Recently, we received a phone call from a fiber optic instructor we know asking an unusual question. Can you run single-mode fiber to the desktop? Of course, we told him, why not? His confusion was based on his understanding of the TIA-568-B standard for structured cabling, which only includes single-mode fiber or multimode fiber in the backbone and multimode fiber only to the desktop. Just because the standard was written before such an application became practical—or necessary—is no reason it should not be done.
In fact, the issue came up because several of his students were installing single-mode fiber to the desktop in a customer’s new building to support Gigabit Ethernet today and 10 Gigabit Ethernet in the future. Both GbE and 10GbE have versions that use 1,310 nm lasers for transmitters with either multimode or single-mode fiber.
Using these 1,310 nm laser transmitters with FDDI-grade 62.5/125 micron fiber or 50/125 micron fiber will allow longer links than the less expensive 850 nm VCSEL lasers, but require some finesse to match the single-mode lasers to the multimode fibers. Special patchcords that offset the small single-mode core in the larger multimode fibers are often required to prevent bandwidth problems over longer distances. Using the same transmitters in single-mode fiber results in distance capabilities well beyond the scope of any premises networks.
Many users—perhaps 40 percent, according to recent industry reports—are choosing to use single-mode fiber instead of multimode fiber in backbone networks. Single-mode fiber means no bandwidth worries and lots of power margin, but brings up new issues that some contractors and installers may not be familiar with.
Cable identification is very important. Just as you must be careful to not mix 62.5/125 micron fiber with 50/125 micron fiber because the unequal core sizes cause up to 4 dB excess loss at terminations, mixing single-mode fiber with either type of multimode fiber can cause up to 20 dB excess loss.
Color coding and tagging cables can help prevent confusion. Single-mode fiber can be cabled just like multimode fiber, but premises cables and patchcords that contain only single-mode fibers are generally made with yellow cable jackets to allow easy identification. Many multimode cables use orange jackets or sometimes gray. Backbone cables with both multimode and single-mode cables, called hybrid cables, may be black or gray, requiring careful reading of the information printed on the cable jacket to identify the fibers in the cable.
Keeping patchcords properly color-coded (yellow for single-mode, orange for 62.5/125 and aqua for 50/125 micron fiber is the new industry standard) and color coding patchpanel ports is the best possible way to prevent confusion. Tags on the cable, preferably color-coded, can help also, as well as provide connection and test data.
Installing single-mode cable is not much different from multimode except single-mode cables are much more sensitive to bending losses. Bends around tight corners that would not be a problem with multimode cables can cause serious loss with single-mode, so greater care is needed during and after installation. As always, do not use cable ties to hold cables together tightly. Use loose hook-and-loop fasteners for holding cables in place instead.
Termination is the big difference. We know installers who terminated single-mode cable just like they had been doing multimode and were shocked to find their terminations had losses of 1 to 2 dB. They would be even more shocked if they measured the reflectance of the connectors, too. Reflectance is a major problem with single-mode fiber, as reflections can adversely affect the noise performance of laser transmitters and create “optical background noise” in short links that will prevent proper operation of gigabit links.
Single-mode fiber requires special polishing procedures using diamond polishing film and a wet polish, preferably with a special single-mode polishing “slurry.” While it’s no problem in a patchcord factory, it is tricky to get good performance in the field. That’s why most users specify using factory-terminated pigtails and fusion-splice them onto the fibers to get the kind of performance these high-speed links require.
Even the connectors are likely to be different. For gigabit links, the new de facto industry standard connector is the LC, replacing the SC or ST still common in multimode links. The duplex LC is practically the only connector the transceivers’ manufacturers support, so networking equipment uses LCs. Using LCs on the cable plant prevents the necessity of keeping hybrid patchcords around, and if the current cable plant has SCs or STs, LCs on the single-mode cables will also help prevent mismatching cables.
As networks get faster, contractors should be expecting to install much more single-mode fiber, so now is the time to get educated on the differences. Before the time comes for doing the first install, get trained or find someone familiar with single-mode to help your crews with the new and unfamiliar processes. EC
HAYES is a VDV writer and trainer and the president of The Fiber Optic Association. Find him at www.JimHayes.com.