dialing in the 1920. The senders made it fairly easy to add other features.
DDD was later added to many all or predominantly step offices, not just in Los Angeles but all over the country. Oklahoma City and Tulsa both got DDD th9s way; I think Dallas, too. It was called Centralized Automatic Message Accounting (CAMA). The Dial 1 for DDD rule fit this application exactly, since on the "1" dial pull the call was trunked directly to the CAMA equipment which handled all subscequent digits.
Wes Leatherock snipped-for-privacy@yahoo.com snipped-for-privacy@aol.com
Yes, but... You have to change your encoding to use more distinct signal levels per Baud, and almost all modems were (and almost all still are) designed with *fixed* D/A encoding (and AA/D decoding) schemes. One notable exception was the Telebit trailblazer, which chopped the ~3 KHz analog phone line up into many narrow subcarriers, and could adaptively choose each sub carrier's modulation among 1, 2, 4, or 6 bits/Baud, depending on current S/N and IM distortion. It was the exception to the rule.
+--------------- | 1.54Mb/s shouldn't require anything like 1.54MHz of bandwidth until | the S/N ratio approaches unity, and that's an incredibly noisy channel. +---------------But T-1 didn't use an adaptive modulation system -- it used a single,
*fixed* transmission method, AMI (Alternate Mark Invert), which is effectively no more efficient than NRZ binary.[1] Thus the only thing that happens when the S/N ratio gets better/worse is that the *error rate* gets better/worse -- the spectral efficiency doesn't change.-Rob
[1] Well, except that AMI allows an average D.C. disparity of zero, which makes it nice for transformer coupling. But to keep the clock density high enough for good sync you have to either use "bit-robbing" on the LSB, or use B8ZS encoding [which came later in the evolution of T-1, when "clear-channel 64"-kb was introduced]. (And, yes, I know that bit-robbing was also used for E&M until CCIS came in.)----- Rob Warnock
627 26th Avenue San Mateo, CA 94403 (650)572-2607***** Moderator's Note *****
The ones density requirement for AMI is 1 in 13 bits, and the "stress test" for such a line is 1-in-13 to this day. Although robbed-bit signalling _sometimes_ provided greater ones density, it could fall short on a busy carrier with most of the trunks in service, since the low-order bit was robbed only during idle state, and when the trunk was seized the low-order bit was part of the voiceband enconding.
This probably seems like a difference that makes no difference, because a trunk that's in use would be expected to produce a fair number of ones just from modulation. However, pauses between sentences are surprisingly long, and of course only one side of a conversation is talking at once anyway, so two adjacent channels with a fairly "quiet" conversation going on could produce more than 13 zero signals in a row. It was also very common to have E&M channel units which were carrying SF signalling, and that could mean a "all zero" state during pauses between outpulsing, even when the connection wasn't completed yet. More to the point, it was common practice to "busy out" trunks by forcing them into seizure, which also sometimes resulted in an all zero state.
ESF changed the signalling away from robbed-bit, which made B8ZS necessary, and that solved the problem. The only remaining issue is one of semantics: even experienced techs and engineers are prone to refer to a DS1 span as "1.536" Mb/s, even though B8ZS obviated the need for the 1-in-193 framing pulse that was used on the original D-1 banks, so a "data pipe" DS1 is really running at the full 1.544 Mb/s rate, and throughput can actually exceed that by a very small amount, due to the shortening of the words which B8ZS entails.
Bill Horne Temporary Moderator
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