From: Phil Karn <karn@qualcomm.com> (Phil Karn)
Date: Wed, 10 Feb 93 21:14:12 PST
To: sdw@sdwsys.lig.net (Stephen D. Williams)
Subject: Re: Compressed/Encrypted Voice using Modems
Message-ID: <9302110512.AA09668@servo>
MIME-Version: 1.0
Content-Type: text/plain


At  0:19 2/10/93 -0500, Stephen D. Williams wrote:

>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,
>stop).

As somebody else mentioned, you do want to turn off LAPM/V.42bis when
running a speech application, because speech is a real-time application
and LAPM retransmissions introduce delay. Compression is not likely
to add much but delay either, since the vocoder itself is already
compressing the speech much more effectively than V.42bis, which is
designed specifically for textual information.

Almost all modern modems support synchronous terminals, although the
feature is rarely used. This is how you get around wasting 2 bits out
of every 10 on start and stop bits.

Phil