Modem Over IP: Problems in connecting two modems using a Modem Passthru type of connection

Hi All,

I have a system where a modem is connected to a residential voip
gateway using a two wire connection. This residential gateway is
talking to an H323 - ISDN gateway over a satellite connection. In
normal voice calls I use G.729. But whenever there is a modem call,
both the gateways upspeed to G.711 and I m using G711 A-Law right now.
Disabling of Echo Cancellation in both the gateways is configurable
(by default echo cancellation is disabled whenever there is a modem
call). I am using modems from a spanish manufacturer called SATELSA
and need to make just the low data rate modems work (V.21, V.22,
V.22bis, V.23)

The problems is that when I try to establish a call between two
modems, calls go through using some models and at some specific data
rates/modulation schemes but not other combinations of models and data
rates. I tried to experiment with disabling echo cancellation just at
one end and keeping it enabled at the other. Sometimes it helps to an
extent that I see the circuit 109 (line signal detected) on at one end
(e.g. at the PSTN gateway end when it is the called end and the echo
cancellation is kept enabled at this end) but not at the other end.
The call still fails.

Now searching for ideas on how to improve this performance.
Any help in this regard will be greatly appreciated.

Warm Regards
Manish

 

Is silence suppression disabled? Is the jitter buffer frozen? What
is the packet size for G.711 after upspeed? I have found that 20ms
works well in this case.

-Greg in MA