L.3 Session setup
26.1143GPPIP Multimedia Subsystem (IMS)Media handling and interactionMultimedia telephonyRelease 18TS
L.3.1 Session setup for any MTSI client supporting facsimile transmission
An MTSI client supporting facsimile transmission shall support facsimile transmission in stand-alone sessions without any other media types.
NOTE: This does not prevent supporting facsimile transmission also in other session types, for example in speech+facsimile sessions, but this is not described here.
An MTSI client supportings facsimile versions (T38FaxVersion) higher than 0 shall be capable of downgrading the session to any lower facsimile version, if indicated by a received SDP message.
An MTSI client sending an SDP for a facsimile session shall include the following in the SDP (offer or answer):
– MIME media type and subtype names as defined in [94];
– bandwidth information, both on media level and session level;
– T38FaxVersion attribute, if the offered version is higher than 0;
– T38FaxRateManagement attribute, with the value according to the offered method.
Absence of the T38FaxVersion attribute indicates that only version 0 is supported.
SDP examples for facsimile calls can be found in clause L.7.
L.3.2 Session setup when the recommended profile is supported
When the MTSI client supports facsimile transmission according to the recommended profile in Annex L.2.3 and initiates a session for UDPTL-based facsimile transmission then:
– the following SDP lines shall be used in the SDP offer:
– b=AS with the bandwidth set to at least 46 kbps for IPv4 or 48 kbps for IPv6;
– T38FaxVersion attribute indicating at least version 2;
– T38FaxRateManagement attribute with value ‘transferredTCF’;
– the following SDP attributes should be included in the SDP offer:
– T38MaxBitRate, the value should be set to 14400;
– T38FaxMaxBuffer with value 1800;
– T38FaxMaxDatagram with value 150;
– T38FaxUdpEC with value ‘t38UDPRedundancy’.
Other SDP attributes defined in ITU-T T.38 Annex D may be included, if supported.
When the MTSI client supports facsimile transmission according to the recommended profile in Annex L.2.3 and accepts an offer for a session initiation for facsimile transmission then:
– the following SDP lines shall be included in the SDP answer:
– T38FaxVersion attribute indicating at least version 2;
– T38FaxRateManagement, the value shall be the same as in the SDP offer;
– T38FaxUdpEC, the value to include depends both on what error correction schemes the MTSI client supports and what error correction schemes that are declared in the SDP offer;
– b=AS, the value indicates the bandwidth needed for facsimile transmission and should be aligned with T38MaxBitRate (if included);
– and the following SDP attributes should be included:
– T38MaxBitRate, the value should be set to 14400;
– T38FaxMaxBuffer, the value indicates the receiver buffer size.