The IMS: IP Multimedia Concepts And Services, Second Edition

3.16. SIP compression

The IMS supports multimedia services using the SIP call control mechanism. SIP is a client server, text-based signalling protocol used to create and control multimedia sessions with two or more participants. The messages also contain a large number of headers and header parameters, including extensions and security-related information. Setting up a SIP session is a tedious process involving codec and extension negotiations as well as QoS interworking notifications. In general, this provides a flexible framework that allows sessions with differing requirements to be set up. However, the drawback is the large number of bytes and the many messages exchanged over the radio interface. The increased message size means that:

• Call setup procedures using SIP will take much more time to be completed compared with those using existing cellular-specific signalling, which means that the end-user will experience a delay in call establishment that will be unexpected and likely unacceptable.

• Intra-call signalling will, in some way or another, adversely affect voice quality/system performance.

Therefore, support for real-time multimedia applications requires particular attention when SIP call control is used. To speed up session establishment, 3GPP has mandated the support of SIP compression by both the UE and the P-CSCF [3GPP TS 23.221]. Although the support of compression is mandatory, 3GPP was not happy to mandate its usage because in the future Wireless Local Area Network ...