[Asterisk-Users] Does anyone have faxes working well with X100P and TDM40B cards?

[Nicolas Bougues]

On Tue, Mar 16, 2004 at 02:16:01PM -0600, Jim Sneeringer wrote:
> 
> Why does Asterisk not work as well with fax modems as any other phone
> system?
> 

Because Asterisk does TDM in software. It's handled by 64 bits chunks,
instead of bit by bit on TDM hardware.

It has two consequences :
- if for some reason (timing, load, bad luck...) a chunk is lost, you
  loose 1 ms of sound. This matters for modems.
- handling sound 1ms at a time induces some slight delay (>1ms), but
  which can easily produce echo on analog loops (you don't hear it
  when it's 1/8000th of a second later, but if can matter if it comes
  back a few ms later). Thus Asterisk uses echo cancellation, which
  may alter data transmission. Hopefully tone detection deactivates
  it though.

-- 
Nicolas Bougues
Axialys Interactive