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

Discussion in 'VOIP' started by Manish Bhasin, Jun 9, 2004.

  1. 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 Bhasin, Jun 9, 2004
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  2. Manish Bhasin

    CCGolfer Guest

    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
    CCGolfer, Jun 10, 2004
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