Gigabit Ethernet standard details optical-fiber specs

May 1, 1997

Gigabit Ethernet standard details optical-fiber specs

GEORGE KOTELLY

Member companies of the Gigabit Ethernet Alliance achieved a key milestone during the recent ieee-802 Plenary Meeting of the Institute of Electrical and Electronics Engineers (ieee), when the feature set of the Gigabit Ethernet standard was finalized. In fact, no new features will be added to the standard unless a major technical flaw is discovered. Balloting on ieee-802.3z, the basic Gigabit Ethernet standard, is slated to begin this summer, and final approval is expected during the first quarter of 1998.

The second draft of the Gigabit Ethernet standard, ieee-802.3z/D2, was distributed last February for review by the full membership of the ieee 802.3 Working Group. This draft incorporates additions and refinements generated by the review of the first draft, especially in detailing transmission media specifications for optical fiber and copper wire. In addition, the new draft management sections consist of updated portions of the 802.3u Fast Ethernet standard to accommodate Gigabit Ethernet`s tenfold increase in speed.

"The various sub-task groups of ieee-802.3 made significant progress in the detailed specifications of the various physical media options for Gigabit Ethernet," says Bob Grow, vice president, industry relations at alliance member xlnt Inc. "The different [physical] types support a broad range of distances, including various options optimized for important cost/distance design points."

Adds Bruce Tolley, manager, business development, at alliance member 3com Corp., "Last November, the core proposals for the Gigabit Ethernet standard were finalized. Some key proposals dealt with the support of carrier sense multiple access/collision detection rules in half-duplex environments, the approval of carrier extension and packet bursting features, and the consideration of the three main physical layer types: short-haul distance copper, called 1000base-cx; short-wavelength fiber, called 1000base-sx; and long-wavelength fiber, called 1000base-lx."

The copper cable is meant to support the interconnection of equipment clusters; the short-wavelength multimode fiber is targeted at horizontal building cabling; and the long-wavelength singlemode fiber is anticipated to support backbone building cabling and campus interconnections. (For market forecast information, see page 98.)

Adds xlnt`s Grow, "The short-wavelength fiber is a physical-layer standard based on Fibre Channel parts but adapted to the higher speed of Gigabit Ethernet. Fibre Channel runs up to 1062.5 Mbaud, and Gigabit Ethernet runs at 1250 Mbaud--a nominal 20% increase in speed. The short-wavelength trans ceivers would run over a basic range of frequencies, typically 850 nm. They are tailored for use on multimode fiber, with the specifications referencing both 50- and 62.5-micron multimode fiber. The long-wavelength fiber operates at a nominal 1300 nm. Multimode fiber can be used, but 1300 nm is more suitable for singlemode fiber."

Comments Steve Haddock, chief technical officer at alliance member Extreme Networks, "The range of the 1000base-sx short-wavelength fiber is 770 to 860 nm. The transmission distance depends on the diameter of the optical fiber, with 62.5-micron multimode fiber specified for a distance of 300 meters maximum and 50-micron multimode fiber specified for a distance of 550 meters. Singlemode fiber is supported by the 1000base-lx long wavelength specification at 1270 to 1355 nm for distances to 3 km."

These numbers were taken from existing and approved building wiring standards. In actual installations, these distances are expected to be much longer.

The 1000base-cx specification for copper wire is based on Fibre Channel twin-axial, or TW-style, cable. It calls for a distance to 25 meters using a balanced shielded cable with two pairs of wire and an equalizer device. Because of the short distance limitation, copper wire is expected to be used mostly as jumper cables for inter- and intra- cabinet connections.

ieee-802.3 physical interfaces take advantage of the existing technology developed for the Fibre Channel standard, both to accelerate the development of the Gigabit Ethernet standard and to improve the component availability for Gigabit Ethernet product developers. Following this philosophy, the ieee-802.3z Task Force adopted the basic electrical characteristics for the Gigabit Media Independent Interface (gmii) that are compatible with most Fibre Channel serializer/deserializer chips. The gmiiwill provide a consistent electrical interface for the development of future Gigabit Ethernet physical interfaces.

Stability and interoperability

The Gigabit Ethernet standard is being structured to ensure interoperability. Alliance member companies are working to verify this by promoting cooperation among vendors and interoperability testing, and by making sure that multivendor equipment passes interoperability tests. Although the alliance is made up of competing companies, the members freely exchange information to ensure multivendor product interoperability.

Certified interoperability testing will be done by independent test laboratories. Specific plans are not available from the alliance, but there will be a multivendor product interconnect exhibit demonstration at Networld+Interop `97 this month. The exhibit will not establish full interoperability, but it will be the first step toward it.

According to Extreme Network`s Haddock, "Standards and interoperability are critical to the success of any technology. However, customers do not need to wait until the completion of both before adopting the technology. End-users are safe as long as the selected vendor stands behind its products and delivers a standards-compliant upgrade, if necessary."

Several alliance member companies are already working on Gigabit Ethernet products, and many products are expected to be ready upon approval of the standard. The standard has been stable since last November. It has therefore given vendors an opportunity to develop products that provide a high probability of adherence to the final standard.

The Gigabit Ethernet Alliance was formed to accomplish four common objectives: support Gigabit Ethernet standards, facilitate consensus on technical specifications, demonstrate product interoperability, and foster communication between suppliers and consumers of Gigabit Ethernet products.

The alliance membership, comprising prominent companies in the networking, semiconductor, and computer industries, cites that widespread support for Gigabit Ethernet technology continues to expand with the recent addition of ten new companies. According to Mike Taddei, worldwide manager of lan switching products at alliance member ibm Corp., "The total membership to date is 108, including 34 steering committee members and 74 participating members." q

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