October 1999

CONVERGENCE ON A GRAND SCALE
The Role Of The Slowly Evolving SS7 Network

BY BROUGH TURNER

As the public telephone network evolves, and as the IP revolution rages, new development opportunities emerge. These opportunities manifest both the practicalities and politics inherent in accomplishing convergence on a grand scale, that is, throughout the public network. The existing network is of such magnitude, and the conventions supporting it are so entrenched, that the anticipated migration — from the traditional telephone network to an all-purpose public network based on IP — is likely to be gradual, and fraught with complexities and compromises.

Many of these complexities and compromises pertain to signaling, which will no doubt interest developers, particularly computer telephony developers. After all, computer telephony developers are already familiar with the challenges peculiar to convergence. And a convergence mindset may stand computer telephony developers in good stead as we begin entertaining convergence scenarios — of interim solutions or hybrid or inter-working schemes.

Typically, convergence scenarios, if they involve signaling at all, account for Signaling System #7 (SS7). While it is hardly new, SS7 retains considerable potential for developers — a point I made in my August 1998 On The Horizon column. In that column, I described an opportunity for computer telephony developers to use open systems to create enhanced services platforms that could be deployed in the traditional telephone network, that is, in the central office (CO).
But the IP revolution is opening up other opportunities that are going to change the way telephony signaling is done. This change will take many years, but some new business opportunities are already emerging.

SS7 FOR THE UNINITIATED
SS7 is a signaling protocol used within public telephone networks to control call set-up and call routing and to provide services such as 800 numbers and caller ID. The protocol is used on a separate, fully redundant data network that runs within the public telephone network. This SS7 network carries data packets much like an IP network, but only the highly specialized packets needed to make telephone service work.

It isn't a very high-capacity network; links run at 56 Kbps in the United States and at 64 Kbps in other parts of the world. On the other hand, the SS7 messages are very efficient, and the SS7 network is fully redundant, making it extremely reliable, much more so than most data networks. Typically, SS7 technology supports telephone services that are faster, more reliable, and more advanced than previously possible.

Using the SS7 network, COs and other switching systems communicate with each other to set up calls and allocate trunks. The SS7 network also provides access to databases for call routing information if a telephone number is not associated with a physical destination.

The classic example of this occurs in 800 number service, or "free phone" service as it is known in other parts of the world. But beyond these applications, the SS7 network is now used to access a variety of special-purpose databases for such purposes as validating calling cards, locating mobile phones, and delivering calling party names (a more advanced form of caller ID). More recently, under the names "Intelligent Network" or "Advanced Intelligent Network," database computers, called Service Control Points, have been extended to execute the service logic (the software) for new services such as personal incoming call management.

The ability to offer new services is not the only force shaping the evolution of the SS7 network. Politics also comes into play. In the United States, the FCC is trying to make it easy for competitive local exchange carriers (CLECs) to provide local telephone service, in competition with the Regional Bell Operating Companies (RBOCs) or other incumbent local exchange carriers (ILECs). Among other things, the FCC has mandated that customers be allowed to change their local service provider while keeping the same telephone number. This is called local number portability (LNP).

It sounds simple, but in fact it poses a major technical challenge. With the exception of 800 numbers and mobile phone exchanges, all telephone numbers in the United States, or the world for that matter, are associated with a specific geographic location. For LNP, all of the routing assumptions, built into the telephone network over 120 years, have to change. Like calls to mobile phones, virtually all calls to any U.S. telephone number will now require a database look-up. Of course, this cannot happen overnight. But over the past three years, the technology to support local number portability has been rolled out for more than half of all U.S. subscribers, and work continues.

SIGNS OF THINGS TO COME
SS7 networks are extremely reliable, but the technology is several stages behind the state of the art. The equipment is specialized, it’s manufactured in relatively low volumes, and it is very expensive. The advent of open telecom SS7 components has brought costs down substantially, but nowhere near the costs of IP networking gear. So why not replace SS7 with IP technology? The Internet was originally designed to survive an atomic bomb attack, so there should be a way to use IP technology to provide reliable services. And with IP’s ubiquity, it should be more flexible and cost much less. Could this happen?

Dial Offload Of Modem Calls
The Internet is already having one major impact on the legacy telephone network. Many subscribers are calling their ISPs and staying on the line for long periods. But telephone facilities were designed for typical voice telephone calling patterns, assuming, for example, that an average call would last three minutes. Suddenly some people are staying on the line for two hours or even staying off-hook 24 hours a day — dramatically increasing the load on local COs and tandem-switching centers.

One way to avoid having to increase the amount of voice-switching capacity is to move the ISP’s modems closer to the customer, off-loading the modem traffic from the voice switches. Of course, it would be an extremely expensive proposition to put modems in every CO, but putting them in busy switching centers is manageable. If calls to an ISP can be identified, they can be routed to the closest modem port on a remote access server (RAS). There the data can be extracted and sent to the ISP using an IP network.

With this scheme, whenever a call is placed to the number of a participating ISP, a database dip determines the nearest modem bank, and the other SS7 signaling information determines how the IP data must be routed. Deploying separate SS7 links to each modem bank, however, would be extremely expensive. Since the RAS boxes are connected with an IP network anyway, there’s a clear interest in routing the SS7-type information over the IP network from a single SS7 gateway.
Standards for such an SS7 gateway are in development, but systems are already being deployed using proprietary signaling arrangements on the IP side. The need to offload modem calls is pressing, and IP-based SS7 bypass is a key part of these systems.

Wireless
Another area in which IP is encroaching upon traditional SS7 territory is in wireless infrastructure. Wireless networks traditionally use SS7 not only for all normal landline purposes, but also for communicating information about where a subscriber is currently located.

Complete subscriber information is maintained in a database called the home location register (HLR). Temporary records for subscribers who are visiting from other areas are maintained in a visitor location register (VLR). As a subscriber travels, each visited system has a dialog, using SS7, with the subscriber’s home system and then temporarily registers the visiting mobile unit in its VLR. At the same time, the home system marks its HLR record for that subscriber so those calls are automatically routed to the visited location.

With so many new mobile telephone systems being deployed, it’s pretty clear that finding a less costly way to support this database infrastructure would be desirable. And again, in advance of standards, major wireless equipment vendors are designing systems where the HLR and VLR are based on commercial database technology running on highly available redundant CPU clusters accessed over redundant private IP networks. This next generation of wireless equipment will use call control messaging over IP with just a single pair of gateways to connect to the legacy SS7 network.

STANDARDS WORK UNDERWAY
Standards are being developed in the Signaling Transport (SIGTRAN) working group of the IETF that should provide a common form of SS7 bypass. The working names are SIGTRAN Common Transport Protocol (SCTP) running on top of Multi-network Datagram Transmission Protocol (MDTP). But the names could change. In any event, what’s being defined is a way to carry the upper layers of the SS7 protocol stack on top of UDP/IP.

The real question is how quickly will lower-level and physical layer SS7 components be replaced by IP technology. And the answer is: very gradually. There are more than 1.2 billion conventional telephones in the world today, supported by a widely deployed SS7 infrastructure. As yet, there are almost no pure IP telephones and, even with gateways, less than one percent of voice traffic is carried by IP. Replacing this installed base is likely to take 10 to 20 years. What we will see soon is IP-based technology in new deployments, interconnected with the existing SS7 infrastructure.

POLITICS AS USUAL
A web of politics and existing vested interests will delay the full implementation of new technology, even after SIGTRAN releases a standard. Consider the calling name databases (CNAM) and the line information databases (LIDB) in North America. CNAM contains the name of the subscriber associated with a telephone number, and LIDB includes the subscriber’s calling card information, if any.

Advanced caller ID services that show the caller’s name do so via access to the CNAM database. If the caller and called party are with the same telephone company, then the CNAM information is local and can be retrieved economically. But if the caller is another company’s customer, then a complex set of agreements must be in place.

Typically, no carrier is willing (or technically able) to allow others to maintain up-to-date copies of their databases, so one company must access the other company’s database. This requires agreement in advance. Typically, phone companies will negotiate agreements, at least with some parties, for access on a per-transaction basis, frequently charging the better part of a penny per look.

Establishing the minimum necessary set of agreements with RBOCs, inter-exchange carriers, independent telephone companies, and existing CLECs can be a major stumbling block for a new CLEC or budding Internet telephony provider. More typically, emerging telephone companies negotiate an agreement with a third-party SS7 network service provider, such as Illuminet (www.illuminet.com), which already has the needed interconnections.

Illuminet is currently the largest independent provider of SS7 and IN services in North America. Besides providing basic SS7 services for telephone companies, they have established interconnect agreements with all of the RBOCs and major inter-exchange carriers such as AT&T, MCI, and Sprint. Here complexity and politics have created an opportunity for those who can provide neutral inter-working functions.

These access issues hold for all SS7 databases, except for the LNP databases, for which the U.S. government has mandated access arrangements and access fees. With the LNP databases, individual telephone companies can actually obtain the update information needed to maintain their own copies of the LNP locally. In this one case only, the database could be located on a commercial high-availability server and accessed over Ethernet, if so desired. But LNP is the exception to the rule.

Until a significant number of the world’s telephones have been replaced with totally new audio appliances, it seems safe to say that SS7 and IP will coexist. During a 10 or 15 year transition of SS7 to IP networks, there will be many opportunities for computer telephony developers, with their expertise in converging networks, to create innovative solutions that ease the migration.

Brough Turner is senior vice president of technology at Natural MicroSystems, a leading provider of hardware and software technologies for developers of high-value telecommunications solutions.

For more information, call Natural MicroSystems at 508-620-9300, or visit the company’s Web site at www.nmss.com. E-mail to the author ([email protected]) is also welcome.