August 1998

BY BROUGH TURNER

A common theme in many of my articles has been the transition of traditional circuit-switched telephony to packet-switched architectures, that is, IP telephony. I’ve also pointed out how the transition to IP telephony is being driven by the need to overcome the built-in limitations of today’s telephone network.

All the same, today’s network, despite its limitations, and despite its eventual demise, still retains latent potential. And this potential, if exploited by applications developers, can translate into interesting business opportunities, even while the transition to IP telephony continues apace.

The original design of today’s telephone network was driven by one application — voice telephone calls. Today’s voice quality remains that of a 19th-century microphone. And, the digital PSTN uses 64-Kbps circuits because that’s what it took to digitize voice with 1960s transistors. These are some major limitations. However, there is yet another, more basic limitation, one that is, perhaps, more constraining than any other: All the intelligence needed to support voice calls is built into the network.

True — the telephone network is the world’s largest network. It’s stable. It’s reliable. And the industry is working to support new applications under an initiative called the "Intelligent Network." But the fact remains, the PSTN is riddled with constraining assumptions that hinder the deployment of new applications.

In an article for the ACM’s netWorker magazine (February/-March 1998) titled, "The Dawn Of The Stupid Network," David Isenberg describes how the Internet avoids constraining assumptions by merely providing dumb routing of bits. The intelligence that supports useful applications is built into clients and servers connected at the edge of the Internet. It is this flexibility that facilitates new applications, and it is new applications that will drive the telephone industry toward IP at a rapid rate. Look for significant changes in the next two to five years.

However, there are roughly one billion traditional telephones in use today. And therein lies an opportunity for the CTI industry. These telephones won’t be replaced overnight. It will take decades. In the interim, these "legacy" telephones will need to interconnect with the new network. Moreover, as new services are rolled out for IP subscribers, tremendous rewards will flow to those who can provide connectivity and even a subset of the new functions for those with older telephones.

Yes, we’ll need voice over IP (VoIP) gateways. But the dumb routing of bits is only the first step. How will applications developers bridge these two worlds? The answer is with tools that help them reach into the traditional telephone network and partially defeat some of its constraints. The most powerful approaches use Signaling System 7 (SS7) protocols and emerging interfaces defined as part of the ITU’s Intelligent Network (IN) efforts.

THE INTELLIGENT NETWORK
Until the advent of IN, the central office (CO) switch contained the information about how an individual call should be routed based on the number dialed. Originally this information was hard-wired into the switch; later it was programmed in. In either case, it was built-in and difficult to update. Continual efforts to make this routing function more flexible are leading to the Intelligent Network.

The Intelligent Network is an architecture for adding advanced functionality to the existing telephone network. Instead of adding software into the proprietary CO switch, the CO switch is equipped with the ability to defer some decisions to a separate computer system. To do this, messages are passed over an SS7 network. Consider a relatively simple example, "800" service. The local COs are programmed to detect all calls with the 800 code and to initiate a trigger function. When a trigger is activated, the local CO switch sends a message over the SS7 network to a remotely located database system, referred to as a Service Control Point (SCP). The SCP determines the actual number of the destination telephone and sends this information back to the local CO switch. The switch then routes the call on that basis.

800 number service preceded IN and spurred its development. Once fully deployed, IN has the potential to allow PSTN operators to implement new services, such as "one-number, follow-me," in a uniform manner across a diverse installed base of CO switches. The CO switch delivers dial tone, absorbs digits, and generates IN triggers. These triggers send messages over the SS7 network to applications processors. Having centralized applications processors and databases eliminates the need to update tables or programs in each local CO switch whenever a service or number changes. The IN separates some of the intelligence from the basic network.

Of course, transactions must be very fast and highly reliable. If the process is too slow, there will be a noticeable delay; if the process is unreliable, the call will either be lost or mishandled. The SS7 protocol suite provides the reliability and speed necessary for passing call control information.

THE SS7 PROTOCOL
SS7 was designed in the late 1970s and early 1980s for exchanging call control information between the various network switches and databases in the PSTN. It replaced an earlier Common Channel Interoffice Signaling system (CCIS, or SS6 in ITU terms) and offered several important advantages, including greater speed and flexibility. SS7 is now widely deployed and is used to support services such as ISDN (Integrated Services Digital Network).

The SS7 network is a packet network operating in parallel with the voice telephone network. While there are common elements to all packet networks, a major difference between the SS7 network and an IP network (like the Internet) is that the SS7 network was designed for extremely high reliability using traditional telephone company techniques, that is, full redundancy. The Internet achieves high availability with routing protocols that route around failures. The SS7 network achieves high availability by having two copies of everything — every router and every connection (link). SS7 networks are typically engineered so that no link is loaded to more than 40 percent of its capacity. In the event of a link failure, the remaining link can carry all the traffic with margin to spare.

Another difference between SS7 and IP is that SS7 uses relatively small messages that are efficiently packed. This choice was driven by the high cost of data circuits in the early 80s. The precursor of SS7, CCIS, ran on 2400- or 4800-bps data links. SS7 typically runs on 64-Kbps links. Because the messages are extremely terse, one such 64-Kbps link set (that is, pair of links) is capable of carrying the message traffic for over 50,000 calls per hour.

Everything about SS7 is a little different. SS7 allows a single flag byte between packets to denote both the end of the preceding packet and the beginning of the next packet. It requires the constant transmission and reception of small "fill-in" signal units (packets) —which can bombard an HDLC processor with interrupts. And, with SS7, one must constantly monitor the error rate on each to identify links that may need to be removed from service. In combination, these features generally require special-purpose hardware and/or firmware — rather different than that used with Ethernet, X.25, or traditional IP transports.

Having said all that, an SS7 network is just a packet network. But it’s a packet network that controls the guts of the PSTN and potentially provides access for new services in the traditional telephone network.

A DECADE OF TRANSITION
Since SS7 networks are critical to traditional telephone service, they are tightly controlled. For a long time, only the traditional telephone equipment vendors built systems with SS7 connectivity. Now this restriction is being relaxed, as carriers come under increasing pressure to deliver new services, and vendors from the computer telephony industry move their applications into the public network.

With growing numbers and types of service providers in the United States and elsewhere — from the major LECs (Local Exchange Carriers) and IXCs (Inter Exchange Carriers), to the wireless operators and smaller carriers —opportunities are opening up for those vendors who can connect to the SS7 network. In addition, corporate users are gaining indirect access to SS7 networks via gateway services offered by the major IXCs. As an example, a corporation with multiple, geographically dispersed call centers can improve operating efficiency by using their own computers and an SS7 gateway to specify routing for their inbound traffic on a call-by-call basis. SS7 is the interface to connect this new application logic to the traditional circuit-switched network.

Telephony backbones will move to IP within the next few years. But the more important battle will be for subscriber access — who delivers IP access and who delivers the new services that become possible with IP access. Today, the RBOCs and PTTs control the subscribers, but their competitors are increasing in number. Given their current business structures, from board of directors on down, it is a fair bet the RBOCs will lose out or be taken over by more innovative management.

During a decade or more of transition, SS7 and IN triggers are the best way to bolt new services onto the existing subscriber access dinosaur — the local COs and dial tone. Not every CTI developer is well versed in SS7. It’s complex and has its own vocabulary. But, new applications and equipment can gain access to the traditional telephone network via an SS7 gateway.

BRIDGING THE GAP
Today, a highly available SS7 gateway can be built using two PCs, connected to the SS7 network with 64-Kbps circuits carried on T1 or E1 digital telephone trunks. Such a gateway terminates one or more SS7 links — most likely redundant SS7 links — and runs the SS7 signaling protocol stacks. It exposes access to the SS7 User Parts which are the useful messages carried on an SS7 network. Most importantly, the gateway provides a simple API or protocol for other computers on an IP network. In a typical configuration, the gateway may communicate over Ethernet to connect to the multiple PC chassis of a CO voice mail system.

It is not even necessary to install a separate SS7 gateway for each CO voice mail system or each IP telephony gateway. One SS7 gateway can be used to support multiple new systems. For example, to implement an IP telephony service with multiple points of presence across New England, a single SS7 gateway could connect to Bell Atlantic. Then a private IP network could handle call control information between the gateway and the new equipment at multiple points of presence.

In one sense, SS7 and the IN are both dinosaurs, but they are so pervasive that it will be decades before they are completely replaced. In the meantime, the CTI industry and the emerging IP telephony industry can get access to one billion existing phones by leveraging SS7 signaling and the IN. There are product and service opportunities in the existing network today. The IP transition has barely begun. Look to SS7 and open telecommunications to enable a wealth of interesting business opportunities.

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.