When implementing a FoIP solution, the transport method used (T.38 or G.711), in addition to several fax settings such as fax speed, redundancy, and error correction, can have a significant impact on FoIP performance for common network impairments.
T.38 vs. G.711
T.38 fax relay is an ITU-T recommendation that allows for fax data to be carried over IP networks. Data is transmitted directly in T.38 without being converted to an audio stream, thereby significantly reducing the amount of bandwidth needed. T.38 also supports data and controls redundancy to mitigate the effects of packet loss. Some disadvantages of T.38 are that gateway support for fax parameters, such as V.34 transmission speeds and Error Correction Mode (ECM), are not universal. Also, in the current mixed network environment of packet-based and circuit-switched (that is, PSTN) connections, T.38 often has a transcoding overhead that can add latency and cost to fax services.
G.711 is an ITU-T recommendation for Pulse Code Modulation (PCM) of voice frequencies. It uses an uncompressed format and requires high bandwidth, typically about 64 kbps. Using G.711 as the transport method for FoIP is an extension of traditional PSTN audio-based faxing. The digital fax data is converted to a PCM audio stream and then sent as G.711 Real-time Transport Protocol (RTP) packets. G.711 has not been optimized for fax transport over IP networks, and does not typically support packet redundancy.
Having been developed for voice, G.711 allows the transmission of missing audio because any gaps would be filled in by a human listener. But when used to transmit modem data, any loss of packets is significant, because the receiver has no way to recreate the missing data. Some packet networks may not be fax-aware, and may optimize the G.711 stream for voice with the use of silence suppression, echo cancellation, or transcoding to a higher compression codec. Such optimizations can cause a loss of data and prevent FoIP from operating. This may force the use of a dedicated G.711 fax trunk to provide reliable fax performance. However, G.711 is an inherently simpler approach to fax than T.38, so interoperability issues between different vendors’ products is less common with G.711 than may be encountered with T.38. The cost for a G.711 approach may also be lower than T.38 because it can leverage voice data infrastructure.