4 Protocol Examples

A typical example of the SDP negotiation under Microsoft Office Communicator would appear similar to the following.

 m=audio 51712 RTP/AVP 114 111 112 115 116 4 8 0 97 101]
 ...
 a=rtpmap:114 x-msrta/16000 
 a=fmtp:114 bitrate=29000 
 a=rtpmap:111 SIREN/16000 
 a=fmtp:111 bitrate=16000 
 a=rtpmap:112 G7221/16000 
 a=fmtp:112 bitrate=24000 
 a=rtpmap:115 x-msrta/8000 
 a=fmtp:115 bitrate=11800 
 a=rtpmap:116 AAL2-G726-32/8000 
 a=rtpmap:4 G723/8000 
 a=rtpmap:8 PCMA/8000 
 a=rtpmap:0 PCMU/8000 
 a=rtpmap:97 RED/8000 
 a=rtpmap:101 telephone-event/8000 
 a=fmtp:101 0-16

The RED/8000 line uses the default redundant payload type mapping for Microsoft Office Communicator (PT=97). Given this if the negotiated encoding is RT-Audio 16Khz, the payload containing the main block + redundant block would appear as follows.

  
 0                   1                    2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3  4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |V=2|P|X| CC=0  |M|      PT     |   sequence number of primary  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |              timestamp  of primary encoding                   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |           synchronization source (SSRC) identifier            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |1| block PT=97 |  timestamp offset         |   block length    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |0| block PT=114|                                               |
 +-+-+-+-+-+-+-+-+                                               +
 |                                                               |
 +                RT-Audio encoded redundant data (PT=97)        +
 |                (38 bytes)                                     |
 +                                                               |
 |                                                               |
 +                                                               |
 |                                                               |
 +                                                               |
 |                                               +---------------+
 |                                               |               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               +
 |                                                               |
 +                                                               +
 |                RT-Audio encoded primary data (PT=114)         |
 +                (38 bytes)                                     +
 |                                                               |
 +                                                               +
 |                                                               |
 +                                                               +
 |                                                               |
 +               +-----------------------------------------------+
 |               | 
 +-+-+-+-+-+-+-+-+
Show: