IMS: a new spin on convergence: wireless and wireline carriers seek the "eBay" of flexible, responsive user experience

Telecommunications Americas, March, 2005 by Sean Buckley

RELATED ARTICLE: Creating a Service-Driven Architecture

As the telecom version of the time-worn phrase should go--it's all about the services, stupid. Taking a page from the wireless world, the IMS architecture as defined by the 3GPP and 3GPP2 standards is an all-packet core network that creates an access-agnostic environment to deliver a wide range of multimedia services that a user can access using any device or network connection. Leveraging the SIP protocol, IMS supports IP-to-IP sessions over any wireline connection (e.g., DSL, cable) or wireless network protocol (e.g., Wi-Fi, GSM or CDMA). The IMS infrastructure allows a carrier to interwork between the TDM and IP networks to provide a seamless service experience.

Built as a unified architecture to support a plethora of services, IMS distributes what is referred to as multiple logical functions into three common layers:

* Connectivity Layer: Comprises routers and switches, both for the backbone and the access network.

* Session Control Layer: Comprises network control servers for managing calls or establishing sessions and modifications. The two main elements of this layer are the CSCF (call session control function) and the HSS (home subscriber server). Sometimes called the SIP server, the CSCF performs end-point registration and routing of the SIP signaling messages to the application server related to a particular service. In addition, the CSCF interworks with the access and transport layers to guarantee QoS for all services. The HSS database maintains each end user's service profile. Stored in a central location, this information could include registration information (IP address), roaming, voice mailbox and IMS information (buddy lists).

* Application Server Layer: Utilizes application and content servers to provide various value-added services. At the heart of this layer are what media server vendor Convedia calls the three workhorses of the IMS services platform: the AS (application server), MRFC (multimedia resource function controller), and the MRFP (multimedia resource function processor). The AS is responsible for the execution of service-specific logic, for example call flows, database dips, and user interface interactions with subscribers, while the MRFP--more commonly known as the IP media server--provides adjunct media processing for the application layer. Through the media server, a service provider can deliver various non-telephony services (e.g., push-to-talk) as well as speech-enabled services, multimedia messaging and video services, and other more mainstream services such as conferencing, prepaid card and personalized ringback tones.

One of the earliest services enabled by the IMS architecture is push-to-talk over Cellular POC. OEMs such as Nokia and Motorola have gained notoriety for POC, while other vendors such as Ericsson, Lucent and Siemens have developed an ecosystem of applications that span fixed line, mobile and enterprise. Beyond PTT, the standard can enable operators to take advantage of multiple, value-added services including traditional telephony services as well as instant, video and multimedia messaging.