October 2008 | Volume 11 / Number 10
Next-gen Network Migration: A Signaling Perspective
By: Ravi Ravishankar
Within the diverse global conversation around IMS, one point is common: the move to IMS will be a gradual transition over time, requiring the interoperability and integration of different network applications, technologies and protocols as the networks evolve. The ideal architecture for the transition is one that provides flexibility to service providers by enabling them to deploy new revenue-generating services on IP based technologies, while keeping costs in line by leveraging existing services and Signaling System 7 (SS7) infrastructure as long as it makes sense. The key to success in this complex and competitive environment is to understand how the evolution of the network will impact a service provider’s business and signaling network, and how to create a practical transitional strategy.
Most people are now beginning to realize that large-scale IP Multimedia Subsystem (News - Alert) (IMS) deployments won’t be occurring in the near future. While many operators are still trying to determine when exactly IMS will be a now-technology, rather than something in the very distant future, they’re not letting this time go to waste. Operators are beginning to work with vendors to start deploying components of an IMS network (Session Initiation Protocol (News - Alert) [SIP], SIP routers, Service Capability Interaction Manager [SCIM], etc.) by laying the groundwork for a standalone SIP signaling framework as defined by the 3GPP specification. The SIP signaling framework allows operators to integrate different types of SIP-based traffic and to capitalize on that traffic to increase their revenues, without having to deploy a full IMS architecture — with all of its components.
The migration to IMS has thus far proven to be an evolutionary rather than a revolutionary transition, which has generated a significant impact on the signaling layer. Signaling is embedded in every service today. When you turn your mobile phone on — you generate signaling messages. Every text message you send or receive generates 4-6 signaling messages. This won’t change any time soon. Therefore, signaling is the obvious starting point of the network’s evolution. As the network evolves, the signaling protocol will migrate from SS7 to SIP and the Signal Transfer Point (STP) of the Public Switched Telephone Network (PSTN) will transition to the Call Session Control Function (CSCF) of the IMS architecture.
Such evolutionary steps to IMS are becoming more popular. In addition, operators face the challenge of interworking many technologies and protocols to enable seamless service delivery across a variety of network types, interoperability between new products and different vendors, and legacy system connectivity to newer systems. Even though operators aren’t fully deploying IMS networks, they can begin using IMS components to solve network problems today. However, before carriers begin deploying IMS network components, they first need to anticipate and overcome a few technical challenges:
Since operators are starting to see a downturn in voice revenue, they’re beginning to look for ways to further leverage already existing components of their networks to offer new multimedia services for customers.
There exists a multi-prong strategy for helping operators migrate to the next-generation service delivery model at their own pace and via the path that best suits their needs, including: gradually moving to IMS by supporting service orchestration and mediation across the Intelligent Network (IN), Next-Gen Network (NGN) and IMS domains; continue building out their NGN to support VoIP growth, while integrating some IMS technologies to experience some of the benefits of IMS without the cost of deploying the entire architecture; and carriers who want to go straight to IMS or who want to deploy IMS-based applications.
Implementing a SIP Signaling Layer in the NGN
The core signaling and session control layer has proven its importance in the SS7 signaling network and is identified in the IMS network architecture. A major issue with the NGN architecture is the lack of core-signaling infrastructure to assist NGN elements with signaling and session routing activities. Without a hierarchal session control layer, each NGN network element must handle all control layer related tasks such as routing, traffic management, redundancy and service implementation. All of this causes a number of barriers to creating an efficient network. For instance, each NGN network element must make application layer routing decisions based upon the destination address (i.e., SIP URI). Thus, all possible routes must be defined at each network element so that each will have one or more signaling routes between them.
The NGN cannot be properly expanded without the implementation of a suitable signaling and session control framework capable of off-loading various SIP signaling and session tasks from the edge NGN elements. With a capable session layer, session-related tasks are migrated from the edge NGN nodes to a centralized core SIP session framework. The resulting architecture allows the NGN network to grow systematically in response to increasing demand for VoIP, while avoiding the various limitations previously mentioned.
This session-based framework also presents an ideal opportunity to introduce the benefits of an IMS architecture into the NGN environment. Essentially, the IMS session management technology is a perfect candidate for implementing a signaling layer in the NGN. With the appropriate signaling and session control framework the NGN network can realize many of the attributes promised by the IMS architecture, such as access independence, Home Service Control model, subscription-based service orchestration, and multimedia support.
Moreover, with an independent control layer, a robust and bearer independent signaling and session control network can be implemented to offer highly available signaling that provides session setup for any type of multimedia service. Therefore, an operator can offer not only VoIP but any other type of media with the reliability and scalability of an SS7 network.
Service Orchestration and Mediation among SS7, NGN and IMS Networks
As carriers transition their networks, an important consideration is how to inter-work their existing networks with future IMS networks to deliver a seamless service experience to subscribers regardless of their access technology. Operators want to leverage their investment in current technology and avoid duplicating services in multiple domains. In addition, providers need the ability to mix services from multiple domains to create unique service packages.
Operators can use SCIM, an IMS-defined technology, to bridge TDM, NGN and IMS networks, providing the orchestration and mediation to enable SIP-based application servers and IN service platforms to inter-work. This allows carriers to deliver SIP-based services such as presence, location, enhanced VPN and IP conferencing to SS7-based subscribers. Conversely, IMS subscribers have access to SS7-based applications like number portability, directory assistance and calling-name delivery.
After the SCIM functionality is deployed at the STP in the SS7 network, operators can extend the SCIM capabilities to the pre-IMS/NGN and IMS domains with a SIP interface. This is a logical interconnection point since the SS7 network is the backbone for intelligent service delivery, data and application interaction, and flexible routing in circuit based networks.
With SCIM deployed in an NGN or IMS network, its functionality can be extended to an SS7 network using SIGTRAN, an SS7 over IP signaling protocol. Deploying SIGTRAN brings the IP service infrastructure into the core signaling network, allowing SIP and SS7 signaling to be processed over the same IP signaling framework. This arrangement allows users on SIP-based terminals to access legacy network services and interact fully with legacy network users.
While the transition to IMS appears to be the end goal, the path and length of time to get there are unknown at this point in time. The reality is that operators are borrowing the best of IMS today to get the most benefits possible, without deploying a full-blown IMS infrastructure, which causes the IMS business case not to work that well. The incremental approach being taken by operators allows them to prove-in the IMS business case much easier than just building the entire IMS infrastructure and waiting for the subscribers and revenues to come.
One thing is clear: the move to IMS will be a gradual transition over time, requiring the interplay of different network applications, technologies and protocols as the networks evolve. The key to success in this complex and competitive environment is to understand how the evolution of the network will impact a service provider’s business and signaling network, and how to create a good transitional strategy.
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