Complete Communications Engineering

In order to improve on evolutive compatibility and avoid confusion, standards must continually be updated and fixed. The ITU standard for real-time facsimile communication over the IP network, T.38, falls into this category. The definition in the current draft document holds procedures for falling back to V.17 when a T.38 gateway does not support V.34 that have been improved. Updates to the SDP syntax have also been put forward. There are minor fixes such as updated references to RFC documents and corrected spelling and grammar.

To better distinguish the types of T.38 gateways, two definitions have been introduced. “T.38/V.34G3” refers to a T.38 endpoint that supports Group 3 fax and V.34, while “T.38/G3” refers to a T.38 endpoint that supports Group 3 fax but not V.34. The “T38ModemType” parameter has also been added to the SDP definition to allow a T.38 gateway to specify if it supports V.34. SDP negotiation using this parameter allows two T.38 gateways to determine if a V.34 transmission can be performed. The fallback procedure utilizes the V.8 dialogue used when starting a V.34 transmission. The V.8 CM message, which describes the capabilities of the sending fax machine, is received by the transmitting T.38 gateway and suppressed. If V.34 is not supported by one of the T.38 gateways, the V.8 JM message is used to force the sending fax machine to fallback to Group 3 fax. Otherwise, the V.8 CM message suppressed earlier is delivered to the remote T.38 gateway and the dialogue proceeds as per T.38.

In addition to the changes to SDP made to enable the V.34 fallback procedure above, other changes to the SDP definition have been proposed. These changes allow specifying multiple possible configuration profiles to choose from, while continuing to provide backwards compatibility with the previous SDP syntax. This enables a T.38 gateway to define an explicit and ordered preference for its available configurations. Also, specifying a port number 0 is no longer required to reject a configuration. In addition, the “T38FaxUdpEC” parameter can now be set to “t38UDPNoEC” to indicate that no error correction is desired. In the T.38 standard draft itself, the example SDP encodings have been updated to include the new syntax. Annex H has been added as well, which contains a table of the T.38 SDP parameters.