The electrical contractor’s role in fiber optic design/build projects is important. In the last few years, there have been many changes in optical fiber technology to accommodate the need for faster network speed for transfering increasingly large amounts of data. The old days, where every application used one multimode and/or one single-mode fiber, are over, and now, you must choose the appropriate fiber for the system from a number of different types.
Most customers are not that familiar with optical fiber systems. Many probably don’t even know the difference between multimode or single-mode fibers, and certainly, few know the difference between OM1, 2, 3 or 4 multimode fibers or the various types of single-mode fibers. If they happen to be information technology (IT) people, they are likely more familiar with computer networking issues and standards than cabling, even though many problems in networks can be traced to cabling. And while they may be used to routine upgrades in category-rated unshielded twisted-pair (UTP) cables, they generally assume that the fiber installed will last practically forever. As a result of this lack of customer knowledge, the contractor must understand the fiber type options to advise customers properly.
The one fiber that was once routinely used in premises networks, 62.5/125 micron multimode fiber, was fully capable of handling every network during its 15 years of popularity. But with the advent of gigabit Ethernet with its laser (VCSEL) transceivers, the modal bandwidth of 62.5/125 fiber (since renamed OM1 fiber) proved to be too limiting for most networks. Therefore, an earlier fiber design with higher bandwidth capability, 50/125, now called OM2 fiber, was brought back because it allowed longer gigabit Ethernet links.
It wasn’t long before fiber manufacturers engineered new versions of 50/125 with higher bandwidth, introducing first OM3 and then, recently, OM4 fiber. Now, there are four multimode fibers to choose from.
The difference between these grades of multimode fiber, other than the core size of OM1 fiber, is the bandwidth, specifically bandwidth with the 850 nm VCSEL sources now used for high-speed fiber optic links. The attenuation coefficients of all the fibers are not very different because the manufacturing processes optimize the fibers for attenuation. The design of the fiber’s core, which optimizes the fiber’s modal bandwidth, is different, since the focus of the fibers is to allow longer links at higher speeds, particularly for 1,000 Base-SX and 10 GBase-S Ethernet used in local area networks (LANs). As you can see from Table 1, the possible link lengths increase substantially with the higher bandwidth fibers, making them a logical choice for LANs that will be upgraded to faster networks in the future.
As usual, higher performance costs more, so the price of OM2, OM3 and OM4 fibers reflects the higher bandwidth specification. Oddly, OM1 fiber actually costs more than OM2 fiber, a result of manufacturing costs and lower marketplace volume.
Recently, another highly useful option was introduced for multimode fibers: bend-insensitive fibers. Bending normal fibers causes loss, making it important to carefully install cables to avoid tight bends, which can also cause long-term degradation. Patchcords were another problem for bending because they can get really crowded around patch panels. The new bend-insensitive fibers can be treated with much less care. One manufacturer likes to demonstrate its cables by attaching them to wall studs with a staple gun, but please, don’t do that in the real world! These bend-insensitive fibers make so much sense that fiber manufacturer Corning has standardized them for all OM2, OM3 and OM4 fiber cables. Check with other vendors about their offerings.
At first glance, it would seem that one should install the highest bandwidth fiber, OM4, for all premises applications, but be careful jumping to that conclusion. Many customers already have extensive OM1 networks and only need to support gigabit Ethernet for desktops and wireless access points. Expanding such a network may not require a higher bandwidth fiber, and the customer may want to avoid mixing 62.5/125 and 50/125 fiber in their network, as that is a potential cause of high connection loss if the different sized fibers are mixed up. Color coding can help keep them separated, but not every user is careful about keeping cables and patchcords color-coded and marked appropriately.
Also, some applications may work better on OM1 fiber. The larger 62.5 micron core size and higher numerical aperture (light acceptance angle) of OM1 fiber is more efficient at coupling to light-emitting diode (LED) sources, which are used in many low-speed and closed-circuit television video links. The additional light captured by the fiber translates into longer links when attenuation, not bandwidth, is the major concern. Therefore, OM1 fiber may work better for many building automation and security systems, especially large facilities, such as campuses or airports.
With the exception of a few applications where links are short, such as campus networks, local security cameras and utility substations, most outside plant links use single-mode fibers. Because one type of single-mode fiber has been most used since the beginning, many contractors are not aware other types exist, which is understandable, as the cables’ applications are very specialized. The issue is made even more confusing because three international standards groups have different designations for the same fibers. Table 2 lists the standard designations for these fibers from TIA (U.S.), IEC (worldwide components standards) and ITU (international telecommunications standards).
The ITU designation is most widely used for single-mode fibers, and G.652 is the most widely used single-mode fiber. Here is a summary of what is different among the types.
• ITU G.652 is a single-mode nondispersion-shifted fiber optimized in the 1,310 nm wavelength range. This fiber is in most of the cable that has been installed to date. A variation of G.652, called “low water peak fiber,” has been specifically processed to reduce the water peak at 1,400 nm to allow use in that range by wavelength division multiplexing systems. There are four additional subcategories (A, B, C and D) that cover grades depending on the attenuation, bend sensitivity and dispersion.
• ITU G.653 covers single-mode dispersion-shifted optical fiber. Dispersion and attenuation are minimized in the 1,550-nm wavelength range, so longer distance links are possible. Use this fiber for long links using only one transmission wavelength.
• ITU G.654 has the zero-dispersion wavelength around 1,300 nm wavelength, which is cut-off shifted and loss-minimized at a wavelength around 1,550 nm. G.654 is optimized for use with dense wavelength division multiplexing in the 1,500–1,600 nm region.
• ITUG.655 is a nonzero dispersion-shifted fiber, which supports high-power signals and longer distances, as well as closely spaced dense WDM (DWDM) channels at rates of 10 gigabits per second or higher. This fiber is for long-distance cable runs, such as transoceanic cables.
For most applications—such as campus networks with a single-mode backbone or when hybrid backbone cables with both multimode and single-mode fibers are installed—G.652 or OS1 fiber is the single-mode fiber of choice. If the fiber is going into a metropolitan link where wavelength division multiplexing may be used in the future, a low water peak fiber (OS2) might be called for. And for a utility running long links, as part of a smart grid project, for example, the lower loss at 1,550 nm of G.653 fiber could affect the choice.
As with multimode fiber, there is a new category of bend-insensitive fiber primarily designed for running cables in buildings and patchcords. Bend-insensitive single-mode fiber is particularly good for fiber to the home in multiple dwelling units and passive optical LANs (POLs), since running cables inside buildings sometimes requires bending tightly around corners (see Electrical Contractor, February 2010, or the archives at www.ecmag.com, for more on POLs ).
Making the right choices
For the new project, one of the first steps is to choose the fiber appropriate for the network. The choice will be determined by the link length, required bandwidth and the transmission equipment. We have already discussed the length and bandwidth issues, but what does the transmission equipment have to do with the choice?
Some equipment, such as that used for traditional telephone networks, Internet routers or fiber to the home and POL systems, is designed for use only with single-mode fibers. Data network equipment, such as Ethernet LANs or data center links using Fibre Channel, are usually able to operate on either multimode or single-mode fiber. Building automation, security and surveillance camera systems may only be able to run on multimode fiber or even copper cabling.
If the fiber necessary to meet the needs of the project and the equipment are incompatible, it is still possible to use them together. Media converters are available for just about any application, designed to the needs of the data center, and have inputs and outputs to convert between single-mode and multimode fiber and, when necessary, UTP or coax copper cabling.
When in doubt, or even before finalizing the specifications for the project, check with several fiber and cable manufacturers for their recommendations. Their applications’ personnel have probably seen every type of installation and will be able to share their experience and knowledge to help you make the best choice.
HAYES is a VDV writer and educator and the president of The Fiber Optic Association. Find him at www.jimhayes.com.