L.7 SDP examples
26.1143GPPIP Multimedia Subsystem (IMS)Media handling and interactionMultimedia telephonyRelease 18TS
L.7.1 Facsimile-only session
This example shows the media scope of the SDP offer and SDP answer for a facsimile-only session when the recommended configuration in Annex L.2.3 is offered by both end-points.
Table L.2: Example SDP offer for facsimile-only session
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SDP offer |
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m=image 49150 udptl t38 b=AS:46 a=T38FaxVersion:2 a=T38FaxRateManagement:transferredTCF a=T38MaxBitRate:14400 a=T38FaxUdpEC:t38UDPRedundancy a=T38FaxMaxBuffer:1800 a=T38FaxMaxDatagram:150 |
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SDP answer |
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m=image 49154 udptl t38 b=AS:46 a=T38FaxVersion:2 a=T38FaxRateManagement:transferredTCF a=T38MaxBitRate:14400 a=T38FaxUdpEC:t38UDPRedundancy a=T38FaxMaxBuffer:1800 a=T38FaxMaxDatagram:150 |
Comments:
The session bandwidth is set to 46 kbps based on the following calculation:
– the maximum fax bitrate is 14.4 kbps;
– it is recommended that the MTSI client supports 200% redundancy;
– 14.4 kbps and 150 bytes in each IP/UDP packet gives 12 packets per second;
– IPv4 and UDP gives 28 bytes overhead for each IP/UDP packet (IPv6 and UDP gives 48 bytes overhead).
This gives 3*14400 +12*28*8 = 45888 bps, which is rounded up to 46 kbps for IPv4 (48 kbps for IPv6).
ITU-T Recommendation T.38 states that only ‘T38FaxRateManagement’ is mandatory to include in the SDP. There are however other reasons to include the other SDP lines:
– b=AS is included both because this is useful information both for resource allocation in network nodes and the remote end-point, and because it is required by the current specification, see Clause 6.2.5.1;
– the ‘T38FaxVersion’ attribute needs to be included to declare that version 2 is supported, since the other end-point would otherwise assume that only version 0 is supported;
– the ‘T38MaxBitRate’, ‘T38FaxMaxBuffer’ and ‘T38FaxMaxDatagram’ attributes are very useful to ensure that the remote end-point sends fax media within the limitations of the local end-point;
– The ‘T38FaxUdpEc’ attribute is very useful information for the remote end-point in case of bad channel conditions.
The SDP attributes ‘T38FaxFillBitRemoval’, ‘T38FaxTranscodingMMR’ and ‘T38FaxTranscodingJBIG’ are not included since the MTSI client is not required to support these options.
The SDP attributes ‘T38FaxMaxIFP’, ‘T38FaxUdpECDepth’ and ‘T38FaxUdpFECMaxSpan’ are not included since these attributes are not defined for T.38 fax version 2.
Annex M (informative):
IANA registration information for SDP attributes