Fax over IP (FoIP) is considered an essential feature in telephony solutions. In the past, people send faxes through fax machines using T.30, handled over the PSTN or Publicly Switched Telephone Network. FoIP has become more and more common nowadays, and this solution basically eliminates the need of sending faxes through PSTN, with the help of additional bandwidth. Regardless of the transport methods used by FoIP, one key factor to ensure the reliability of FoIP is a sufficient level of network performance.
There are two fax transport methods used by FoIP -
1. G.711 (PCMA/ PCMU)
G.711 is an ITU-T recommendation for Pulse Code Modulation (PCM) of voice frequencies. This codec requires high bandwidth (~64kbps) and uses an uncompressed format. The digital fax will be converted to a PCM audio stream and sent as G.711 in RTP packets.
2. T.38
T.38 fax is an ITU-T recommendation that allows fax data to be carried over IP networks. If you are sending digital fax with T.38, the data will be directly transmitted in T.38 without any conversion to an audio stream.
Which fax transport method is better?
Both of these two methods have their pros and cons. T.38 consumes less bandwidth and supports data and control of redundancy, which then lowers the chance of packet loss. T.38 depends a lot on the fax parameters configured on the gateway or ATA. These parameters include ECM and transmission speed. Now, these parameters are not universal, which means that that particular set of settings might work for you but not for others. And this is one of the downsides of using T.38. G.711 is cheaper as it can leverage voice data infrastructure and has fewer interoperability issues between different vendors. Although G.711 is a widely used voice protocol, it is not optimized for FoIP and it does not support packet redundancy.
In my opinion, I would say go with T.38 if the environment and costs are allowed. Although tweaking the correct workable fax parameters might be a little time-consuming, this method has a lower chance of packet loss with its redundancy support.
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