As speeds increase, which multimode optical fiber is the right choice for the local area network?
Paul Kolesar Lucent Technologies and Preston Buck Corning
Before the development of the Gigabit Ethernet standard, there wasn't that much discussion about what type of multimode fiber to install. Most fiber-based networks today deploy 62.5/125 Fiber Distributed Data Interface (FDDI)-grade multimode fiber in their backbones and risers and, in some cases, all the way to the workstation. However, as backbone speeds have increased, new questions have surfaced about the differences between 62.5-micron and 50-micron multimode fibers. Users want to know which fiber is better for their applications and what factors to consider when they choose the cable for their networks.
Which fiber is better? The answer depends on the parameters of the network: the applications the network will need to support over the next few years and the length of the links. It also depends on whether you are evaluating fiber for a new installation or planning to upgrade from an installed base.
The good news is that both types of multimode fiber available today offer the bandwidth to support such protocols as Ethernet, Token Ring, and FDDI over the distances specified in the application standards (see Table). Both multimode fibers have proven performance over decades of use. Physically, these fibers have the same cladding diameter and virtually identical mechanical properties. Operationally, the fibers provide similar bandwidth at 1,300 nm. The standards bodies accept both fibers, and both offer migration paths up to gigabit-level speeds. However, there are some important differences that will affect the migration paths to higher speed and distance goals.
In terms of physical properties, the difference between these two fiber types is the diameter of the core-the light-carrying region of the fiber-signified by the numeric nomenclature. In 62.5/125 fiber, for example, the core has a diameter of 62.5 microns and the cladding diameter is 125 microns.
In terms of performance, the difference lies in the fibers' bandwidth, or information-carrying capacity, and in the power-coupling efficiency to light-emitting-diode (LED) sources. Bandwidth is actually specified as a bandwidth-distance product with units of MHz-km. The bandwidth needed to support an application depends on the data rate. As the data rate goes up (MHz), the distance that rate can be transmitted (km), goes down. Thus, a higher fiber bandwidth can enable you to transmit at higher data rates or for longer distances. Therefore, 50-micron multimode fiber offers nearly three times more bandwidth (500 MHz-km) than FDDI-grade 62.5-micron fiber (160 MHz-km) at 850 nm. However, the smaller core of 50-micron fiber can cause a reduction in power budget for LED-based applications. A lower power budget reduces the number of connections permitted in a link and can reduce the sup portable distance for power-limited applications like 10-Mbit/sec Ethernet (10Base-F) and Token Ring.
So, while fiber bandwidth is a critical factor in determining link length and data rate, it is not the only one. Transmitter and receiver characteristics also play a critical role. Any statements on the distance capabilities of a particular fiber must be made in the context of the full suite of specifications for a given application. For 850-nm Gigabit Ethernet, these bandwidth values support link lengths of 220 m over 62.5-micron fiber and 550 m over 50-micron fiber.
Today, the 850-nm operating window is increasingly important, as low cost 850-nm lasers such as verti cal-cavity surface-emitting lasers (VCSELs) are becoming widely available for network applications. VCSELs offer users the ability to extend data rates at a lower cost than long-wavelength lasers. Since 50/125 multimode fiber has higher bandwidth in the 850-nm window, it can support longer distances using these lower-cost VCSELs. Thus, 50/125 is more suitable for fiber backbones running Gigabit Ethernet and higher-speed protocols over longer distances. Either fiber provides sufficient bandwidth for cable lengths up to 300 m. For many users, that includes their building backbones as well as the horizontal cabling portion of their networks. There are several new 62.5-micron fibers that provide 300-m-and even up to 500-m-guarantees for Gigabit Ethernet.
Does this mean you need to re-cable today if you have old 62.5-micron fiber in your backbone? Not necessarily. The fact that 50-micron multimode fiber performs better with VCSELs at higher speeds is not in itself sufficient reason to re-cable your current infra structure. Users with an installed base of 62.5/125-multimode fiber can still migrate to higher-speed protocols by utilizing 1,300-nm lasers, which will give them the same 550-m link length as 50-micron fiber.
If you have an installed base of FDDI-grade 62.5/125 fiber, you have an infrastructure with bandwidth capabilities to support applications up to 155 Mbits/sec over distances of 2 km. If, however, you need to exceed the standard capabilities of your infrastructure, you have several migration paths open to you. As discussed earlier, Gigabit Ethernet already supports the use of 62.5-micron multimode fiber at distances of up to 550 m using 1,300-nm lasers. While this is not the lowest-cost laser option, you should evaluate it against the costs of re-cabling all, or part of, a network.
In addition, new technologies are emerging, such as wavelength-division multiplexing (WDM), that potentially can increase the capacity of a system and extend cable lengths. WDM increases system capacity by simultaneously carrying multiple wavelengths through the same fiber. Please keep in mind that this technology is not yet supported by standards, but its technical feasibility has been demonstrated in the telephony market where it is widely used over singlemode fiber. Several companies are now developing versions of this technology for multimode-fiber systems.
Finally, you have the option of extending your current 62.5-micron plant by deploying 50-micron multimode fiber to new parts of the local area network (LAN). In these cases, the most conservative strategy is to connect these two portions of the LAN through a switch. Extensive modeling and statistical analysis have been conducted that demonstrate the compatibility of 50-micron and 62.5-micron fibers using LED-based technologies. Research also shows that in laser-based systems, the coupling loss between the two fiber types is negligible. However, the launch conditions for VCSELs are still widely variable. Until these specifications are in place, it is difficult to predict the actual insertion loss and resultant modal noise that applications might experience.
If you are deploying fiber in a new installation, you have several options. You should start by evaluating the system's data rate and distance requirements. For applications where the link lengths are as long as 550 m and gigabit-level speeds will be needed, 50/125 fiber offers a cabling infrastructure that supports both short-wavelength (SX) and long-wavelength (LX) solutions, making it a cost-effective choice with excellent upgradability.
Since 10-Gigabit Ethernet is already being discussed, it's important to note that multimode solutions are already under development, although the standard isn't scheduled to be completed until early 2002. Specifications for a "next-generation" 50-micron multimode fiber are currently under development by the cabling-standards committees of the International Organization for Standardization/International Electrotechnical Commission (ISO/IEC) and the Telecommunications Industry Association (TIA-TR 42). The current draft specifications propose a new multimode fiber that has enhanced bandwidth at 850 nm sufficient to support 10-Gbit/sec transmission using a single 850-nm laser and simple two-level encoding in links up to 300 m. This solution would enable the use of short-wavelength VCSEL technology, making it a logical extension of the lower-cost, higher-bandwidth solutions that are already driving the LAN market. The Institute of Electrical and Electronics Engineers' Gigabit Ethernet standards committee (IEEE-802.3) is currently addressing proposals using this technology and others for 10-Gbit/sec Ethernet.
Wherever you are on the network continuum-upgrading an installed base, extending fiber into the horizontal, or planning a new installation-multimode optical fiber continues to offer users the most reliable network performance and a straightforward migration path from today's protocols to tomorrow's. With technology that is both backward-and forward-compatible, network managers can build their networks with confidence, knowing that they have a technologically robust solution that will provide superior performance for all future applications.
Paul Kolesar is a distinguished member of the technical staff at Lucent Technologies' Bell Laboratories (Holmdale, NJ). Preston Buck is the marketing manager for premises cabling at Corning Inc. (Corning, NY). Kolesar and Buck co-authored this article on behalf of the TIA Fiber Optics LAN Section. Member companies include 3M/Volition, Allied Telesyn International, AMP Netconnect, Belden Wire & Cable, Berk-Tek, CommScope, Corning, LANcast, Lucent Technologies, Micro Linear, Ortronics, Panduit, the Siemon Co., Siecor, Sumitomo Electric Lightwave, and Transition Networks. Visit the Fiber Optics LAN Section's Website at www.fols.org.