Complete Communications Engineering

T.38 Prioritization in a Universal Gateway has become an important aspect of a complete communications solution for real-time facsimile transmission over IP networks. To this end, T.38 fax protocol is the most commonly implemented solution to provide fax over IP (FoIP) functionality. However, when implemented in a universal gateway, T.38 communication must compete with other types of traffic for network resources.

Certain constraints on the network conditions are necessary to ensure a reliable T.38 Fax over IP transmission. Depending on the facsimile transmission speed and other factors, such as the amount of error correction data sent by T.38, the maximum bandwidth used by a T.38 transmission can range from approximately 2400 bps to around 8.6 mbps. With the most common amount of redundancy being 1 to 3 packets, a T.38 transmission should not be expected to exceed 150 kbps.

Certain requirements on the latency of the packets are also necessary. By adding delays in the facsimile dialogue through careful use of the T.30 specification, T.38 gateways can increase the amount of acceptable latency to allow usually 3-5 seconds or longer delay. Packet loss is also problematic and can often cause transmission failure. Tolerance to some loss is possible through the use of Error Correction Mode (ECM) and redundant T.38 packets, at the expense of increased bandwidth consumption. Packet jitter can cause problems with a T.38 transmission by causing a T.38 gateway to fail to meet the timing constraints of the T.30 dialogue. However, this can be remedied with buffering techniques as long as the total delay remains within acceptable limits.

In a situation where other forms of traffic are present and the available network bandwidth may be insufficient, T.38 must be prioritized in order to ensure reliable delivery of the facsimile image. T.38 requires a certain quality of service in order to correctly follow the T.30 specification when communicating with the local facsimile machine, while also providing data to the remote T.38 gateway so that it may adhere to the T.30 specification as well. Compared to audio and video data, T.38 can usually accept a larger amount of latency and commonly requires much less bandwidth.

In contrast, while most other forms of communication can tolerate some packet loss resulting in lower quality or throughput, T.38 may not be as forgiving. If a packet is lost and cannot be recovered through error correction, the resulting image may have corrupted or blank lines or the T.38 transmission may fail. Since the maximum bandwidth required by a T.38 transmission can be easily determined and can be arbitrarily lowered by the gateway, there is some flexibility in deciding the bandwidth required for a given T.38 transmission.

Therefore, to achieve reliable delivery of the facsimile image, T.38 communication will need prioritization sufficient to ensure an acceptable amount of latency, as well as a dedicated amount of bandwidth that can be determined depending on the current network conditions. Allowing T.38 priority in this respect may slightly increase the latency encountered when other forms of data are being transmitted concurrently. Also, the increased bandwidth usage may increase packet loss if the available network bandwidth is consumed which would cause, for example, degradation of audio/video quality or decreased throughput for Internet traffic.