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MPLS gives frame relay new extranet life

Frame relay is a great wide-area technology except for one thing: It doesn't make any sense for 21st century networking.

Let me modify that before the e-mails start flying. The graying founders of the zippy and bursty variable-length packets we call frame relay always knew the technology had one potential drawback: What happens if all the sites on the network wanted to talk to each other?

Twice the frame relay community came up with an answer that didn't pass the market test. First, there was the idea of full meshing: pre-establish a permanent virtual circuit between every pair of sites.

A little math combined with the per-circuit pricing culture of the telecom industry showed why this failed. The number of site pairs grows exponentially as the network sizes up. Did you think carriers were going to give away the extra permanent virtual circuits (PVC)?

The second idea was switched virtual circuits (SVC), basically a standby mapping of all network sites with the promise of charging only for the usage sent by any one site to another. But usage-based billing systems for SVCs have taken forever to mature, and SVCs have been adopted by few carriers.

Perhaps the real issue all along was urgency. The number of frame relay ports rose above one million on the strength of classic terminal-to-host and client/server enterprise networking. Now along comes the IP revolution with the promise of bringing the world onto the network, and companies are thinking of extending what are essentially private networks into the public sphere.

That's why the third time may be the charm for frame relay carriers. First AT&T, and now MCI WorldCom, have launched frame relay services based on Cisco's implementation of the emerging Multi-protocol Label Switching (MPLS) standard in their carrier boxes.

AT&T's IP-Enabled Frame Relay and MCI WorldCom's Business Class IP are based on the idea of requiring each network site to send only a single frame relay PVC into the carrier's ATM network core. From there, the Cisco MPLS software assigns a label to each packet that contains information on what type of closed user group - or virtual private network (VPN) - the packet belongs to. Via IP addressing, the packet can then be sent to any other site on the network.

Among the beauties of this system: Companies can create VPNs for multiple extranets - say, different classes of suppliers or customers. Pure IP VPN security measures are not needed because the network retains the more-inherent security of frame relay and ATM. Router utilization can be optimized because the carrier switch does more of the work. And users get more life out of their existing frame relay implementations.

Sure, this system may be a weird hybrid. It's not a real IP VPN, and not just anyone can dial in, because they'd still need a frame relay interface. But when have corporate networks not been characterized by hybrids? These new services may be just the ticket for a lot of users extending their networks - at least for their next three-year contract.


Rohde is a senior editor with Network World. He can be reached at

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