Re: Secure voice software issues

[jthomas@kolanut.mitre.org (Joe Thomas)]

Phil Karn <karn@qualcomm.com> wrote:

[Re: squeezing 13.3 kbps data w/o start&stop bits over modem] 

> 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.

Well...  How much latency does LAPM really add?  Little enough that  
full duplex keystrokes echo back nearly instantly on my connections.   
People talk over satellite links with more delay than that all the  
time.  Since we're not going to get toll quality voice out of the  
vocoders anyway, and the error correcting stuff is especially useful  
for encrypted data streams...  I think V.42 is probably a good idea  
for a cryptophone project. 


> 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).

I'm in favor of getting a minimal version that will run on the lowest  
common denominator hardware first.   (Might have something to do with  
the fact that I just spent a couple hundred dollars on an internal  
V.32bis modem that doesn't do synchronous :^)

Joe