Tài liệu Maintaining fiber-optic cable at the demarc doc

T o the traveler, the border crossing is invisible: no armed guards, snarling guard dogs, passport checks, or luggage searches. To the signals crossing this border, it might as well not even exist; but to your network man- agers and technicians, it’s a critical crossing. The border is the demarcation point, or “demarc,” between your fiber network and that of your ser- vice provider. On the outside, the service provider handles maintenance; on the inside, the responsibility is all yours. Every installation involves tough choices. Users need service, nobody wants to wait, and cost is a constant issue. Faced with the compet- ing demands for service, speed, and savings, you may be tempted to do the job “well enough for now,” with the rationalization that it can be fixed later. If, as a network builder, you find yourself asking, “Why not?” The answer is simple: “Y2K.” The impending Y2K nightmare is a classic result of short- term thinking. Programmers came up with what they consid- ered a safe, simple fix to a relatively small problem. They as- sumed that the fix would be replaced long before it, in turn, became an issue in its own right. In fact, the “fix” stayed in place and became a ticking time bomb that will likely cost bil- lions of dollars before it is resolved. In much the same way, we are now in the early days of fiber networking. It may seem that we are installing enormous amounts of capacity, but experience tells us that demand al- ways grows faster than expected. Chances are excellent that over relatively few years, • The bandwidth carried by existing fiber will increase more than expected. • The amount of fiber cable in fiber troughs will grow faster than we assume. • Much more equipment will be needed to support fiber infra- structure. • The density of fiber on that equipment will increase. • The complexity of fiber management will grow geometrically. It is impossible to accurately anticipate future needs, but the responsible approach is to be conservative without skimping on details that could jeopardize network performance. Making an entrance: the fiber demarc There are two basic styles of fiber demarc: those at which inside and outside fibers are spliced together and those at which connectors join them. Which type you use depends on your internal architecture. If there is a main crossconnect (MC) relatively close to the demarc, you might prefer splicing incoming fiber at the entrance cabinet located there. Moves and changes can be accomplished at the MC, as can rerouting around failed fibers. But if your architecture brings fiber to multiple locations (telecommunications closets, for example) directly from the demarc, the situation is very different. If a splice-based en- trance cabinet were used, many moves, changes, or reroutings could be accomplished only by resplicing. In this case, a con- nector-based fiber entrance terminal would be far more prefer- able. The demarc’s crossconnect capability allows connection around internal or external failures, taking the place of the oth- er architecture’s MC. In both cases, the demarc should do more than merely ter- minate fiber; it should also protect it from damage and prevent excessive bending. In addition, it should provide protected storage for excess cable to simplify access and prevent damage. A wall-mounted unit saves space and provides flexibility in in- Maintaining fiber-optic cable at the demarc installation Who does what at the junction of the public and private networks? Reprinted from the April 1999 edition of CABLING INSTALLATION & MAINTENANCE Copyright 1999 by PennWell The network architecture influences whether you should splice or connectorize fiber at the demarc. In networks with a main crossconnect near the demarc, splicing is recommended (a). If the fiber goes directly from the demarc to telecommunications closets, it is best to use connectors at the entrance facility (b). IC a) TC IC TC MC PBXDemarc b) TC PBXDemarc Network Architectures TC IC - Intermediate crossconnect MC - Main crossconnect PBX - Private branch exchange TC - Telecommunications closet Joel K. Matthews / ADC Telecommunications stallation. If the location is not secure, a lockable demarc is preferable. Finally, it should provide features for clear routing and labeling to help manage growth and change. Yet another kind of demarc is a terminal, or cabinet, that accommodates both splicing and connectorization. The joy of splicing By itself, splicing is relatively simple, reliable, and perma- nent. In reality, however, splicing tends to take place in dif- ficult places and times, making the process much more complicated. First of all, it can take place in con- fined spaces with relatively little room to work. Sec- ond, it tends to take place where there are lots of oth- er fibers, which can be dis- turbed in the process. Third, it can take place un- der time pressure, either in the course of a multifiber installation or to remedy a failure. There are several ways to simplify and speed splicing. • Don’t use splices if you don’t have to. You can’t remedy to- day’s splice problems this way but you can prevent tomor- row’s. You may not be planning to grow, change, or repair, but Murphy’s Law suggests that you should. • If you must splice, use a system that is splicing-friendly, pro- viding easy access, splice trays, work space, fiber protection, lighting, and room to grow. • Give yourself a break. Leave plenty of slack for convenient handling. Lay out your fibers so that repairing one won’t damage others. • Allow enough time to do the job carefully. Rushing through a job increases the likelihood of damage now and sets you up for problems the next time you have to access the same splices. Typically, splicing personnel (splicers) are skilled above and beyond the normal premises technicians. Splicers are often sub- contracted out at considerable cost. Making connections: frames and panels Both during and after installation, most of your contact with the fiber will be at the frames and panels. Depending on the ar- chitecture you choose, these will be in some hierarchy of MC, intermediate crossconnects, and telecommunications closets (TCs). These are all critical places for your fiber. • Connectorization allows greater flexibility than splicing, but just as a cut or scrape provides the opportunity for infection, a connector provides an opening for dust or dirt, which can disrupt data-signal flow. • Complex routings, sharp turns, and untended slack provide ample opportunity for bend-radius violations, which can cause attenuation or even breaks in the fiber. • Frequent handling affects both the fiber being handled and those around it. • Poor fiber management can increase the amount of han- dling, resulting in slow in- stallation or repair and in- creasing the likelihood of damage. All of these problems can be alleviated with plan- ning. The first step is to es- tablish fiber-specific proce- dures. Once procedures are established, train your staff; many of the over- sights that lead to prob- lems will disappear. Standardizing on equip- ment helps to standardize procedures. A single sys- tem throughout the facili- ty will simplify inventory and shorten the learning process. Such a system should be modular and inclusive and should provide a range of options such as crossconnect or intercon- nect. It should cover all levels of facilities, from demarc to fiber frame to TC to the desktop. An integrated system also elimi- nates the risk of damage to fiber in transit from one piece of equipment to another. A system should be completely free of sharp corners and pro- vide physical management for cable from end-to-end. Conve- nience features like front and rear access speed up the installa- tion process, reduce the chances of damage to fiber, and simplify connections. Attention to detail: connectors Properly selected and used, connectors can provide nearly per- fect signal transmission, along with the convenience of fast, simple reconfiguration. To deliver high-speed, high-band- width service, however, they must provide a near-perfect in- terface. A small gap or misalignment can cause attenuation or total loss of the signal. On the other hand, if the connectors press the fiber end-surfaces together with too much force, the glass can be crushed. Environmental variations are another consideration when selecting a connector and connector manufacturer. Connec- tors must be carefully designed and manufactured to avoid these problems, initially as well as over time and environ- A secure fiber entrance terminal provides for both splicing and crossconnect capability. mental variation. • Fiber ends must be precisely shaped and polished to provide proper mating of the surfaces and avoid pits and scratches. • Epoxies that hold the fiber in place must withstand extremes of temperature and humidity. If they do not, the fiber can “piston” or withdraw, leading to fiber damage or poor physi- cal contact at the connector. • In the event of failure, the ability to identify the manufactur- er’s lot number can help identify other potential problems before they occur. • To help eliminate any potential problems, components must be carefully tested using the best available equipment, which should be recalibrated frequently. Installation and handling also affect the performance of con- nectors. Cleaning is critical and should be part of every installa- tion or move. Careful handling of the installed connector as well as adjacent fibers and connectors can improve contact of the mating surfaces and prevent unnecessary fiber damage. Going ’round the bend The cabling installer who looks at a fiber and asks, “What could go wrong?” is not a pessimist, but a realist. First of all, fiber is made of glass. Sec- ond, it can be subjected to some very trying conditions. Third, a fault can cause seri- ous problems and be difficult to isolate and repair. A leading cause of problems is the vi- olation of fiber’s bend radius. Many of these problems occur long after installation but could probably have been foreseen and prevented. Here are three examples. 1) A fiber is run loosely around a sharp, sheet-metal bend. The natural stiffness of the fiber keeps the turn gradual and larg- er than the minimum bend radius. Later, the addition of other fibers adds bulk or weight, pressing the original fiber against the sharp edge and into a tight bend, causing attenu- ation or breakage. The only reliable preventive measure is the absolute avoidance of sharp edges throughout the sys- tem. 2) Connectorized fibers plugged straight into a fiber frame or panel at a 90 o angle are pulled sideways across the panel to allow visibility and access. The bend just behind the connec- tor may not violate minimum radius, but a hard pull on the fiber or careless handling can eventually force it into a tight right-angle bend. This design could cause signal attenuation or contribute to jumper failure. A simple solution to this problem is to make use of angled retainers, which reduce the sideways pull and improve both visibility and access. 3) Unmanaged slack in cables or jumpers allows them to form loops. Jostling of these loops can easily cause kinks that vio- late minimum bend radius. The solution is to take up all slack, even over short distances, with systems specifically de- signed to control bending. Fiber trough systems Continuous fiber running in fiber trough systems faces a whole different set of problems. While it is not “open” as it is at con- nection points, it can be more exposed, traveling through pub- lic areas of the facility. And while a fiber may go for years with- out being touched, it is also subject to accumulated pressure as newer cables are added to the run, increasing both weight and physical pressure that can lead to unanticipated problems. For example, within a fiber trough system, fibers may run horizontally across an open “downspout” without any prob- lem, until the weight of accumulated fiber starts pressing down over the opening. The bottom fibers will then be pressed against the angled edge between the horizontal and vertical runs and eventually bend. The best trough systems provide rounded corners and baffles that separate nonexiting fibers from those making the turn. Systems should also provide the widest range of options: size, joints, and materials. Even color may be an issue, for both aesthetic and management reasons. Some less obvious issues may also be important. For example, a system that requires fewer supports may cost more to buy but less to install. Snap- together systems can simplify installation, but only if they do not sacrifice strength or require additional support that eats up the time and cost savings. In-house network builders face the classic dilemma: Fight al- ligators or drain the swamp. The bad news is that even if you can keep up, the system itself will betray you as higher data rates make greater demands than existing systems can meet. The good news is that better equipment costs only a little more and can save a lot in installation and ongoing support costs. The same ar- gument can be made for judicious overbuilding. The better news is that the payback begins almost immediately. Joel K. Matthews is product manager, fiber-optic products, at ADC Telecommunications (Minneapolis, MN). For more information, visit www.adc.com. Fiber trough systems such as this one in a small telecommun- ications closet should be adaptable to different-size spaces. C MI . can cause attenuation or total loss of the signal. On the other hand, if the connectors press the fiber end-surfaces together with too much force, the glass. demarc installation Who does what at the junction of the public and private networks? Reprinted from the April 1999 edition of CABLING INSTALLATION &