Modem over VOIP

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If you are replacing an analog PSTN line with VOIP service, in addition to ordinary phones, there may be other devices that contain data modems that were using the PSTN line. Examples are: Tivo, POS Terminals, FAX machines, etc.

The problem is that the codecs used by VOIP ATAs are designed to compress voice, not the analog signals sent and received by modems. A second problem is if a non-compressing codec is used, the transmission will be very sensitve to network QoS, i.e. packet loss, jitter, and latency will be issues. To sucessfully use data modems over a VOIP connection you will need a minimum of:
  • A non-compressing codec - ITU G.711 is the usual choice
  • A very high quality network connection

In order to make modem connections less sensitive to network QoS problems, rather than passing through the modems signals over the VOIP connection, the signals can be converted (de-modulated) at the VOIP ATA and sent as data over the network to the far end where they are converted (re-modulated) back to their original form. This method has the advantage that the data transmission over the network does not require a high QoS.

For FAXs T.38 is the standard for relaying FAX data accross IP networks.
Many VOIP endpoints/ATAs now support T.38 but many VOIP Service Providers do not support T.38 or are in the process of implementing it.

For Modems the ITU approved ITU V.150.1 (also know as V.MOIP) in January 2003. This standard defines how to relay modem data accross IP networks. This standard is not implemented yet in most VOIP equipment.

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Created by: jht2, Last modification: Sat 22 of Oct, 2011 (12:28 UTC) by faxvoip


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