VSR optics achieves working concept and rough consensus

Technology

By MEGHAN FULLER

The Optical Internetworking Forum's (OIF-Fremont, CA) recent passage of the very-short-reach (VSR) optical-interface specification represents the culmination of 18 months of work to substantially lower the cost of OC-192 by providing an interconnection between collocated networking equipment in the central office (CO) or points-of-presence (PoPs). Just days before the OIF vote, Cisco Systems Inc. (San Jose, CA) and Ciena Corp. (Linthicum, MD) broke new ground when they completed the first successful 10-Gbit/sec OC-192/STM-64 VSR interoperability test.

"One of the things we wanted to look at," explains Rob Redford, senior director of marketing at Cisco, "was how can we reduce the cost of connecting [optical-networking] devices that are very close together and how can we help service providers deploy this more rapidly. That's what VSR is all about."

As optical-networking devices scale in capacity, service providers must connect them with higher-speed links to prevent a bottleneck from developing at the PoPs. Currently, service providers have two viable options for scaling this link. They could use multiple OC-3, -12, or -48 ports to in crease the bandwidth between equipment, but this is costly and results in wasted ports on each device. The other alternative is to use one OC-192 port and a SONET/SDH serial interface, which is the option many of today's service providers have adopted to accommodate exponential bandwidth growth. While this option does minimize wasted ports, it has proven to be very expensive. Such interfaces are designed for wide-area networking and optimized for distances of up to 2 km, but most service providers need only 300- to 500-m links within their COs and PoPs.

"One of our customers told us that 75% of the connections they would need to make for OC-192 are all within 100 m of each other," says Redford. Service providers have found that it simply doesn't make sense, from an economic standpoint, to deploy long-haul optics to connect equipment within one central office.

Using VSR optical interfaces in lieu of the SONET serial interface should yield a 40% savings, predicts Redford, and that's his conservative estimate. "Obviously, as the technology grows and more vendors are available, that percentage could increase in the future," he adds.

VSR leverages technology developed for Gigabit Ethernet. It employs an array of 12 850-nm vertical-cavity surface-emitting lasers (VCSELs), which are all packaged on a single chip. VCSELs use less power and are less expensive than the single high-powered laser used in SONET/SDH serial interfaces, effectively giving the service provider 12 times the capacity for 1/12th the cost of packaging. From the chip, the transmission signal is then striped across 12 1.25-Gbit/sec ribbon fibers; 10 are transmission channels and two are reserved for protection and error detection. The signal is then recombined and transmitted out at 10 Gbits/sec.

"It's very ordinary technology," admits Russ Tuck, chairperson of the OIF's physical and link layer (PLL) working group, "but now it is being used to get lots of data across a few hundred meters."

The passage of the specification came on the heels of the first successful interoperability test, performed over 300 m of multimode ribbon fiber between Cisco's 12000 Series Internet Routers and Ciena's MultiWave CoreDirector intelligent optical-switching system. At press time, the test had been up and running, passing different types of traffic nonstop without errors. Redford is encouraged by its reliability, even in this first instantiation of the technology.

"Whenever you have a new technology," he says, "one of the things that's important to the new specification is 'can someone take that spec and build a product?' What we've demonstrated here is that you can."

While Cisco representatives admit they are not ready to ship a product, they are nevertheless optimistic about the OIF vote, which puts them 16 to 18 months ahead of the curve, they say. "It's a level of endorsement," says Redford. The OIF's passage of the standard should pave the way for additional interoperability tests and initial product shipments.

When asked if the standard has any significance beyond it's cost metrics, Tuck cites the new application of VCSELs. "The same technology has been used in millions and millions of CD-ROMs and Gigabit Ethernet and so forth," he says, "but to use an array of them to get high-bandwidth communication at low cost is definitely a new application."

Aaron Graham, senior manager of public relations at Ciena, believes the collaborative nature of the project is also significant. "That's been real common in the data-networking world," he says, "but it's kind of a new concept with some of the new players in the telecom world. Hopefully, with joint initiatives like this, we can continue to push that desire for open standards across the telecom network, too."

In addition to Cisco and Ciena, vendors taking part in the development of the specification include Pirelli Optical Systems, Avici Systems, Juniper Networks, Corvis Corp., Qtera Corp., Conexant Systems, Siecor, Vitesse Semiconductor Corp., US Conec Ltd., and W.L. Gore & Associates. Representatives from service providers Enron Communications, Global Crossing, and Level 3 also added input to the draft specification.

Concurrent with the passage of the VSR specification, the OIF also passed another specification, a 1,310-nm serial interface used for larger facilities or in campus environments where optical-networking devices may be farther apart. This interface is optimized for 600 m and uses singlemode fiber commonly installed in structured cabling. It is less expensive than the 2-km reach that was the norm, but more expensive than the 300-m parallel-optics option because it still requires 10-Gbit electronics and 10-Gbit edge-emitting lasers.

"The reason for having both," says Tuck, "is to give [service providers] two degrees of cost savings, depending on what [their] requirements are."

The VSR standard may have passed, but the OIF's work is far from over. It is currently working on a serial 1,310-nm interface with enough loss budget to be used with optical crossconnects.

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