Survivability has taken on a new meaning in the wake of the attack on New York's World Trade Center — especially for communications networks expected to provide vital connections following such a disaster.
For cable-system designers, network survivability has increasingly become a critical element, as technology allows such advanced services as Internet access and telephony to become part of the mix.
How hard can a cable system be hit and still provide services to most of its customers? Those questions remain largely theoretical, with a different answer for each specific system and scenario.
But for some time, cable engineers have had to factor in potential disasters — such as tornadoes, earthquakes and the so-called millenium bug — and more mundane events, like rolling blackouts, into plans to rebuild analog, one-way systems into two-way, hybrid fiber-coaxial networks.
As a result, historically nonredundant cable systems are becoming more like fiber-based telecom networks that use synchronous optical network (SONET) technology to reroute traffic around trouble spots within milliseconds of determining that there's a problem.
"Some level of priority has been placed on it already, but more along the lines of cable cuts and vandalism," said Ken Wright, a former MSO executive who is chief technology officer for cable systems vendor C-COR.net. "Natural disasters and unnatural disasters, I don't think, have been as prevalent in planning or spending.
"What happened [on Sept. 11] could have operators stand up and take note and think, 'Maybe we should plan for that while we're rebuilding our networks anyway?' "
Fortunately for Time Warner Cable — which already has rebuilt the networks for about 94 percent of homes passed — its fiber architecture in lower Manhattan was able to function when disaster struck.
Other than a lack of power and the need to clean ash and soot from air filters in the hub that serves the area, the effects on the MSO's local system were limited to the immediate area of the World Trade Center collapse, spokesman Michael Luftman said.
Proof of that survivability was underlined when Time Warner was asked to wire 1 Police Plaza for high-speed Internet access, replacing phone lines that had been cut off.
"It's because of the amount of the fiber in the system, especially in the case of Road Runner, which is built on SONET technology," Luftman said. "But the rebuilt networks use the same architecture."
Such planning goes beyond the fiber ring design to other elements, such as the interconnection of headends, to ensure multiple signal sources.
Wright noted that when his former MSO, Intermedia Partners LLC, was rebuilding in Nashville, Tenn., the system interconnected several standalone local cable systems using digital transport gear so they could be served from a single headend.
Headends that then become unnecessary in the new configuration might be left "hot" to serve as backups that are ready to go if the primary headends become unable to function.
Another, more cutting-edge strategy aimed to make cable networks more resilient is the elimination of hub buildings in favor of smaller, remote terminals distributed throughout the network.
The distributed-terminal scheme costs more in terms of equipment, but significantly less with respect to facilities, noted Scientific-Atlanta Inc. transmission sector vice president of marketing Paul Connolly. It also offers the greater redundancy of a fiber-ring setup, he said.
"Instead of having intermediate hub sites that might service 20,000 to 40,000 subscribers as a single building, we take that functionality and distribute it around a fiber ring," Connolly said. "In place are small, uncooled, unmanned cabinets" that are interconnected via fiber.
"Certainly, from a technology point of view, we can double source the data and transport it from site to site," Connolly said. "Most operators, as headends have gotten bigger and more sophisticated, don't want to have a single vulnerability point that could affect 100,000 customers."