IMS - Special Focus: The Role of SIP in IMS

IMS promises access to all the rich services available on the Internet, including Web, e-mail, instant messaging, VoIP, and IPTV, on handheld devices via ubiquitous wireless networks. IMS provides a set of standards that enable the convergence of the Internet and cellular networks and a unified platform for creating and running these multimedia services regardless of the network or device used to deliver them. The key call control (and session control) protocol for IMS is SIP. The evolution of SIP has become the focal point of the VoIP revolution, driving advances in VoIP services. Its advances and acceptance in the VoIP world should provide a significant boost to the number of services available on IMS architectures. SIP is a text-based, client/server protocol that resembles the HTTP and SMTP protocols; SIP is used to create sessions in an IP network. SIP has proven to be extremely flexible. SIP can create sessions as diverse as a simple two-way telephone conversation or as complex as a multimedia conference involving many participants.

In a simple session, for example a two-way telephone conversation, two SIP user agents (UAs), each with a unique address or uniform resource indicator (URI), are connected so that information can be passed between the UAs. A session is created through a straightforward set of requests and responses. One UA (the caller, in our example) initiates a session by sending a request with the SIP URI of the other UA (the called party in our example). If the IP address of the called party is known, the request can be sent directly to that address. If the IP address isn’t known, the request is sent through one or more proxy servers, which attempt to locate the called party and forward the SIP message. Once the called party is located, the SIP message, which carries text information and describes the type of session being established, is delivered. At this point, the called party can send back an acknowledgement and a session between the caller and the called party will begin.

SIP Within IMS

The IMS architecture separates the signaling plane from the media plane. SIP has been selected as the protocol used by the signaling plane. SIP signaling packets within IMS are normally processed by a collection of SIP servers or proxies, collectively referred to as the Call Session Control Function (CSCF). The CSCF includes:

• P-CSCF (Proxy-CSCF) — a SIP proxy that is the initial interface in a visited network. It provides security, authorization, and translation services.

• S-CSCF (Serving-CSCF) — a SIP server within the IMS signaling plane that controls sessions, deals with registration, and triggers and executes services.
• I-CSCF (Interrogating-CSCF) — a SIP proxy that deals with registration, routing and forwarding of SIP messages and charging.

SIP is currently used as the basis of session control for myriad Internet services such as VoIP, conferencing, video calls and video conferencing, presence management, instant messaging, collaboration and location-based services. Since the goal of IMS is to provide ubiquitous access to all existing and future Internet services, adopting SIP as the control protocol for IMS brings immediate access to some of today’s most popular and fastest growing Internet services.

Innovation and investment in the SIP standard and SIP-based services is expected to continue, or even accelerate. Currently, the standards community is active in creating new standards or protocols based on SIP to enable various kinds of services such as instant messaging and presence management (e.g., SIMPLE and IMPP). It is clear that SIP is the protocol of choice for session-based Internet services in the future. The presence of SIP in IMS will not only allow IMS users to benefit from these additional services, but will also allow IMS users the flexibility to combine multiple services from different vendors to form entirely new services.

With its flexibility, simplicity, and broad adoption, SIP provides IMS with solid call/session control capabilities. Continued innovations in SIP and SIP-based applications will allow IMS users to introduce a wide range of innovative, multi-media services to their customers. opportunities.

Sanjeev Sawai is vice president of research and development at Envox Worldwide (news - alert). For more information, please visit the company online at www.envox.com.