Media Coverage

The Path Less Traveled: A New Method for Optical Ethernet Transport

By Ed Gubbins

Though the consensus of most major carriers has for some time seemed congealed around IP and multiprotocol label switching (MPLS) technologies as the basis for Layer 3 core networks, a small but growing group of vendors is trumpeting a Layer 2 technology, Provider Backbone Transport (PBT) as an alternative to MPLS for certain applications. One of those vendors, Meriton Networks, is taking things a step further, using PBT to challenge another widely accepted notion: the presumed convergence of transport and service delivery networks.

In many optical transport networks, Meriton explained, traffic is forced to make frequent pit stops along the way. Wavelength-division multiplexers (WDMs) pool together lots of traffic from a variety of different services to efficiently ship them along as gigabit Ethernet (GigE) circuits. But the distinct services within the GigE pipes of the WDM network have a range of different destinations. So the GigE pipes are occasionally pulled out of the WDM network (usually at an add/drop multiplexer, or ADM) and cabled a few feet away to an Ethernet switch or router, which takes a deeper look inside the signal to see which services need to go where. Much of that traffic then goes through another cable back to the ADM. This ducking into and out of the service delivery network--sometimes called "tromboning," or "hair-pinning"--is what Meriton hopes to reduce.

"You're using two client interfaces on two different ADMs and two client interfaces on the service delivery switch just to switch the GigE across the [central office] to move it across the network," said Ken Davison, Meriton's vice president of marketing and business development. "That's a lot of client interfaces just to do switching. You're paying for an overlay you don't want. We said, 'That doesn't make sense."

Meriton's approach is to keep the transport and service delivery parts of the network as separate as possible. Its newest products create "tunnels" of dedicated bandwidth in the transport network for traffic to follow, moving head-down through the network as services, undisturbed until it resurfaces on the other side of the tunnel. So instead of blindly pulling traffic out of the transport network in big pipes for the service delivery network to sort through and switch, the transport network would switch those service tunnels intelligently. Today's traffic enters the network as virtual local access networks (VLANs, tagged for identification as a particular service), and under Meriton's plan, those VLANs are tagged again to allow them to flow quickly through the transport tunnel without making pit stops in the service delivery network along the way.

An upcoming version of Meriton's 7200 optical switching platform, available in the second half of this year, uses PBT technology--a Layer 2 technology, like Ethernet--to provision those transport tunnels. A subsequent version of the product will support transport-MPLS, which addresses some of MPLS's complexity for transport networks.

"That really ties [Meriton's] fortunes to the willingness of carriers to adopt the [PBT] approach in the near term," said Jason Marcheck, Current Analysis analyst. "A lot, if not most, of the market is still more comfortable with MPLS at this point."

PBT has gained more attention since British Telecom announced last year it would deploy the technology in equipment from Nortel Networks and Siemens. That gear applied PBT to metro Ethernet switches. Other vendors followed, promising PBT plans of their own, including Extreme Networks and Hammerhead Systems. Earlier this year, core router vendor Avici Systems announced work on software that would use PBT to create what it calls a "virtual control plane"--in essence decoupling the network control plane from the hardware network elements to make the control plane more dynamically malleable.

As a well-entrenched supplier of MPLS technology, Cisco Systems is not only touting the benefits of that technology over PBT, it is trumpeting the charge toward convergence of optical and IP networks while Meriton is vowing to keep them segregated. Cisco has been beating the drum ever harder this year that service providers need to resign themselves to the necessity of integrating optical and IP.

"The unnecessary layers in the spaghetti need to be removed from the network," said David Ward, the architect of Cisco's CRS-1 core router, said during a panel discussion at this year's Optical Fiber Communications conference in March. Cisco introduced IP-over-dense wavelength division multiplexing (DWDM) capabilities for its CRS-1 in late 2005, naming cable operator Comcast as a customer.

Glenn Wellbrock, director of network technology development of Verizon Business, sees the inevitability of network convergence on the horizon but sees obstacles along the way as well. As for Cisco's proposition, he said, "The back-to-back portion of that, the gray interface [Cisco is] eliminating, is not the most expensive part. So it's a marginal cost savings, but it is beneficial in that it's fewer things to manage and spare."

One of the challenges Wellbrock sees in integrating optical and routing is that the leading optical equipment vendors and the leading router vendors aren't the same folks. "The switch/router guys don't necessarily own the transport gear," he said. "So then you have to operate it in what's called an alien wavelength, which means the transport system is just accepting a signal it knows nothing about necessarily. It can do power management on it, but it can't do the same type of controls it would have over its own wavelength, so the performance is a bit limited."

Another potential barrier to optical/IP integration is that those functions are typically managed by different personnel groups in big carriers. The cultural differences between them have their roots in an old tug-of-war between telecom's past and its future, sometimes described as the friction between "Bell heads" and "Net heads." To Meriton, it's one more reason to keep transport and service networks separate: it's what carriers do today. But it won't be forever, Wellbrock said.

"[Optical and routing] are managed by different teams," he said. "Will we overcome that stuff as carriers? Yeah. It's a matter of survival and being competitive. Convergence is changing a lot of the way we do business."

As for Meriton's proposal, Wellbrock isn't religious. Whether it's MPLS or PBT, Layer 2 or Layer 3, the most important feature is the bottom line, and the same questions apply. "Is it more cost effective to do it that way?" he said. "Then we're interested in doing it. But you can't charge a premium for doing it that way. It has to be a lower-cost overall solution."