At the heart of IMS is Session Initiation Protocol (News - Alert)
(SIP). SIP is a lightweight protocol, Revenue is the key to success of any infrastructure, and applications are the key to revenue. The IP Multimedia Subsystem (News - Alert)
(IMS) was developed by 3GPP (3rd Generation Partnership Project) to address wireless services, the
rich capabilities offered by voice over IP (VoIP) technologies. In order to increase Average Revenue per User (ARPU), IMS allows multiple applications to be delivered to users and also allows the applications to be retained by the users as they roam throughout the world.which is transport-independent, and offers a vehicle for rich media interworking between end points. SIP has been used in next generation-networks (NGN) since the late 1990s. In 2000, 3GPP adopted SIP as the protocol standard and as the framework for the IMS control. As part of the IMS model, SIP extensions have been added to accommodate a rich service model that assumes multiple applications interacting with each user session.
There are several major advantages to building a new SIP-based feature or service:
• Simplicity — SIP is based on a straightforward requestresponse interaction model, making it a simple and comprehensible protocol for developers to implement.The messages are also text-based, which makes them easy to parse, create, read, understand and debug.
• Extensibility — SIP can set up sessions for any media type, such as voice, video and application sharing.
• Flexibility — SIP allows developers to interact with individual protocol messages without breaking interaction boundaries.This allows developers to concentrate on creative application development within the context of IP communications.
• Familiarity — SIP borrows heavily from HTTP and other Internet standards from the IETF, which allows SIP applications to be developed using web-like technologies. SIP development looks and feels a lot like Web development.
Applications in IMS Environments
The IMS architecture introduces several new capabilities to allow for advanced applications.The IMS model specifically was designed to address multiple applications assigned to users and unique invocations for registration, origination and termination events.
In the IMS network, the Home Subscriber Server (HSS) stores each user’s registration profile and
application filters, such as the following:
• Registration — As endpoint devices register, third party registration filters indicate which applications deployed in the IMS network should be informed of originations.The operational systems populate the HSS filters and the Service Call Signaling Control Function (S-CSCF) uses the data to inform appropriate applications of the condition.
• Origination — As devices within the IMS network originate sessions, the Serving CSCF uses the filter information to determine where to route the originating SIP sessions and in what order. In the case of the girlfriend with the prepaid application (see next page), the origination session routes to the Prepaid application server first, and then to the Synergy application server.
• Termination — Sessions destined for a user’s device or application is signaled separately with their
own set of filters.Again,multiple applications can receive control of the terminating SIP sessions..
Consider an example of two users: a boyfriend who is not able to dial long distance; and his girlfriend who has Find-me,
Follow-me and a prepaid service. (See Figure 1.)
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From a signaling perspective, call setup in IMS environments involves many SIP sessions.The simple call from the boyfriend to his girlfriend involves the following SIP signaling call flows:
• Boyfriend’s CPE — Proxy CSCF.
• Proxy CSCF — Serving CSCF.
• Serving CSCF — Application Server, based on boyfriend’s originating filter criteria.The Application feature server applies the originating logic for the boyfriend. Based on the example scenario, the logic consists of dialing policies to ensure that the boyfriend does not make a call outside of the local calling area.
• Application Feature Server — Serving CSCF.This is the response to the boyfriend’s origination processing.
• Serving CSCF — Prepaid application server, based on girlfriend’s terminating filter criteria.The prepaid application server applies the appropriate logic and allows the call to continue.
• Prepaid application — Serving CSCF.
• Serving CSCF — Application Feature Server, based on the girlfriend’s terminating filter criteria.The Application Feature Server applies the terminating logic for the girlfriend.This includes playing a Color Ringback tone to the boyfriend and then applying the Find-me, Follow-me applications.
• Application Feature Server — Serving CSCF.This is the response to the girlfriend’s termination processing.
• Serving CSCF — Proxy CSCF of girlfriend’s phone.
• Proxy CSCF — girlfriend’s phone.
The flexibility of the IMS model is that each independent session at the application level is managed separately.The prepaid application can terminate the call to the girlfriend at any point when funds are depleted, drop the call in the direction of the caller, and connect the girlfriend to a media server so she can hear about payment arrangements.The Application Feature Server can pull back the call to the girlfriend’s phone and reroute it anywhere depending on the settings of Find-me, Follow-me feature.
Adding new applications, such as an external third party control SIP application server, is easily implemented by updating the
filter criteria.
Roaming in IMS Environments
Today’s wireless networks offer limited services when a mobile phone user roams into a visiting network.This limited functionality also means that the user receives inconsistent service while roaming and the service provider loses control of the call. IMS architects have solved this issue by leveraging SIP signaling.
Referring again to the residential application example above, if the girlfriend is roaming in another network and uses her phone to place a call, the IMS network uses the I-CSCF in the visiting network to communicate to her Home network. In the Home network, the services operate identically as if they were originating from the girlfriend’s phone directly.This means that the originating and prepaid applications can receive control and process the calls, even when the call is placed in a remote network.This provides complete and consistent feature delivery for the girlfriend’s phone services and retains application revenue in the home service provider’s network.
In Conclusion
Applications continue to drive opportunities within new architectures. IMS has leveraged the SIP signaling to allow delivery of multiple applications to users across any access network. Combine this with the rich media capabilities that SIP introduces, and you’ve got a foundation established for significant future service provider success. 
Frank Salm is Vice President of Marketing for
Sylantro Systems (News - Alert)
. For more information, visit the company online at www.sylantro.com
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