SIP Protocol

Defined in IETF RFC 3261.

“... an application-layer control (signaling) protocol for creating, modifying, and terminating sessions with one or more participants. These sessions include Internet telephone calls, multimedia distribution, and multimedia conferences.”

SIP is to the Internet what SS#7 is to telephony.

In IMS, SIP is extended to include extra functionality.

• E.g. 3GPP TS 23.228.

At the core of IMS there are several SIP proxies:

• I-CSCF, S-CSCF, P-CSCF.

• The Call Session Control function (CSCF) is the heart of the IMS architecture.

• The main functions of the CSCF:

  • provide session control for terminals and applications using the IMS network,

  • secure routing of the SIP messages,

  • subsequent monitoring of the SIP sessions and communicating with the policy architecture to support media authorization,

  • responsibility for interacting with the HSS.

Serving - CSCF.

• Controls the user’s SIP Session.

• very few per domain.

• Located in the home domain.

• Is a SIP registrar (and proxy).

Proxy – CSCF

• IMS contact point for the user’s SIP signaling.

• Several in a domain.

• Located in the visited domain.

• Terminals must know this proxy (e.g. DHCP used).

• Compresses and decompresses SIP messages.

• Secures SIP messages.

• Assures correctness of SIP messages.

Interrogating – CSCF

• domain’s contact point for inter- domain SIP signaling.

• one or more per domain.

• In case there are more than one S- CSCFs in the domain, locates which S-CSCF is serving a user.