Secure voice software issues

[karn@qualcomm.com (Phil Karn)]

>> do not use error correction or compression.  (they will slow you down)

>huh?!?  my file transfer times would *double* if i turned off v.42bis
>(modem compression).  *that* would slow me down.  and you can't run
>v.42bis without running the error handling protocol (v.42), for obvious
>reasons.  what's more, v.42 gives an immediate 20% (or so) increase in
>throughput, by eliminating start and stop bits.

I think there's a confusion here between the conflicting goals of low
delay and high throughput. Depending on which is more important to
your application, V.42bis will either "slow you down" or "speed you
up". Unfortunately, most modems don't give the user any way to say
whether delay or throughput is more important to you at the moment;
the modems themselves try to infer this automatically from the data
streams you send them. And they bias the decision in favor of greater
throughput. This is right for most computer applications, but not so
for real time voice.

Phil