New Multimode Fiber Spec Approved

Previously, I informed you about ongoing activity concerning 10 Gbps Ethernet and a new multimode fiber. In March 2002, TIA approved the standard for this new multimode fiber and probably by the time this article is published, IEEE will have approved the 10 Gbps Ethernet specifications.

To get to this point, two product-testing standards had to be established, plus concurrence had to be reached between transmitter and fiber manufacturers to ensure that the result was achieved. The resulting two product standards are TIA-492AAAC Detail Specification for 850-nm Laser-Optimized, 50-μm Core Diameter/125-μm Cladding Diameter Class Ia Graded-Index Multimode Optical Fibers and TIA/EIA 568-B.3-1 Addendum 1 Additional Transmission Performance Specifications for 50/125-μm Optical Fiber Cables.

TIA-492AAAC provides the detail specifications for the new 50/125-μm fiber. The most significant development is the specification of an additional bandwidth parameter called effective modal bandwidth (EMB). Previously, multimode bandwidth was only specified using an overfilled launch condition. This procedure tests bandwidth with light fully filling the fiber core and is directly applicable to the bandwidth experienced with an LED transceiver, such as with 10 and 100 Mbps Ethernet. However, the overfilled launch condition does not accurately predict the bandwidth of the fiber when using multimode lasers, VCSEL, necessary to achieve gigabit data rates. The minimum EMB of a 50/125-μm fiber when using a laser can be ensured when two parameters are specified. The first is the distribution of the laser launch power, known as encircled flux, specified by TIA FOTP 203 and the other is the differential modal delay (DMD) of the 50/125-μm fiber specified by TIA FOTP 220.

To achieve the required EMB needed to support 10Gbps for a distance of 300 meters, a compromise between the transmitter and fiber specifications had to be obtained. If the transmitter-encircled flux were allowed to be extremely loose, then the fiber DMD would be extremely difficult to achieve and result in extremely high fiber cost. Likewise, the opposite is true. These two specifications were developed in conjunction with each other to ensure the minimum EMB would be obtained by the system.

TIA did extensive studies, using fibers and lasers contributed by several different manufacturers, and performing extensive simulations. The results show that the two combined specifications will yield a minimum an EMB of 2000 MHz•km. You may hear discussions about six DMD templates with inner and outer mask specifications. The tradeoff between transmitter and fiber properties resulted in these templates. The transmitter specifications ensure that a minimum amount of light is carried in the very center of the fiber (< 4.5-μm) and that a minimum amount of light is carried in the outside edges of the fiber (> 19-μm). This resulted in the fiber DMD performance being specified both within the region that carries the most light (5-μm to 18-μm) and across the entire core.

Not to worry. All of the above is taken care of by the fiber and fiber cable manufacturer. Just as overfilled modal bandwidth did not have to be tested in the field, neither does EMB or DMD. The end-user and installer must simply specify that the 50/125-μm fiber has an EMB of 2000 MHz•km at 850μm. Easier than this, you can specify that the fiber cable meets the requirements of Addendum 1 of TIA/EIA-568-B.3. The end-user and installer must still choose the fiber type to be used in the system. For the horizontal, the choices are standard 62.5/125-μm, standard 50/125-μm and now 850nm laser-optimized 50/125-μm. For the backbone, they are these three plus single-mode optical fiber cable. The 850nm laser-optimized 50/125-μm fiber is backward-compatible and fully supports lower data rate transmissions over LED transmitters such as 10 Mbps and 100 Mbps Ethernet. However, it is optimized at 850nm to support the higher data rates of 1 Gbps and 10 Gbps using the cost-effective serial lasers (VCSEL).

A significant number of manufacturers of fiber and fiber cable have introduced and made available not only cable meeting the requirements of Addendum 1, but also patch cords and no-polish, no-epoxy connectors. In the near future, 10 Gbps Ethernet products will be introduced using VCSELs also meeting these requirements, providing the industry with a new high-bandwidth multimode system. EC

BEAM is director of systems marketing at AMP NETCONNECT Systems. He can be reached at 336.727.5784 or

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