“IMS and SIP” and the IMS Forum Plan at NXTcomm

At the highest level, the IETF defines the SIP family of protocols, while for IMS the 3GPP, To better understand how SIP became the protocol of choice for both peer-to-peer (P2P) communications and IMS architectures, we need to refer to the decisions made by the 3GPP and 3GPP2 task forces in establishing liaisons with the IETF (RFC 3113 and RFC 3131). The 3rd generation wireless task forces decided not to ‘reinvent the wheel’ and use an existing IETF set of standards defined for the Session Initiation Protocol, or SIP.ETSI and CableLabs (News - Alert) defined the network and service enabling architectures. That separation is very important, and emphasizes that there’s only one SIP standards body — the IETF. In the past there was a lot of confusion between what is considered the ‘original SIP’, and the ‘IMS SIP’. This confusion is hard to understand as SIP has only one source, the IETF. To look at SIP within IMS in the proper light we should consider the rationale behind moving to the IMS architecture. In contrast to traditional IP networks, IMS offers end-to-end Quality of Service (QoS) and supports standardized interfaces (using SIP and Diameter as the underlining protocols) to deploy all types of IP-based services, support flexible billing and consistent operational and management interfaces.
SIP is used in IMS to support ‘service capabilities’ as opposed of supporting ‘services’ in the way PRI and SS7 were used in the Advanced Intelligent Network (AIN) to support fixed or mobile voice.

A key factor in the decision for the IMS group to support of SIP comes from the way SIP supports multimedia. SIP was specified by the IETF as a protocol to establish and manage multimedia sessions over IP networks. SIP inherits many of the characteristics of the SMTP (Simple Mail Transfer Protocol) and HTTP (Hypertext Transfer Protocols), together representing the most widely-deployed Internet protocols. SIP, which is text-based, is very easy to extend; hence it is ideal for adding functionality related to QoS and new services which combine any type of media.

SIP is the most-used protocol in IMS. It is defined by IETF RFC 3261. For IMS the SIP protocol was extended by adding support of QoS, Session control, security and administration. For IMS SIP added a negotiation mechanism using three header fields: supported, required, unsupported. The negotiation is done through a dialogue where the user agent client lists all the names of the required and supported fields. In response, the user agent server sends back an error response for the unsupported fields.
In summary, SIP, now a mature, widely-deployed protocol, is used in many forms of IP services, including IMS and P2P. While the protocol extensions and the scope of the applications make SIP implementations different, it is expected that the two architectures will continue to co-exist in the future.

IMS Forum at NXTcomm plan:
The IMS Forum® is a global, non-profit industry association devoted to interoperable IP Multimedia Subsystems services and solutions. IMS Forum’s mission is to accelerate the interoperability of IMS applications and services, enabling enterprise and residential consumers to quickly benefit from the delivery of quadruple play voice, video, Internet and mobile services over broadband via cable, wireless and fixed networks. The IMS Forum is the creator and organizer of the IMS Plugfest™, the industry’s only event focused on IMS services interoperability verification and certification.

T hrough its organized Plugfests, working group interaction, and other activities, forum members are able to develop cost-effective technical frameworks for converged IP services over wireline, cable, 3G, WiFi and WiMAX (News - Alert) networks. Plugfest II will be held June 4-8, 2007 at the IMS Forum UNH Interoperability Lab. Plugfest II results will be released at NXTcomm, June 18 — 21, 2007 in Chicago at the IMS Forum’s booth #3669. For additional information or to join the IMS Forum and the IMS Plugfests.