January 2002

Signaling For The Next-Generation Wireless Network

BY RAVI RAVISHANKAR

This column examines how the signaling network will evolve to deliver voice and multimedia services. It is the final installation in a three-part series that explores the evolution of the wireless network. 

Signaling is a very critical component of telecommunication networks. Signaling System #7 (SS7) is universally used in all of the current telecommunication networks, including wireless. SS7 enables basic voice services and wireless-specific services such as roaming and handoff. It supports advanced features such as toll-free, one-number service, local number portability, and wireless data services like short message service (SMS). Most wireless carriers have built their SS7 network as an overlay to the voice network, using specialized SS7 routers known as signal transfer points (STPs). 

On the other hand, a wireless data network such as general packet radio service (GPRS) is built using Internet protocol (IP) technology. Though these networks use an IP backbone for transporting end-user data, they use a blend of SS7 and IP technology for signaling. Many capabilities used in the Internet world such as DHCP servers (for dynamic allocation of IP address) and domain name servers (DNS) are a natural fit in the GPRS world. However, services such as roaming and handoff will continue to use the proven SS7 signaling, instead of new technologies such as mobile IP. The interoperability requirement between voice and data networks also makes it essential to support SS7 in the GPRS network. For example, a single and simultaneous registration of a roaming subscriber to both a GPRS data network and a GSM voice network requires SS7 signaling interaction between the GPRS nodes and the mobile switching center (MSC).

Further evolution in signaling technology will involve many new protocols. SS7 continues to evolve at the application layers, with protocols such as customized application of mobile enhanced logic (CAMEL) and wireless intelligent network (WIN), which are standards for wireless networks that enhance the provision of intelligent network services. Session Initiation Protocol (SIP) is gaining wide popularity for delivering multimedia services over an IP network. SIP is a lightweight protocol, well suited for the data networks given its heavy borrowing from technologies used in the Internet world, such as HTTP. H.323, another protocol that is widely used in voice over IP networks, will also co-exist for a long time because of its legacy status.

While these new protocols will enable the delivery of innovative services, it is essential to realize that many of the industry practices developed for the SS7 signaling network still apply. It is a common misconception in the industry that SS7 is simply a protocol. Rather, SS7 defines an umbrella of protocols, and also includes a definition of the network architecture, methods, procedures, and practices for the deployment of highly reliable signaling networks, applications, and services.

SS7 was greatly enhanced following recommendations made by the Network Reliability Council (NRC). Organized by the FCC in 1992 following a series of major service telephone networks outages, the NRC undertook a detailed study of the causes of outages based on historical patterns. The NRC concluded that the signaling network is one of the critical elements concerning reliability, and recommended a series of industry best practices (see �Network Reliability � A Report to the Nation,� published by the NRC). As the industry adopts new protocols, the lessons learned regarding reliability will prove invaluable. Also, security will take on additional importance with the open architecture of an IP-based signaling network.

Steps In Evolving Signaling Networks
Given the existence of multiple protocols, as well as the adoption of IP as the universal transport network for voice, data, video, and signaling, the three principal cornerstones for signaling evolution are:

• Support of, and interoperability between multiple protocols such as SIP, H.323, CAMEL, WIN;
• Modular, layered architecture, separating transport, switching and application � where protocols and technology can be mixed and matched at different layers; and
• A blend of SS7 and IP technology � preserving the industry best practices developed for SS7, coupled with the open system architecture of IP.

The very first logical step in evolving the signaling network is to build a protocol-independent, packet-based signaling transport network, which meets or exceeds the reliability, security, and performance of the current signaling networks. The Internet Engineering Task Force (IETF) is working on common transport protocols, under the SIGTRAN working group, for transporting signaling packets. The transport adaptator layer interface (TALI) is a pre-SIGTRAN, open-source standard developed by Tekelec, which is widely deployed by many wireless and next-gen carriers. This has not only enabled the industry to develop early expertise in this area, but also helped the operators to overcome some of the current network overload problems that they are facing due to the unprecedented explosion of SMS traffic.

3G Signaling
The STP has evolved from providing simple, network layer routing to advanced application-aware routing and firewall screening capability. The current-generation STPs support policy-based proxy or redirect routing and filtering for several SS7-based applications, such as local number portability, SMS roaming, and prepaid SMS. The STP functionality will continue to evolve to become multi-protocol signaling routers, supporting next-gen protocols such as SIP and H.323. In addition these signaling routers will support several emerging translation, routing, and session management functions like ENUM for telephone number mapping, telephone routing information protocol (TRIP) for telephony routing updates, and call state control functions (CSCFs) for service mobility in 3G networks. These are currently being defined by the IETF, ITU, 3GPP/3GPP2, and other standards organizations.

Conclusion
Signaling is a key enabler for delivering new, innovative services in the 3G network. SS7 signaling will undoubtedly evolve, and new, IP based technologies will be included. In fact, several operators have already deployed or started deploying an SS7 over IP network. Clearly, signaling will continue to be the glue that will provide a seamless migration to tomorrow�s 3G network. c

Mr. Ravi Ravishankar is director, Advanced Technology Planning, Tekelec. His focus is on defining signaling solutions and products for the next-generation packet telephony and 3G wireless networks. Tekelec is a leading developer of telecommunications signaling infrastructure, softswitches, testing and diagnostic solutions, and service applications. Please visit their Web site at www.tekelec.com.

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