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Industry Insight
November 2000

Jim Machi

Why SS7 Will Stay In The Picture


Internet protocol (IP) is being touted everywhere as today's unifying technology. Poor Signaling System 7 (SS7) languishes in contrast. If you walk through a public network convergence tradeshow you might not even hear SS7 mentioned. It would be easy to conclude that SS7 is boring and out of the picture -- but it's not. In fact, SS7 has a huge role to play in IP telephony. This established and evolved technology will enable the public switched telephone network (PSTN), wireless networks, and IP networks to unify.

The traditional PSTN isn't going away just because IP telephony is a cool new technology. The PSTN exists, it works, and it represents a tremendous investment. Convergence may be the latest buzz word, but the new IP networks will actually interoperate with the PSTN -- giving us multiple networks to manage as we roll out new services. SS7 has evolved from its simple original purpose, requesting voice circuits between two offices. Today it is a primary unifying technology and enabler for enhanced IP services. Wireless services also depend upon SS7 for most of their functionality.

In the changing network, the Internet dominates because of the rich content it provides. But while the content is rich, the IP network's delivery -- or lack of delivery -- for data, video, and voice applications clearly shows it is missing something as a reliable communications technology. Conversely, reliability and quality of service are the hallmarks of the traditional PSTN -- at least in most parts of the world. The simple content this reliable network delivers is voice, along with fax and slow data access.

Integrating these worlds opens up a whole new set of opportunities, allowing the industry to deliver innovative end-user services based on the IP infrastructure. It also makes it possible to leverage the huge installed based of traditional telephony users and services and the growing wireless community.

The challenge is to integrate three separate paradigms -- one based on delivering rich content through an unreliable network, one based on delivering simple content through a reliable network, and one based on the ability to roam freely without wires -- while communicating with everything else. SS7 will enable this integration.

Originally designed to carry end-to-end messaging to set up switched circuit connections, SS7 largely drives the circuit-switched PSTN world. SS7 has evolved to include capabilities that make the network increasingly intelligent, including:

  • The ability to access remote databases to support toll-free or free phone service;
  • Management, location identification, and call delivery through wireless networks;
  • Number portability; and
  • Caller ID display.

SS7 has defined the intelligent network architecture, providing the ability for distributed nodes to have specific functionality that enables enhanced, intelligent services. It is not without problems, notably the fact that there are many different variants in different countries and regions. At the same time, the SS7 software has continued to add capabilities that allow more intelligent and wireless networks to rely on SS7 for both basic and enhanced functionality.

To build on these existing services, and to leverage today's huge infrastructure investment, we need a method for seamless communication between the PSTN and IP worlds. By allowing SS7 traffic to be carried over IP networks, it's possible to extend today's architecture and services without having to replace existing equipment. Providers can host existing telephony applications in the IP world -- including routing voice calls through IP networks. This opens up the possibility of better tariffs to existing circuit-switched networks.

Also, in the IP world it's possible to host the databases required for wireless operation or intelligent networking services like number portability. This enables a gradual integration of the two technologies and, ultimately, service sets. IP also adds the capability of back-hauling signaling traffic for processing far away from the home network. Services can be operated remote from the end user. For example, a remote network without such a capability can use spare capacity on an existing local number portability (LNP) database.

In recent years there have been numerous proposals for conveying SS7 information through IP networks. These generally replace the lower SS7 layer (message transport layer) with an IP transport. The most successful and widely adopted standard is being defined by the Internet engineering task force (IETF) signaling transport (Sigtran) working group. The IETF has defined an end�to-end protocol known as stream transmission control protocol (SCTP), which exists within the IP protocol stack alongside transmission control protocol (TCP) and user datagram protocol (UDP). SCTP provides reliable, non-blocking communication between two IP end points. This is modified by adaptation layers to provide an SS7 protocol-like interface, allowing the higher SS7 protocol layers to seamlessly intercommunicate through an underlying IP backbone.

Using such protocols enables different vendors to offer standard equipment for deploying converged SS7 and IP networks, like SS7-to-IP routers or gateways. These -- when combined with media gateways that packetize the TDM voice and media gateway controllers that provide a softswitch function -- can be used to deploy voice-over-IP solutions built from different suppliers' components. It's also possible to deploy a public network-type system with a lower initial investment -- ideal for areas with low population or for small-scale deployments such as a corporate private wireless network based on public network principles or a competitive local exchange carrier (CLEC).

Access to SS7 network resources enables emerging IP-based services to route to existing wireline or wireless users. Wireline subscribers can be managed through local number portability (LNP) databases containing subscriber information such as text strings for caller display. Wireless users are connected through either Interim Standard 41 (IS-41) or Global System for Mobile Communications (GSM) networks, both based on SS7.

Connecting to these resources gives IP service providers access to information on subscribers' locations and the ability to contact them by voice or text messaging, whether the subscribers are in a fixed location or on the move. From the user's perspective, IP integration promises greater flexibility and control, plus the possibility of programming a personal subscriber profile to route calls to home, a mobile phone, or the office through a Web interface.

SS7 is the enabler that will allow IP and PSTN service providers to unify the PSTN, wireless, and IP networks. The huge installed base of PSTN customers will be able to use IP-based enhanced services. The growing base of IP network customers will be able to access enhanced PSTN services. Wireless customers will be able to communicate with everyone -- and, someday, to roam the globe. In fact, SS7 will enable competitive tariffing, low-cost service rollouts, and communication between disparate networks. In short, it will unify the world.

Jim Machi is director, product management, CT Server and IPT Products, for Dialogic Corporation (an Intel company). Dialogic is a leading manufacturer of high-performance, standards-based computer telephony components. Dialogic products are used in fax, data, voice recognition, speech synthesis, and call center management CT applications. 

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