|Dan Lanciani wrote: |+--------------- || firstname.lastname@example.org (Scott Dorsey) wrote: || |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. |+--------------- | |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.
Interesting; I thought V.34 used a variable constellation subset to adjust bits/baud according to line quality.
Anyway, I may have snipped too much. A longer quote:
|Now, back in the days when this stuff was being done, there wasn't any |digital audio transmission. Telephone trunk circuits were created with |frequency division multiplexing of 4 KHz audio channels. I don't recall |the various Telpak channel designations or the exact channel spacing |and guard bands any more, but figure that you can put more than 500 but |no more than 1000 4 KHz channels onto a 4 MHz pipe. I bet a google search |on 'telpak carriers' gets you something a bit better. | |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 assumed we were discussing the bandwidth requirements within a multiplexed carrier, not on a baseband T1 line.
|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.
I still have a small pile of one of the last generation (WorldBlazer) that supported Telebit's PEP. Actually, some of them are T3000s (sent as replacements for T2500s with the one-way data loss) with WorldBlazer firmware but without whatever board change was supposedly required to make PEP reliable. Unfortunately, the T3000 and WorldBlazer suffered from extreme latency in V.32 mode and the ring-no-answer bug that was never (as far as I know) resolved. The last days of Telebit were no fun...
Dan Lanciani ddl@danlan.*com