Phone Outages Show Hazards of New Technology
The massive telephone failures in the Los Angeles and Washington areas on Wednesday stemmed from glitches in a sophisticated new technology that is now being installed at local telephone companies across the country.
This technology, a specialized computer network that shuttles information about calls between telephone company switching offices, forms the basis for a broad range of exotic new telephone services that are expected to become available in the next few years.
But the inherent complexity of an increasingly software-based phone system that uses this type of computer network--which was also the culprit in the collapse of American Telephone & Telegraph’s national long-distance network in January, 1990--raises the prospect that the public telephone service may be inherently less reliable in the future than it has been in the past.
“Any time you have something as sophisticated as this you’re going to have the potential for this type of problem,” said Greg Masniaeff, an analyst with Northern Business Information, a New York-based telecommunications research firm. “It could happen again.”
Wednesday’s glitches knocked out Pacific Bell’s Los Angeles-area telephone network and shut down service throughout Washington, D.C., Virginia and Maryland. Service for millions of Southern California customers was disrupted for nearly three hours, and problems arose in Los Angeles’ 911 emergency center.
Pacific Bell said Thursday that it had suspended further deployment of the computer network, known as Signaling System 7, until the exact cause of the problem could be identified. It appeared Thursday that the problems at Pacific Bell and at C&P; Telephone, which serves the District of Columbia, Maryland, and Virginia, were not identical, but both companies attributed their breakdowns to SS-7 equipment supplied by DSC Communications of Dallas.
A DSC spokesman said the company was “working diligently with our customers” to find the cause of the problem, but declined further comment.
In a traditional phone system, calls begin at a local switching office. A dial tone indicates that the office is “listening,” and the call is then sent on its way according to the number dialed. The originating office doesn’t know what will be found on the other end, but simply sends the call along for further switching or completion by another local office.
With a signaling system such as SS-7, however, the call is in effect held for an instant at the local switching office while information about the call is relayed ahead on a separate data network. The data network finds out about conditions on the other end and relays the information back to the originating switching office, telling it what to do with the call--send it along, or give a busy signal, for example.
This type of design has a number of benefits. John Seaholtz, vice president of technology planning at Bell Atlantic, parent company of C&P;, said the system was initially envisioned as a way to make the network more efficient. Because calls are never actually sent through the network if the receiving end is found to be busy or if there is no answer, there is no waste of valuable transmission capacity. Calls can also be sent through much more quickly.
In addition, the system improves security, because information about the origination and destination of the call is carried separately from the call itself. By allowing the many different switches in a network to “talk” with one another via a separate communications link, moreover, SS-7 is supposed to enable the entire network to deal with problems better by re-routing calls around trouble spots.
SS-7 also opens up a host of new service possibilities, because information about the origin of the call actually arrives at the destination before the call itself. That allows services such as the controversial Caller ID, which makes it possible to determine who is calling as the phone rings.
Other new services include selective call waiting and selective call forwarding. A customer using a touchtone phone can “program” the local switching office to route specific numbers to another destination, or to prevent certain numbers from interrupting an on-going conversation.
The flip side of all this, as Pacific Bell, AT&T; and C&P; have all discovered, is that if the SS-7 system malfunctions, it begins sending incorrect information all over the network. Ross Ireland, general manager for network services at Pacific Bell, said Wednesday’s incident was caused by a signaling system unit in downtown Los Angeles that inexplicably began sending out a flurry of wrong information about problems in the network, and ultimately shut itself down.
Then there was a cascade effect, in which the other signaling system units began acting on the incorrect information. Finally, when people tried to make calls and couldn’t, they kept trying, which created an abnormally high level of calling traffic and thus further exacerbated the problem.
Because a phone network is so tightly integrated--akin to one big computer--it’s very hard to locate and fix problems. That’s especially true of problems--such as those Wednesday--that are supposed to be precluded by the redundant design of the signaling system.
Still, phone company officials said they were not questioning the fundamental value or reliability of SS-7 systems, which are produced by several different vendors and have been in use in U.S. long-distance networks and in many European countries for several years.
