A.13 SDP examples for MTSI client in terminal using fixed access
26.1143GPPIP Multimedia Subsystem (IMS)Media handling and interactionMultimedia telephonyRelease 18TS
A.13.1 General
This clause includes SDP examples that may be applicable to an MTSI client in terminal using fixed access. The SDP examples in Annex A.13.2 to A.13.6 show SDPs including PCM, G.729, G.722 and G.729.1. Examples of SDP offers for the AMR and AMR-WB codecs are described in Annex A.1 and the corresponding examples of SDP answers are found in Annex A.3. SDP examples for EVRC, EVRC-B and EVRC-WB are found in [97].
Examples of SDP offer and answer for video are described in Annex A.4.
An example for an SDP offer for real-time text is described in Annex A.5.
These examples also include bandwidth information which is calculated assuming IPv6 and 20 ms packetization.
A.13.2 SDP examples for PCM
Table A.13.1 shows an example for the SDP offer and answer negotiation for PCM. The SDP offer uses the static payload type numbers that are defined in RFC 3551 [10] for PCM, i.e. payload type number 0 for -law PCM and payload type number 8 for A-law PCM, see also Clause 18.4.3. Since static payload type numbers are used, as shown on the m= line, then there is no need for adding any a=rtpmap attribute lines. The answerer chooses to accept A-law PCM and therefore sends an SDP answer with RTP payload type number 8 on the m= line.
Table A.13.1: SDP example for PCM
|
SDP offer |
|
m=audio 49152 RTP/AVP 0 8 b=AS:88 a=ptime:20 a=maxptime:240 |
|
SDP answer |
|
m=audio 49152 RTP/AVP 8 b=AS:88 a=ptime:20 a=maxptime:240 |
Comments:
The SDPs further describe that the clients prefer to receive speech with 20 ms packetization (ptime is set to 20) but up to 240 ms packetization is allowed.
Table A.13.2 shows an example for how the PCM codec can be negotiated using dynamic payload type numbers. In this case, payload type number 96 is used for -law PCM and payload type number 97 is used for A-law PCM. The answerer chooses to accept -law PCM.
Table A.13.2: SDP example for PCM
|
SDP offer |
|
m=audio 49152 RTP/AVP 96 97 b=AS:88 a=rtpmap:96 PCMU/8000/1 a=rtpmap:97 PCMA/8000/1 a=ptime:20 a=maxptime:240 |
|
SDP answer |
|
m=audio 49152 RTP/AVP 96 b=AS:88 a=rtpmap:96 PCMU/8000/1 a=ptime:20 a=maxptime:240 |
Comments:
This example is included here to show that it is possible to use dynamic payload type numbers also for codecs for which static payload type numbers have been defined. It is however preferable to use static payload type numbers, see Clause 18.4.3.
A.13.3 SDP example for G.722
Table A.13.3 shows an example for how G.722 can be negotiated using the static payload type number (9) defined in RFC 3551 [10], see also Clause 18.4.3.
Table A.13.3: SDP example for G.722
|
SDP offer |
|
m=audio 49152 RTP/AVP 9 b=AS:88 a=rtpmap:9 G722/8000/1 a=ptime:20 a=maxptime:240 |
|
SDP answer |
|
m=audio 49152 RTP/AVP 9 b=AS:88 a=rtpmap:9 G722/8000/1 a=ptime:20 a=maxptime:240 |
Comments:
The G.722 codec uses an RTP clock rate of 8 kHz even though G.722 is a wideband speech codec that uses a sampling frequency of 16 kHz. This means that the RTP Time Stamp is sampled with 8 kHz.
The SDPs further describe that the clients prefer to receive speech with 20 ms packetization (ptime is set to 20) but up to 240 ms packetization is allowed.
A.13.4 SDP example for EVS, AMR-WB, G.722, AMR, PCM and DTMF
Table A.13.4 shows an example where an MTSI client in terminal using fixed access has been developed to support fixed-mobile interworking without the need for transcoding in a media gateway. It therefore supports the G.722 and PCM codecs that are normally used in fixed networks. In addition, it also supports the AMR-WB and AMR codecs in the same way as an MTSI client in terminal using mobile access would do. The SDP offer includes all these codecs as well as DTMF.
Table A.13.4: SDP example for EVS, AMR-WB, G.722, AMR, PCM and DTMF
|
SDP offer |
|
m=audio 49152 RTP/AVP 96 97 98 9 99 100 8 0 105 106 b=AS:89 b=RS:0 b=RR:4000 a=rtpmap:96 EVS/16000/1 a=fmtp:96 br=64; bw=swb; max-red=220 a=rtpmap:97 AMR-WB/16000/1 a=fmtp:97 mode-change-capability=2; max-red=220 a=rtpmap:98 AMR-WB/16000/1 a=fmtp:98 mode-change-capability=2; max-red=220; octet-align=1 a=rtpmap:99 AMR/8000/1 a=fmtp:99 mode-change-capability=2; max-red=220 a=rtpmap:100 AMR/8000/1 a=fmtp:100 mode-change-capability=2; max-red=220; octet-align=1 a=rtpmap:9 G722/8000/1 a=rtpmap:0 PCMU/8000/1 a=rtpmap:8 PCMA/8000/1 a=rtpmap:105 telephone-event/16000 a=fmtp:105 0-15 a=rtpmap:106 telephone-event/8000 a=fmtp:106 0-15 a=ptime:20 a=maxptime:240 |
Comments:
The wideband codecs (AMR-WB and G.722) are listed as preferred over the narrowband codecs (AMR and PCM). This ensures that a wideband service will be set up whenever possible.
The AMR and AMR-WB codecs are listed here as preferred over the PCM and G.722 codecs, respectively, because of their lower bitrate and also because of their bitrate adaptation capabilities.
The SDP offer includes DTMF with both 8 kHz and 16 kHz RTP clock rate since there are codecs with both clock rates in the offer. An answerer is expected to accept the DTMF variant that has the same clock rate as for the accepted codec. This means that when G.722 is accepted then DTMF with 8 kHz clock rate should also be accepted, even though G.722 is a wideband speech codec. This is because G.722 uses 8 kHz clock rate, see RFC3551 [10].
Since the clock rate of EVS is set to 16 kHz, regardless of the bandwidth in the session, DTMF with 16 kHz RTP clock rate should be accepted when EVS is accepted.