Secure voice software issues

[Phil Karn]

>so I haven't been able to verify which V.42/V.42bis/MNP options will let
>you squeeze out the start/stop bits to let you fit 13.3 kbps of data over it,

Whenever V.42 error correction (LAPM) is enabled, synchronous HDLC
frames are what actually pass over the link. So the start/stop bits are
already removed.  Unfortunately, the packetizing done by LAPM adds delay we
don't want for a real time voice application. And if you turn off LAPM,
you return to sending the start/stop bits over the wire.

Most V.32 and V.32bis modems provide for direct synchronous operation,
which would let us have our cake and eat it too, except that few PCs
can speak synchronously to a modem. This may require some extra
hardware (sigh).

Phil