Compressed/Encrypted Voice using Modems

Stephen D. Williams sdw at
Tue Feb 9 21:23:07 PST 1993

> I don't see how a 28.8kbps (raw) data rate is possible, as the Shannon
> limit for a POTS line is 22kbps.  Certainly parts of the phone system
> no longer impose the narrow bandwidth that are part of the 'spec', but
> one can not always depend on getting a line that exceedes the published
> parameters of the phone system.
My impression was that most of the new systems dealt with variable 
bandwidth automatically.  28.8kbps might only be acheived on a higher
quality line.  The stated rates are max, not nominal.
> I'm also fairly sure that recent Zyxel modems (e.g. the U-1496) use
> this Rockwell chip(set), and not a dedicated DSP.
They told me they use their own design 'datapump', and I know they use
a 68K (I swapped the rom in mine).
> Jim

Also, from a previous note, you wouldn't want to turn off V.42/V.42bis
since that is where the error correction is.  Also, even on compressed
data, you get some additional bandwidth because it does packetized 
synchronous data.  This gets close to 8bits/byte instead of 10 (start,

I get around 1760cps for LZW (zoo/compress) data.
This is Zyxel to non-zyxel (Cerfnet or World).


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