A.2 SDP offers for speech sessions initiated by media gateway
26.1143GPPIP Multimedia Subsystem (IMS)Media handling and interactionMultimedia telephonyRelease 18TS
A.2.1 General
These examples show only SDP offers when the MTSI media gateway does not support the same configurations as for the MTSI terminal in clause A.1. A media gateway supporting the same configurations as for the examples in clause A.1 should create the same SDP offers.
A.2.2 MGW between GERAN UE and MTSI
This example shows the SDP offer when the call is initiated from GSM CS using the AMR with the {12.2, 7.4, 5.9 and 4.75} codec mode set. In this example, it is also assumed that only the bandwidth-efficient payload format is supported and that it will not send any redundant speech frames.
Table A.2.1: SDP example
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SDP offer |
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m=audio 49152 RTP/AVP 97 a=tcap:1 RTP/AVPF a=pcfg:1 t=1 a=rtpmap:97 AMR/8000/1 a=fmtp:97 mode-set=0,2,4,7; mode-change-period=2, \ mode-change-neighbor=1; mode-change-capability=2; max-red=0 a=ptime:20 a=maxptime:80 |
Comments:
Since the MGW only supports a subset of the AMR codec modes, it needs to indicate this in the SDP. The same applies for the mode change restrictions.
An example of a suitable SDP answer to this SDP offer is shown in Table A.3.5.
A.2.3 MGW between legacy UTRAN UE and MTSI
This example shows the SDP offer when the call is initiated from legacy UTRAN CS mobile that only the AMR 12.2 mode. In this example, it is also assumed that only the bandwidth-efficient payload format is supported.
Table A.2.2: SDP example
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SDP offer |
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m=audio 49152 RTP/AVP 97 a=tcap:1 RTP/AVPF a=pcfg:1 t=1 a=rtpmap:97 AMR/8000/1 a=fmtp:97 mode-set=7; max-red=0 a=ptime:20 a=maxptime:20 |
Comments:
Since only one mode is supported, the mode-change-period, mode-change-neighbor and mode-change-capability parameters do not apply.
In this case it is advisable to not allow redundancy since the legacy UTRAN CS mobile does not support any lower rate codec modes and then redundancy would almost double the bitrate on the PS access side. Therefore, maxptime is set to 20 and max-red is set to 0.
If a mode-set with several codec modes was defined and if max-red and maxptime are set to larger values than what Table A.1.8 shows, then redundancy is possible on the PS access side but not together with TFO.
An example of a suitable SDP answer to this SDP offer is shown in Table A.3.6.
A.2.4 MGW between CS UE and MTSI
This example shows the SDP offer when two mode sets are supported by the MGW.
Table A.2.3: SDP example
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SDP offer |
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m=audio 49152 RTP/AVP 97 98 a=tcap:1 RTP/AVPF a=pcfg:1 t=1 a=rtpmap:97 AMR/8000/1 a=fmtp:97 mode-set=0,2,4,7; mode-change-period=2, \ mode-change-neighbor=1; mode-change-capability=2; max-red=20 a=rtpmap:98 AMR/8000/1 a=fmtp:98 mode-set=0,3,5,6; mode-change-period=2, \ mode-change-neighbor=1; mode-change-capability=2; max-red=20 a=ptime:20 a=maxptime:80 |
Comments:
Redundancy up to 100 % is supported in this case since max-red is set to 20.
An example of a suitable SDP answer to this SDP offer is shown in Table A.3.6.
A.2.5 MGW between GERAN UE and MTSI when wideband speech is supported
This example shows the SDP offer when the call is initiated from GSM CS when AMR is supported with the {12.2, 7.4, 5.9 and 4.75} codec mode set and when AMR-WB is supported with the {12.65, 8.85 and 6.60} mode set. In this example, it is also assumed that only the bandwidth-efficient payload format is supported and that the MTSI MGW will not send any redundant speech frames.
Table A.2.4: SDP example
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SDP offer |
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m=audio 49152 RTP/AVP 98 97 a=tcap:1 RTP/AVPF a=pcfg:1 t=1 a=rtpmap:97 AMR/8000/1 a=fmtp:97 mode-set=0,2,4,7; mode-change-period=2, \ mode-change-neighbor=1; mode-change-capability=2; max-red=0 a=rtpmap:98 AMR-WB/16000/1 a=fmtp:98 mode-set=0,1,2; mode-change-period=2, \ mode-change-neighbor=1; mode-change-capability=2; max-red=0 a=ptime:20 a=maxptime:80 |
Comments:
Since the MGW only supports a subset of the AMR codec modes and of the AMR-WB codec modes, it needs to indicate this in the SDP. The same applies for the mode change restrictions.