[asterisk-dev] SIP/H.324M Video Gateway

[Klaus Darilion]

Kevin P. Fleming wrote:
> Klaus Darilion wrote:
> 
>> Ok. This would be possible. But as you obviously know I would also like 
>> to see Asterisk supporting pure digital data connections too, for 
>> example to forward the the stream e.g. using a "clear channel" codec.
> 
> Isn't that just G.711 A-law or mu-law?

Yes and no. Yes, it is 8 bit data - just like alaw or ulaw. But the 
difference is, that "digital" or "clear channel" must not be transcoded. 
  Currently, digital data is tunneled through Asterisk either as alaw or 
ulaw. But sometimes Asterisk thinks that a certain channel should use a 
certain (preferred) codec and Asterisk performs transcoding.

Further, if a digital call is received from DAHDI, there is no user 
information layer 1 which tells alaw or ulaw. Thus, DAHDI (I can't 
remember if it is kernel module or chan_dahdi) uses the default-law - 
which is either alaw or ulaw depending on if the port is T1 or E1.

This leads to strange behaviors like outgoing calls work but incoming 
calls fail (as the wrong default-law triggers transcoding inside 
Asterisk). Further, the transcoding of alaw/ulaw/slinear in the DAHDI 
kernel module makes things worse.

I think a rather easy approach would be to introduce a pseudo codec 
"digital" or "clearchannel" with no transcoder paths - thus transcoding 
by mistake should be impossible.

regards
klaus