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IMS IP Multimedia Subsystem
SIP is the Key

The key technology behind IMS is the SIP protocol; the 3GPP have chosen SIP as the protocol underlying many of the important interfaces between elements in an IMS-based network. Carriers and service providers have been using SIP to build new products for some time.

Why? There are several big advantages to building a new feature or service using SIP:

  • Simple:
    • SIP is based on a straightforward request-response interaction model, making life simple and comprehensible for developers. The messages are also text-based which makes them easy to parse, create, read, understand and debug.
  • Extensible:
    • SIP can set up sessions for any media type, be it voice, video, application sharing or teleportation (once we invent it!)
  • Flexible:
    • SIP allows developers to interact with the individual protocol messages (within limits) without breaking anything. This means that developers can perform a lot of neat tricks that make development much easier.
  • Familiar:
    • In large part because SIP borrows heavily from HTTP and other internet standards from the IETF, lots of web-like technologies can be used to build SIP applications. SIP development looks and feels a lot like web development, and there are a lot of web developers out there.
So clearly, there are many reasons you would want to build your applications in the land of SIP. The only problem with this approach is that few networks, and even fewer phones, speak SIP today (although they will in the future). Most cell phones, office phones and home phones don't understand it. So if we build applications in SIP, how do can we use the new services using our old, non-SIP phones?

The answer is to surround our SIP development 'oasis' with gateways which convert SIP signaling to protocols that the rest of the network can understand, including your cell phone and my home phone. If we put these gateways in the right place, then everyone is happy: developers get a great environment in which to build new applications and we get to use their new features using our existing phones. The diagram below shows this gateway architecture in action. Bob has a new feature, find-me-follow-me, which his service provider decided to build in SIP space and deploy using IMS. Bob isn't aware of SIP or IMS of course: he just gets a great new feature which ensures Alice can always find and speak to him.

The Specification
So what does the IMS standard describe? How does IMS recommend we build our oasis for new applications? The specification essentially describes two things:

  • A selection of logical network functions. IMS defines a set of logical elements that are required to deliver new services: It's rather like a tool box for developers of new services. One of the key aims of IMS is to offer a reusable 'kit of parts' that are shared among applications to ensure new applications don't require separate disconnected silos of data and functions. The kit inventory includes everything an application developer needs, including application servers, media servers, gateways and data stores. A key IMS concept is shared customer data, to which all applications have access.
  • The interfaces between these functions. By making the interaction between the logical elements standard, we can mix and match different components from different manufacturers, or even build our own (at least, that's the idea).
What the User Gets
Of course customers don't care about reusable components, standard interfaces and shared data. They only care about much higher-level behaviors they can interact with directly, such as the quality of their voice call, or easy access to their email and instant messaging contacts. As well as standardizing lower-level functions, IMS specifies higher level behaviors too. These behaviors benefit the customer and make the difference between using a paid IMS service over a similar, freely available service on the Internet. For example:

  • Security:
    • IMS ensures proper user authentication and authorization, and privacy. Users are authenticated once, and this single-sign-on is used to access the entire range of services to which they subscribe.
  • Roaming:
    • Services can be offered across separate networks via roaming mechanisms specified by IMS. Users of GSM phones can already roam between different networks run by different operators. IMS enhances the roaming experience by allowing users seamless access to their services, data and applications from another network.
  • Quality of Service:
    • IMS makes sure that communication sessions are of guaranteed quality. One of the problems with public Internet-based communications is that it is typically 'best effort'. The Internet offers no guarantees of bandwidth or latency and the communication experience varies widely as a result. The IMS specification makes sure there are always enough resources available for your communication session by dynamically reallocating as required, prioritizing your important video conference over my frivolous ringtone download.
People Tech systems Develop applications for SIP using following Java technologies. Opencloud, SIP servlets, Mobicents, Glassfish sailfin.
Sangoma SIP to TDM gateway software (Netborder Express) can be a cost effective solution for your SIP application to connect with PSTN networks.

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