By STEPHEN HARDY
The Optical Internetworking Forum (OIF) added two more implementation agreements to its roster of 10-Gbit/sec very-short-reach (VSR) interface approaches. One is a 4-fiber interface based on parallel optics and the other is a serial approach for multimode fiber. The two new implementation agreements join a pair of VSR interfaces announced this past January (see Lightwave, February 2001, page 1).
The first of the two new interfaces, dubbed OIF-VSR-03.0, "Very Short Reach OC-192 Four Fiber Interface Based on Parallel Optics" (VSR-3) uses eight of the 12 fibers in a 12-ribbon multimode-fiber cable. Four 50-micron multimode fibers carry 2.5-Gbit/sec streams in each direction, leaving four fibers in the ribbon cable unused. Vertical-cavity surface-emitting lasers (VCSELs) operating at 850-nm will provide the horsepower to support transmission distances of 300 m.
The technique is designed to enable the mapping of the OC-192 frame onto the optical link without bandwidth expansion or overwriting of the SONET overhead bytes. The OIF points out that ANSI Fibre Channel and Infiniband industrial consortium working groups are also pushing toward four-channel approaches at similar speeds, optical power, and jitter link budgets.
VSR-3 is the second OIF implementation agreement to operate using parallel optics. VSR-1 uses all 12 fibers in a 12-ribbon cable; 10 fibers transport 1.25 Gbits/sec apiece, one carries CRC error-detection codes, and the final fiber carries parity of the 10 data fibers. Unlike VSR-3, however, VSR-1 calls for 62.5-micron multimode fiber and attempts to build on technology developed for Gigabit Ethernet applications.
Meanwhile, the new OIF-VSR-04.0, "Serial Shortwave VSR OC-192 Interface for Multimode Fiber" (VSR-4), mirrors the 10-Gbit/sec 850-nm VCSEL approach now under review as part of the IEEE 802.3ae 10-Gigabit Ethernet standards process. Like VSR-3, VSR-4 also calls for 50-micron multimode fiber but supports link distances of only 85 m over "typical" fiber of that type. With 2,000-MHz-km high-bandwidth multimode fiber, the link distance increases to 300 m. In addition to the single 850-nm VCSEL at the transmit end, the standard calls for a single PIN photodiode at the receive end.
Rounding out the quartet of approaches is VSR-2, a 1,310-nm serial approach over singlemode fiber that supports links of 600 m.
The VSR interfaces seek to provide a method to link major systems within a central office or point of presence that is more cost-effective than the traditional SONET/SDH approaches now common. According to the OIF, the variety of implementation agreements reflects the organization's desire to meet the various needs of carriers as well as the maturity of different technologies.
The implementation agreements place an emphasis on the use of VCSEL technology as a means of decreasing component costs. While the 850-nm VCSELs called out in most of the VSR approaches are readily available, 1,310-nm VCSELs that would conform with VSR-2 have proven a challenge. At the Conference on Optical Fiber Communications in March, Nova Crystals Inc. (San Jose, CA) announced that it has developed a preliminary version of such a device that currently can provide 7 mW of power. Meanwhile, Cielo Communications (Broomfield, CO) demonstrated a 1,310-nm VCSEL parallel array operating at 2.5 Gbits/sec at the same show. According to Cielo product marketing engineer Mick Wilcox, a 10-Gbit/sec VCSEL would be a natural evolutionary path for the company's product.