QoS and Call Setup

The path which SIP takes may be different that the media path, thus new extensions were added to enable more handshaking:

Early Media - by allowing SDP to be included in the 183 Session Progress response (allows establishment of QoS requirements before call is answered) - may also enable one-way RTP {hence the name “early media”}, formally: “media during early dialog”
Reliable Provisional Responses extension allows detection of a lost 183 Session Progress response based on using Provisional Response Acknowledgement (PRACK)
UAs can use preCOnditions MET (COMET) method to indicate that the QoS requirements can be met and that the user can be alerted by ringing the phone.

SDP in the INVITE contains an attribute-value pair: "a=qos:mandatory".

For further details see: RFC3312 and RFC3262; more about SDP in the next lecture module.

Slide Notes

G. Camarillo, W. Marshall, J. Rosenberg, “Integration of Resource Management and Session Initiation Protocol (SIP)”, IETF, Network Working Group, RFC 3312, October 2002, Updated by RFC 4032, RFC 5027, http://datatracker.ietf.org/doc/rfc3312/ Links to an external site.

J. Rosenberg and H. Schulzrinne, “Reliability of Provisional Responses in the Session Initiation Protocol (SIP)”, IETF, Network Working Group, RFC 3262, June 2002, http://datatracker.ietf.org/doc/rfc3262/ Links to an external site.

Transcript

[slide164] So we had a couple of questions about quality of service in the call setup, and one of the ideas was adding something called early media. So if you've ever called the United Kingdom, one of the things you'll notice is you hear the English ring signal, not your local ring signal, because they're actually providing you early media as feedback to keep you happy and contented while you're waiting for them to answer. So rather than just not giving you anything, they give you some audio already to begin with. So even though the SIP session hasn't been initiated, there's still audio being sent to you, that is early media. We can set preconditions, so we can say, only if this quality is supported will I initiate the call.