The virtual circuit crossroad: Flexibility, scalability bring SVCs forward

Flexible, economic and efficient ways to distribute bandwidth are top priorities for service providers. For many that use ATM networks and private virtual circuits to deliver voice traffic, another technology has been suggested: switched virtual circuits. Although they enjoy only limited implementation, SVCs show value and promise.

Currently, service providers use PVCs to provision new customers through their operations support systems. A PVC is a permanent, dedicated connection between two points, whether it is in use or not. With that technology, the PVCs essentially are "nailed up," rendering them relatively inflexible and difficult to scale. Although PVCs may work well in smaller networks, challenges arise with larger networks, which makes SVCs more attractive.

In contrast, SVCs lessen the provisioning burdens and increase scalability, said Kevin Walsh, vice president of marketing for Accelerated Networks.

With SVCs, only the ATM addresses of the end points and routing tables in the network switches must be provisioned, he said. And as soon as the provisioning step is complete, the end-user devices automatically can establish SVCs when needed. Circuits are set up and then torn down after use, freeing up circuits for later.

"Service providers need a scalable solution, and SVCs do that," Walsh said.

MCI WorldCom is deploying a combination of Accelerated's and Mariposa Technology's equipment for its Smart Bandwidth on Command service, using SVCs and PVCs. At a recent demonstration of the service, Accelerated's integrated access devices were tied into MCI WorldCom's intelligent network policy server for call routing instructions using SVCs.

But Accelerated is trying to take SVCs a step further by proposing switched voice over DSL. Accelerated submitted the proposal to the DSL Forum with the collaboration of MCI WorldCom. Despite its support, MCI WorldCom doesn't appear ready to deploy that technology.

"We are testing to see what different [voice-over-DSL] boxes do, and we are working with our partner Rhythms [NetConnections] on the subject, but there are a lot of cards yet to be played out," said an MCI WorldCom spokesman.

"There is just not enough pressure from service providers for us to support SVCs now, so what's the point," said Martin Taylor, chief technology officer at CopperCom, which develops voice-over-DSL equipment. "There are almost no standard [DSL access multiplexers] to support SVCs," he said.

Service providers have concerns about ease of use with SVCs, Taylor said. "If a customer has a problem with a call, there is no connection trace information. With a PVC, you can look at statistics, know that the connection exists and do trouble shooting as well."

Security issues are also a problem with SVCs, Taylor said. "PVCs are inherently secure, whereas with SVCs, you can't determine if a person is authorized or not."

One of CopperCom's partners, AccessLan Communications, takes a similar yet slightly more extreme view of PVCs and SVCs. AccessLan believes dynamic virtual paths should be used instead of PVCs or SVCs, said Kumar Shah, vice president of marketing for AccessLan.

Although many vendors do not yet support SVCs, the sooner they offer them, the better, said Claude Romans, director for loop access with RHK. Finding ways to bypass Class 5 switches and local exchange carriers, which switched voice over DSL is said to do, are always valuable, he said.

"Anything that allows service providers to avoid handoffs is worth looking at," said Ron Westfall, senior analyst with Current Analysis. Although SVCs have not traditionally had the market attraction, service providers will be using them more in the future, he added.