What is an IP Multimedia Subsystem?

I have read that IMS defines a complete architecture and framework that enables the integration of voice, video, data and mobile network technology over an IP-based infrastructure. Does the framework provided by the SIP network elements like Registrar Server, Location Server and Proxy Server is not sufficient for VOIP telephony? So this IMS is required? I am not able to understand the difference between SIP 开发者_开发问答network and IMS.

IMS is part of the telecom industry's blueprint for the transition to an all-IP network (a core component of the Long Term Evolution (LTE) project). It uses SIP as the backbone protocol for session setup in what is refered to as the Control Plane.

Each IMS installation will have two network elements which are similar to the usual SIP Registrar and Proxy servers. However, Accounting, Authentication and Authorization (usually refered to as AAA) as well as managing mobility and access to and from different network types all add to the responsibilities of the IMS core over and above the simple Registrar and Proxy functions.

For example, the Serving Call Session Control Function server (S-CSCF) in the IMS-core is the SIP server that closely resembles the Registrar performing user registration, managing session control, interacting with the services layer, and is charged with accounting for usage. Users never interact directly with the S-CSCF, instead, they communicate only via the Proxy CSCF server (P-CSCF), which as its name suggests is essentially a SIP proxy server.

There is another CSCF at the heart of an IMS installation, the Interrogating CSCF (I-CSCF), but doesn't lend itself to comparision with the usual SIP servers as much.

So in summary, when interacting with an IMS installation you should be able to treat it in much the same way as a typical SIP Registrar and Proxy server-based network, however, IMS need to accomplish much more on top.

SIP (with other protocols) is, overall, suitable for VoIP. As you said, IMS defines a combination of services other than VoIP. IMS is an architecture that defines VoIP, video calls, presence, instant messaging, etc. So you can see IMS being the next evolution of medias on IP. It is much more than just VoIP.

Some words on definitions may be good here:

• SIP is a signaling protocol.
• IMS is an architecture. It is huge, defining many things at a high level. It uses SIP and many other protocols for parts of its architecture.

I like the answer by jwri, but I'd like to pitch in a less technical one :-)

IMS is an architecture, SIP (and Diameter) are the protocols used.

It should allow mobile service providers to build heterogeneous (multi-vendor) networks. Where this actually works and makes sense is the Application Servers (AS):

You should be able to add independent services like Voice Mail to your IMS core from any vendor. IMS defines a standard way to hook an AS up to the call flow, billing, etc.

So to answer your question, the IMS is just one specific way to set up your SIP Proxies (CSCF) registrar (backed by the HSS), billing, etc.

Just like if you were a 2G mobile provider, you could theoretically deploy a bunch of BSS towers, hook them up to your own implementation of the A interface, and do the switching between them any way you like. Or build your own radios and implement the GSM radio interface only, it's enough to do radio telephony, you don't need all the BSCs, MSCs, HLRs, and so on :-)