FAX Transcoding

FAXes are transmitted in a call as either T.30 and T.38 media. T.30 FAX is binary in-band media carried over G.711. The Oracle® Enterprise Session Border Controller can transcode between T.38 and a faxable codec. The supported faxable codecs are PCMU and PCMA.

T.30 can only be transported in non-compressed codecs. The two non-compressed codecs supported by the Oracle® Enterprise Session Border Controller are PCMU (G711µ) and PCMA (G711A). If a transcoding realm does not support an uncompressed codec, T.30 can not be supported in that realm. Alternatively, G711FB may be allowed specifically for FAX only.

The Oracle® Enterprise Session Border Controller uses an internal codec called G711FB (G711 - Fall Back) that is an umbrella codec of all FAXable codecs. G711FB will default to PCMU for the purpose of offering a faxable codec. You can remap G711FB to PCMA by configuring the media-profile for it appropriately. G711FB’s only use is for FAX transcoding.

FAX transcoding is triggered when you configure the add on egress parameter with either T.38 or G711FB. In a FAX scenario, when the codec policy adds either T.38 or G711FB, a new m= line is added to the SDP. When adding T.38, the new m= line specifies the T.38 codec. When adding G711FB, the new m= line specifies PCMU (or alternatively PCMA).

Once added, m= lines can not be deleted in the context of a call. The Oracle® Enterprise Session Border Controller maintain all m= lines between itself and an endpoint throughout the course of call. All m= lines not in use can be disabled by setting their receive port to 0, but they can not be removed from the SDP.