Knowledgebase Article: SmartMedia: SDP Profile Configuration ...

Contents Introduction Media Announcement Attribute for Media Announcement Dynamic payload type Other media specific attribute Examples References

You can specify the Profile SDP Description to define which voice codecs are supported in the NAP using this Profile.

Here is the default Profile SDP Description.

m=audio 0 RTP/AVP 0 8 4 96 97 18 98 13 a=rtpmap:96 iLBC/8000 a=rtpmap:97 iLBC/8000 a=fmtp:97 mode=20 a=rtpmap:98 telephone-event/8000

Each line of the Profile SDP Description consists of text of the form <type>=<value>. <type> is always exactly one character and is case-significant. <value> is a structured text string whose format depends on <type>. It also will be case-significant unless a specific field defines otherwise. Whitespace is not permitted either side of the '=' sign. In general <value> is either a number of fields delimited by a single space character or a free format string.

In our Profile SDP Description, only 2 types of <type> field are used:

Media Announcement

The format of media announcement is as follows.

m=<media> <port> <transport> <fmt list>

• The first sub-field is the media type. Currently defined media for TMedia is "audio".

• The second sub-field is the transport port to which the media stream will be sent. In SmartMedia we don't specify it in Profile SDP Description and therefore you should insert "0".

• The third sub-field is the transport protocol. For most of the application, you may specify it as "RTP/AVP" - the IETF's Realtime Transport Protocol using the Audio/Video profile carried over UDP.

• The fourth and subsequent sub-fields are media formats. For audio and video, these will normally be media payload types as defined in the RTP Audio/Video Profile. When a list of payload formats is given, this implies that all of these formats may be used in the session, but the first of these formats is the default format for the session. When the transport protocol is specified as "RTP/AVP", the payload format can be specified as either
  • the payload type number for static payload types
  • the payload type number along with additional encoding information for dynamically allocated payload types.

The payload type, which is carried in the actual RTP packet header, is used to identify the type of codec information carried in the packet. A list of payload type values for each codec is defined within RFC3551. Unfortunately, since the payload type field is only 7 bits-wide, all codecs cannot have a permanent payload type value understood universally by all VoIp systems. Therefore, some codecs have dynamic values that need to be negotiated through a call control or session control protocol such as SIP before the actual RTP session can take place.

Here is the list of codec payload type values per RFC3551.

Codec	Payload type value
G.711 uLaw	0
G.723.1	4
G.711 aLaw	8
Comfort Noise	13
G.728	15
G.729AB	18
G.726-40	dynamic
G.726-32	2 or dynamic (depends on the network)
G.726-24	dynamic
G.726-16	dynamic
G.729EG	dynamic
AMR	dynamic
EVRC	dynamic
QCELP	dynamic

When you use dynamic payload types, you need to specify the additional encoding information using the attribute for media announcement.