What is the T.38 standard (G3 Fascimile over IP)?

Last Update: August 9th, 2021

T.38 is the ITU recommendation for “Procedures for real-time Group 3 Facsimile Communications over IP networks”.

The summary of the T.38 recommendation is to provide a procedure where (in addition to ISDN or PSTN) part of the path of transmission happens over an IP network (in most instances the internet)

The latest revision of T.38 clarifies H.323, H.248.1, SIP and SDP call establishment. It also improves the compatibilities between Group 3 facsimile and T.38 Gateways.

The full ITU recommendation is found at http://www.itu.int/rec/T-REC-T.38-201009-P.

Table of Contents

History of T.38

The T.38 standard was put together in 1998 with the intention of transmitting G3 fax signals between individual terminals over IP networks.

Voice over IP (VoIP) networks gained popularity with phone operators and consumers alike. Many of them have found that the method of compression being optimized to voice operations rather than data created a problem for the transmission of fax information. T.38 has become a potential solution.

Overview of T.38

In the majority of situations involving the use of T.38, a PSTN network is at least at least part of the transmission between two T.38 enabled fax machines.

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 times, and UDP is preferred in most cases as it uses redundant data packets to transmit the signal.

It is important to note that T.38 does not include a call setup protocol, meaning the T.38 faxes will need to use regular call setup protocols such as H.323, SIP, or MGCP to make the T.38 call work.

T.38 Operation

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.

With T.37, the fax is enclosed or attached to an email and delivered to its final destination. In this transmission format, it is following essentially the same path and requirements as an email.

Under T.38 (and T.30 converted through T.38), the transmission is directly over IP in the native format of the transmission.

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 transmits at 8000 samples with an 8-bit sample or 64,000 bits per second. A bi-directional fax requires 128,000 bits of bandwidth. As the UDP portion of T.38 will at times send multiple packets across the network to eliminate packet loss, this can mean that the process uses more bandwidth (although other fax formats use less bandwidth)

T.38 is an improvement over a regular modem, which typically transmits at 14,400 bits/second.

Most bandwidth uses 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.

Challenges Involving T.38

There has been mention of problems involving confusion in regards to interoperability between enterprises, vendors, and service providers that have 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.

Additional Resources on T.38

This article was sourced from parts of and additional resources on the T.38 protocol can be found at:

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