From: Eric Blossom <eb@comsec.com>
Date: Fri, 11 Aug 95 15:17:44 PDT
To: rjc@clark.net
Subject: PRZ encrypted voice software release imminent
In-Reply-To: <199508111816.OAA05222@clark.net>
Message-ID: <199508112143.OAA02837@comsec.com>
MIME-Version: 1.0
Content-Type: text/plain



>   If you are forced to abstract above "modems", your algorithms will
>   be designed to work over more general transmission schemes. I fear
>   that coding for modems first will lead to an overall application tuned for
>   modems, but poorly designed for asynchronous networks. The tunning should
>   be done in the driver, not the application/algorithm level. 
>   (for example, modems don't experience much "packet churn and loss",
>   and they usually have a dependable bandwidth. Even if they retrain
>   randomly from 28.8 to 14.4, they can still be counted on to atleast have
>   9600 bps throughput more consistently deliverable than say a slip/ppp line 
>   would)

asynchronous networks are a completely different beast than your basic
point-to-point phone call.  Over.

If you expect people to use a secure voice communication device,
they've got to like it.  Over.

I don't know anybody who prefers more latency.  Over.

I think that it makes great sense to optimize for a point-to-point
connection.  I also believe that it should be an un-error corrected
channel (no V.42 or V.42bis) since many speech coders can tolerate
the errors.

Knowing the channel characteristics also allows you to tailor your
crypto usage.  If you know you've got a raw synchronous channel, and
Pr(bit insert or bit delete) << Pr(bit error) than you can avoid a lot
of overhead.  This does matter where bandwidth is tight.  Say,
sticking a 13,000 bit/s coder down a 14,400 bit/s pipe.  With GSM's
260 bit frame every 20ms, it leaves 28 bits per frame for all
overhead.  This includes any forward error correction, sync
maintenance, crypto IV's etc.  You can't tune this in the driver.

None of this says that you shouldn't also optimize for the packetized
case too.  I think that you can negotiate the right behavior at start
up time based on detected channel characteristics.  I think that the
biggest impact is in the framing overhead, or lack of it.

You are always trading off bandwidth, speech quality and MIPs.

Eric