In the T.38 mode, the gateway DSP devices convert T.30 signals into T.38 packets and send the converted packets to a peer. If the fax endpoint on the far end supports T.30 signaling, the peer converts the packets back into T.30 signals and passes the packets to the fax endpoint. However, if the fax endpoint supports the T.38 protocol, the peer passes the packets directly to the fax endpoint.

T.38 is the preferred industry standard fax protocol. H.323 and SIP trunks support the T.38 protocol.

Communication Manager uses the T.38 protocol for fax transmission over IP network facilities. Communication Manager supports the transition of an existing SIP audio call to a fax call.

During a SIP audio call, when Communication Manager receives a reINVITE message with the audio and image stream, Communication Manager performs one of the following operations:

• If T.38 is administered, Communication Manager accepts the image stream and rejects the audio stream.

• If T.38 is not administered, Communication Manager accepts the audio stream and rejects the image stream.

For more information about FAX over IP and T.38-G711-fallback, see Avaya Aura® Communication Manager Feature Description and Implementation.