Which Fiber, at What Speed, and for What Distance?

Copper technology has evolved to the point where a 100-meter maximum distance is among the golden rules of premises cabling. This rule is rooted in 10BASE-T Ethernet and the first edition of Telecommunications Industry Association/Electronic Industry Association (TIA/EIA)-568 in 1991. This distance remains a rock within the cabling industry upon which to build your network. In the early ‘90s, fiber optics was simple also in that we had 62.5-micron multimode fiber supporting local area network (LAN) applications for 2,000 meters, primarily 10BASE-F Ethernet and fiber data distributed interface (FDDI). While data rates have continued to increase, the 100-meter rule stands fast, primarily because buildings are built so that all users are within 100 meters of a telecommunications room. Therefore, 100 meters is not only a cabling specification, it is also a building design specification. Unfortunately, there have never been solid design specifications for buildings or campuses relative to backbone distances. In 1991, TIA/EIA-568 tried to establish a loose specification for what in practice would be an inside building backbone, even though it was never specifically called that. The distance was 500 meters; however, this was destined never to become a rule, because it actually came from 10BASE-5 Ethernet, a dying technology. More importantly, it was an excessive length for most buildings in North America. Over the last three to five years, there have been significant debates, studies, and surveys within both the TIA and IEEE over what the distance for inside building backbones should actually be. Fortunately, harmonization and agreement seems to be occurring, because the golden distance for inside building backbones with the new release of TIA/EIA-568-B.1 is 300 meters, not 500. Not only is 300 meters stated as being the maximum distance for the inside building backbone, but it is also the maximum distance for centralized optical fiber cabling. This is significant because optical fiber applications; more specifically, multimode optical fiber applications, are being developed with this distance as an objective. This 300-meter requirement, when combined with the 1000BASE-SX (1Gbps Ethernet), was one of the driving factors for including 50-micron fiber as a recommended media in the latest edition of TIA/EIA-568-B.1. With the addition of 50-micron fiber and the growing confusion about how far each fiber would support selected data rates and applications, the new revision contains an application support annex providing distance and channel attenuation information for each fiber. The Maximum Distance Supported is primarily based on the standards-based fiber bandwidth capacity and transceiver characteristics. For data rates up to and including 155 Mbps, the 2,000-meter campus backbone distance is supported by the more cost-effective multimode solution. However, at speeds of 622 Mbps and higher, the distance capabilities decrease. Also, 50-micron and 62.5-micron have equal distance capabilities, except for the most popular implementation of 1 Gbps Ethernet, 1000BASE-SX, estimated at over 90 percent of the 1 Gbps ports sold. Another item is also apparent with the “NST” call out used for single-mode for 10 Mbps and 100 Mbps Ethernet as well as Token Ring. This means that the IEEE standards do not address these speeds with single-mode fiber. Products can be purchased for single-mode, but they are cost-prohibitive. Single-mode fiber’s inability to provide a cost-effective, standards-based migration path from 10 Mbps to 100 Mbps to 1 Gbps for Ethernet is an important reason TIA did not recognize it for horizontal cabling applications and also kept multimode (50 or 62.5) as a recognized cable for the backbone. This is also why TIA/EIA-568-B.1 recommends that users consider using both multimode and single-mode for backbone applications, especially for campus backbones or any backbone greater than 300 meters. The Maximum Channel Attenuation highlights that for light-emitting diode (LED)-based applications, there is a difference between the 62.5-micron and 50-micron power budgets. TIA has agreed that the maximum power penalty when using 50-micron with an LED is 4.7dB. This is the worst case and it rarely occurs. The other significant point is that, while the multimode laser-based applications have a decreased allowable maximum channel attenuation, it is not to a level where there is inability to meet and exceed the requirements with standards-based connectors and cable. The TIA channel attenuation numbers reflect the IEEE agreed-upon position that the unallocated power budget can be used by the cabling system for alternate configurations, which may have more connectors than typical, but not for increased length. Presently, both TIA and IEEE are studying and discussing multimode solutions for 10 Gbps Ethernet for distances up to 300 meters. They are considering both serial lasers with higher-laser bandwidth 50-micron fiber and wave division multiplexing (WDM) with standard multimode fiber. Once the dust settles and the solution set(s) are more apparent, I will provide you with an update on this most important technology. In either case, it appears likely that, in the future, building designers will have to follow a second golden rule: 300 meters. The designer will need to ensure that the inside building backbone distance (telecommunications room to equipment room) does not exceed 300 meters. BEAM is director of systems marketing at AMP NETCONNECT Systems. He can be reached at (336) 727-5784 or tebeam@tycoelectronics.com.

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