Compressed/Encrypted Voice using Modems
Stephen D. Williams
sdw at sdwsys.lig.net
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).
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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