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SS7 makes the switch.

Back-to-the-future technology for Internet telephony.

As William Shakespeare once said, Everything old is new again." Even though Sir William wouldn't know the difference between a tin can and a digital telephone, his pithy aphorism is especially applicable to the convergence occurring in public voice and data networks today. Older technologies with a modem twist are key protocols in the brave new world of Internet-based telecommunications.

Today, most Internet telephony voice traffic is transported in the "core" network, on high-bandwidth facilities over long distances (often overseas). At the "edge," voice calls still originate and terminate on the public switched telephone network (PSTN). Getting digital voice data to hop back and forth between the PSTN and the Internet is a pretty straightforward job. But getting services to work between these two networks is a horse of a different color. The key to bridging services between these two telecom worlds is a resilient technology known as Signaling System 7 (SS7).

SS7 was developed by AT&T in 1975 and adopted in 1980 as a worldwide standard by the International Telecommunication Union (ITU). SS7 defines the procedures and protocols by which elements in the PSTN, namely voice switches (SSPs), routers (STPs) and databases (SCPs), exchange information to handle call setup, billing, routing and control. SS7 messages are carried on a separate digital signaling network, logically apart from the actual voice traffic.

SS7 handles a wealth of functions, from basic call setup, management and tear down to advanced services like local number portability, toll-free ("800") services, call forwarding, caller ID and three-way calling. While SS7 was not specifically designed for wireless networks, its robust and versatile design was easily adapted to support mobile telephony services, such as PCS, wireless roaming and mobile subscriber authentication. Now the venerable protocol is being adapted to support the next generation of services in the converging circuit-switched and packet-switched network environment.


Today, the key driver in routing calls over the Internet is cost savings. Voice traffic is transparently routed from the circuit-switched PSTN onto the packet-switched Internet to take advantage of IP's improved efficiency in sharing bandwidth over the PSTN's dedicated TDM (time division multiplexing) approach. Obviously, the cost savings are highest when calls are carried over long distances, especially overseas. As cost savings drive prices lower and lower, at some point, the cost to bill individual calls becomes prohibitively expensive. As more voice calls are carried over the Internet, the billing of individual long-distance calls likely will disappear in favor of the flat-rate billing model used today by most Internet service providers and many cellular operators. Voice calls also likely will continue to generate the lion's share of telco revenues for several years, or at least until new IP-enabled multimedia services become widely popular.

Next-generation carriers know the PSTN is here to stay for the foreseeable future and existing voice services need to be supported. That means reliable inter-working with the PSTN and the SS7 network is key to the success of IP telephony.

Internet telephony carriers need to use SS7's "back-to-the-future" technology to offer new IP-based services to the PSTN and to access existing services--both wireline and wireless--in the PSTN. Next-generation carriers know they need to provide seamless interoperability of services and features across both networks to achieve widespread acceptance of Internet telephony.

Currently, there are no accepted standards for such inter-working. But the signaling transport working group ("sigtran") of the Internet Engineering Task Force (IETF), the worldwide body responsible for recommending standards for the Internet, has made good progress in defining a suite of new protocols for IP telephony. A low-level transport protocol called stream control transmission protocol (SCTP) is already close to becoming a new Internet standard. However, much work remains to be done on higher-level protocols before inter-working of services and features between the PSTN and Internet telephony networks becomes a reality.


Despite the technical immaturity of Internet telephony, innovative carriers are plunging headlong into the next generation, drawn by Internet telephony's lower cost of carrying calls and promise of supporting valuable new multimedia services never before possible in the PSTN. Already the public Internet is carrying millions of minutes of voice traffic every month from the United States to destinations around the world.

The early success of Internet telephony has not gone unnoticed. Emerging countries, such as China, are keen to gain a competitive advantage over their Western counterparts by leapfrogging over legacy technology and jumping directly to a new network infrastructure based on IP technology. While investments in PSTN infrastructure are still being amortized in many places, some innovative countries, such as Finland and Sweden, are making big investments in next generation IP telecommunications technology. In the United States, the rollout of Internet telephony networks will probably lag behind other countries, at least until IP telephony networks can match the PSTN's reliability, robustness and support for "life-line" services like 911.


While the ITU defined the international standard for Signaling System 7, over time many countries have deployed national variants. Unfortunately, these national variants are often incompatible. For instance, a switch in NorthAmerica cannot send an SS7 message to a switch in Europe. Why? Because in NorthAmerica, point codes (signaling point addresses) are 24-bits in length as per the American National Standards Institute specification. In Europe, point codes follow the ITU standard of 14 bits. China and Japan also use different versions of SS7. IP telephony equipment manufacturers realize that their growth potential in their target markets, (i.e., overseas), will be severely limited if their networks cannot support the local variants of SS7.

SS7 support is already appearing in the latest crop of IP telephony gear. A new device, called a "signaling gateway," provides SS7/IP interoperability between the PSTN and IP telephony networks. To speed the deployment of IP telephony gear around the world, some signaling gateways even provide on-board support for multiple international SS7 variants and conversion between variants as needed. The signaling gateway handles all details of the local SS7 implementation while providing a consistent application program interface to IP switch clients.


The Internet and the public switched telephone network may seem like "strange bedfellows." Despite rumors to the contrary, the Internet can carry voice traffic with a high degree of reliability, provided that network operators handle congestion appropriately. Internet telephony carriers carefully monitor the networks for congestion, rerouting traffic over alternate pathways when congestion occurs. Some Internet telephony carriers guarantee quality at least equivalent to the PSTN and will even reroute traffic onto the PSTN as a last resort if no viable IP network route is available.

Ultimate convergence of voice and data networks is inevitable, but the industry must mitigate equally inevitable interoperability problems, while managing the interim "patched" network world that will get us from here to there. Likewise, an array of "who-owns-it" liability and security questions must be raised and adjudicated.

During the convergence process, vendors, integrators and OEMs must work together toward new standards and champion interoperability to smooth internetworking between PSTN and IP networks. This will beget new revenue-generating services that can satisfy the market's demand for the promised capabilities of the converged Internet telephony future.

Cable is vice president of signaling systems at Performance Technologies, Rochester, NY.

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Title Annotation:Technology Information
Comment:Singaling System 7 (SS7), originally developed by AT&T in 1975 and adopted as a worldwide ITU standard in 1980, defines procedures and protocols used by PSTN elements such as voice switches, routers and databases for exchanging information.
Author:Cable, Reg
Publication:Communications News
Geographic Code:1USA
Date:Oct 1, 2000
Previous Article:Voice over IP--the better choice.
Next Article:Combine the proper elements to create new architecture.

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