Secure voice software issues

This is why you'd really want to run your modems synchronously. You'd just send HDLC frames that encapsulated IP packets (fragmentation may be required). This is how most IP routers work over serial links.

Indeed. And because LAPM uses HDLC framing over the modem, you could be completely compatible with a regular (asynch DTE interface) V.42bis modem on the other end of the call. But by implementing V.42bis yourself, you would have complete control over when frames get sent, and how large they are. Phil