The full ITU recommendation can be found at http://www.itu.int/rec/T-REC-T.38-201009-P
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As voice over IP (VoIP) networks gained popularity with phone operators and consumers alike, many have found that the method of compression being optimized to voice operations rather than data created a problem for the transmission of fax information over VoIP networks. This presented a problem where T.38 became a potential solution.
T.38 can also occur as a fax relay service, where a T.30 fax transmits over PSTN and is then converted by a T.38 Fax Gateway from T.30 to T.38. From there, the signal is either transmitted to a T.38 fax or is later converted back to T.30 for transmission over a PSTN system to a T.30 fax.
The T.38 recommendation allows for the use of either TCP or UDP to carry the information contained in the T.38 packets. TCP is known to result in delays at time, and UDP is preferred in most cases as it uses redundant data packets to transmit the signal.
Difference between T.37 and T.38
The primary difference between the T.37 and T.38 recommendations are found primarily in how the fax information is transmitted.
Under T.37, the fax is enclosed or attached to an email, and delivered to it’s final destination – in this transmission format, it is following essentially the same path and requirements as an email.
How T.38 is Transmitted
With the T.38 recommendation, information is transmitted from one fax machine to a T.38 gateway in T.30 – the gateway then transmits that information to T.38 for sending either to another gateway or T.38 enabled Fax Machine.
Reduced Bandwidth Use
Compared with the T.30 recommendation, T.38 has the potential to save on bandwidth involved.
T.38 transmitts at 8000 samples with an 8-bit sample, or 64,000 bits per second. For a bi-directional fax, 128,000 bits of bandwidth is required. As the UDP portion of T.38 will at times send multiple packets across the network to eliminate packet loss, this can mean that more bandwidth is used in the process (although less bandwidth is still used than in other fax formats)
This is an improvement over a regular modem, which typically transmits at 14,400 bits / second.
Most bandwidth use information that is required can be found on the Session Information Protocol RFC 3261.
Loss of Packets
Under T.38‘s UDP protocol, there is the capability to reduce packet loss that occurs. At times, up to 3 duplicates of the original packets are transmitted – this can increase the required bandwidth of the process.
There has been mention of problems involving confusion in regards to interoperability between enterprises, vendors, and service providers that has slowed the use of IP as a real-time fax transport. As mentioned in SIP Forums’ Fax Over IP Task Group Problem Statement, this has complicated both the implementation and integration of networks using the T.38 recommendation.
There has also been mention of problems arising from packet loss and the T.38 recommendation, primarily in regards to transmissions that involve both T.38 and T.30 communications. Often when packet loss occurs, the gateway is unable to properly reassemble the packets involved in time to properly transmit to the equipment using the T.30 recommendation, resulting in a failed transmission. Once again, this is part of the problem of interoperability among multiple recommendation standards.
This article was sourced from parts of and additional resources on the T.38 protocol can be found at:
Full T.38 ITU Recommendation – The full recommendation and process of T.38 as recommended by the ITU
3cx.com – What is T38 Fax?