Terminations and Connections
| For copper cabling, the main criterion is to provide an intimate, gas-tight joint between the connector contacts and the cable conductor. In reality, this seldom exists and there are two approaches to terminations in premise cabling: crimping the center conductor and using an insulation displacement contact. When doing either type of connection, it is important to use the correct size and type of wire and also the correct tool. For example, if a contact is rated for a 24-gauge solid conductor, using a stranded or smaller wire, such as a 28-gauge wire, would result in a connection that could become loose or could fail. CrimpingWhen a conductor is crimped, the contact is crushed around the center conductor. This cold-welds the contact to the center conductor. Many crimping tools are available today for almost any type of connection and cable type. You must use the proper tool for a successful crimp. Crimping tools are designed to provide the correct pressure by closing the dies a fixed amount. Using the wrong tool or die can result in either an under-crimp or an over-crimp. Under-crimping results in either a high resistance or a loose connection. Over-crimping can crush the wire or the connector so badly that it will be damaged and fail. Insulation Displacement ContactInsulation displacement contact uses a slotted beam. The wire is driven between the slotted beams. The beams are under spring tension and pierce the wire insulation and provide contact to the conductor inside. The contact can be either a flat form bar or a slotted barrel. These terminations are the most common in premise cabling applications. Modular Jacks and PlugsModular jacks and plugs have been around a long time and are familiar to everyone as the connectors that plug into telephone handsets, bases, and wall outlets. The connectors and jacks that are used in premise wiring are different. Residential wiring and equipment use four-position plugs and jacks. The ones used for premise wiring are eight-position and terminate all four pairs of the cable. In typical nomenclature, the plug is the male end and the jack is the female end. Modular jacks and plugs often are referred to as RJ connectors. The RJ comes from the term registered jack, and is specified in the USOC specification. USOC stands for Universal Service Order Code. This is a Bell Telephone specification that was developed for specific wiring connections, patterns, and applications within the telephone system. An RJ-11 is a six-position connector and an RJ-45 is commonly referred to as an eight-position connector. Each of these basic jack styles can be wired for different RJ configurations. For example, the six-position jack can be wired as an RJ-11 C, which is a one-pair jack. It can also be wired as RJ-14 C, which is a two-pair, or an RJ-25 C, which is a three-point configuration. An eight-position jack can be wired for configurations such as RJ-61 C, four-pair, and RJ-48 C. The key eight-position jack can be wired for RG-45 RAS, RJ-46 S, and RJ-47 S. The fourth modular jack style is a modified version of the six-position jack, commonly called an MMJ. It was designed by Digital Equipment Corporation, along with the modified modular plug, to eliminate the possibility of connecting DEC data equipment to voice lines and vice versa. See Figure 6.9 for an example of these types of jacks. Figure 6.9. Several types of modular jacks can be used for network cabling. Modular Plug Pair ConfigurationsIt is important that the pairing of wires in the modular plug match the pairs in the modular jack as well as the horizontal and backbone wiring. If they do not, the data being transmitted might be paired with incompatible signals. Modular cords wired to the T 568A color scheme on both ends are compatible with the 568B systems and vice versa. See Figure 6.10 for a breakdown of jack types and how they are wired. Figure 6.10. A color scheme is used to match up wires at each end of a cable when joining the cable to a modular jack. Common Outlet ConfigurationsSeveral outlet configurations were shown in Figure 6.9; however, it should be noted that the T 568A and T 568B have been adopted by the 568B.1 and 11801 standards. They are nearly identical except that pairs two and one are reversed. T 568A is the preferred scheme because it is compatible with one-or two-pair USOC schemes. Either configuration can be used for an ISDN service or high-speed data applications. Transmission categories 3, 5, 5E, and 6 are applicable only to this type of pair of grouping. As shown in Figure 6.11, USOC wiring is available for one-, two-, three-, or four-pair systems. Pair one occupies the center conductors, pair two occupies the next two contacts out, and so forth. One advantage to this scheme is that a six-position plug configured with one, two, or three pairs can be inserted into an eight-position jack and still maintain pair continuity. The disadvantage is the poor transmission performance associated with this type of pair sequence. None of these pair schemes is cabling-standard compliant. Figure 6.11. USOC wiring is available for one, two-, three-, or four-pair systems. Various other standard schemes appear in Figure 6.12. Figure 6.12. Different network cables require different wire connections to standard jacks. There are a few guidelines you should follow when using the modular jacks and plugs:
Patch PanelsPatch panels provide a means of rearranging circuits so that adding, subtracting, and changing workstations is made easier. Patch panels are where the circuits are connected and reconnected. Several patch panels use a feed-through connector set into which a cable can be plugged on both sides. Some configurations can have the horizontal cables going to the work areas plugged into one side of the panel. Typically, feed-through patch panels are not suited for high-speed operation. Category 5 and higher panels feature IDC contacts on the back and modular jacks on the front. Modular jacks are usually 110-style or barrel style. This configuration offers a better electrical performance to reduce NEXT. Fiber-optic patch panels often offer a transition between different connectors. Transition among S C, FDDI, and S T are common. There are also other ways of connecting and terminating cabling. Two of these use IDC connections. The first is the Type 66 cross connect block. This type of block has 50 rows of IDC contacts to accommodate the 50 conductors of 25-pair cable. Each row contains four contacts. Type 66 blocks represent an older style designed originally for voice circuits. Some of the newer designs meet Category 5 requirements. You should check to be sure that the block is rated for the category you're installing, because older block designs have high cross-talk, which makes them unsuitable for high-data-rate designs. A 110 cross connect consists of three parts: mounting legs, wiring block, and connecting block. The legs provide cable routing management and also hold the wire block. The wire block is composed of small plastic blocks that position the cable with index strips. Conductors are placed in the slot of the index strip. The strip usually has 50 slots to accommodate a 25-pair cable. It is marked every five pairs to help visually simplify the installation and reduce errors. This is also color-coded using the standard blue/orange/green/brown/slate color code. Wires used are punched into place with the 110-installation tool. This, however, does not terminate the conductors; it simply positions them. The device that does the termination is the connecting block. The IDC connecting block has contacts at both ends. One set of contacts terminates the contacts of the wiring block, and the other set on the outside is used for performing the cross connect. This wiring system can accommodate as many as 300 pairs. Each horizontal strip can handle 25 pairs. A 100-pair cross connect requires four index strips. A 200-pair cable requires eight index strips, and so forth. The system can be used as a prewired assembly for specific applications. One variation uses a 25-pair connector. In this situation, the block is prewired to the connector to allow a 25-pair cable from a hub or PBX to simply plug into the cross-connect. There are pros and cons to using cross-connect blocks. They offer higher densities and require less space than patch panels, and also are less expensive. On the other hand, they are the least friendly for making moves, additions, and changes to the configuration. Skill is involved in removing and rearranging cables. When using patch panels, almost anyone can rearrange the system. In both situations security, ease of attachment, expense, and physical space are all considerations. Terminating FiberWhat used to be a challenging task in the past, and is still an important task today, is terminating fiber-optic cable. There is a big difference between terminating electrical wiring and terminating a glass fiber that is only 62.5 microns in diameter. For one, electrical connections require a low resistance connection; the fiber requires a tight tolerance alignment. Misalignment in fiber connections will cause energy to be lost as light crosses a junction of the connector. There are three functions of the termination process:
Several varieties of common connectors are used in fiber optics. The following list does not include all connectors but does include those most commonly used for communication applications:
You can see examples of some of these in Figure 6.13. Figure 6.13. Several kinds of connectors are used with fiber-optic cables. Various techniques are used for installing fiber-optic connectors, but five tasks are common to any termination process:
The correct installation of fiber-optic connectors requires training, specific equipment, and consumable materials. Thankfully, manufacturers are constantly reducing the amount of time and training required to install the fiber-optic equipment they produce. Epoxy terminations have long been used to ensure that the fiber is properly held in the connection ferrule. It does have its drawbacks, however. It is an extra step in the process, it's potentially messy, and it requires curing the epoxy. Curing time can be shortened by utilizing an oven. This requires having another piece of equipment. There is also a possibility of spilling the epoxy on carpets and furniture in the finished building. The epoxyless connectors require only a crimping tool and eliminate the need for epoxy. The simplest connectors use an internal insert that is forced snugly around the fiber during crimping. Inserts clamp and position the fiber while the crimp secures the cable strength members. Another variation uses a short piece of fiber, which is factory-assembled and polished and inserted into the end of the connector ferrule. The inserted cable fiber butts up against this internal fiber, which is enclosed by an index-graded gel, and then the cable is crimped in place. There also is a connector that uses a hot melt adhesive that is preloaded into the connector. This eliminates external mixing and loose components. Next, the connector is placed into an oven for a minute or so to soften the adhesive. The prepared fiber then is inserted in the assembly and is left to dry. Finally, the prepared connector is lightly polished. The main benefit of epoxyless connectors is that they take less time and hence increase productivity. Remember that installing cables is perhaps the most expensive part of your network. Although servers and desktops computers are not cheap, the cost of labor and cabling to connect those computers to an enterprise network can be quite expensive. Fiber-Optic SplicingThe preceding section discussed connecting fiber-optic cables to terminators. This section focuses on connecting segments of the fiber-optic cable itself. The splicing techniques are used only for extending the distance of the cable segment by adding another segment to it, or for repairing a cut or a damaged fiber cable. Now, splicing techniques and products have become user-friendly and often are considered a fast, low-loss alternative to traditional connector terminations. There are two main types of fiber-optic splicing: fusion and mechanical. Fusion SplicingFusion splicing is a process in which two sections of fiber are heated and, in effect, welded together. Fuse splices typically are good connections with attenuation losses as low as 0.1 db. Mechanical strength exhibited by this splice is often as strong as the original fiber. The steps of a fusion splice include the following:
Mechanical SplicingMechanical splicing is the process in which two sections of fiber are aligned and either glued or crimped in place within a permanent hood or shell. Mechanical splices are less labor-intensive than fuse splices. In some cases the splices can be installed in less than one minute per unit. Until recently, mechanical splices were relatively high-loss connections and were used for very limited applications. In the past several years, manufacturing processes have developed very low-loss mechanical splice units. The low-loss connection attributes of today's mechanical splice units created a new technique for terminating fiber. After cleaning and polishing each installed fiber, installers have the option of purchasing fiber jumpers or tails with connectors on one end and quickly splicing these tails to the installed fiber. Fiber-Optic Patch PanelsFiber-optic panels are termination units, which are designed to provide a secure, organized chamber for housing connectors and splice units. The typical termination unit consists of the following components:
Tip It usually is a good practice to design termination units and jumper cables into fiber-optic installation because such units provide for growth and flexibility. The termination unit can use a patch panel in respect to making changes or additions to a system. It also can be a test point for troubleshooting the system. General Considerations for Fiber-Optic CablingFor the insulation of optical fiber connecting hardware, the following recommendation should apply. Connectors should be protected from physical damage and moisture. Optical fiber cable connecting hardware should incorporate high-density termination to conserve space, provide for ease of optical fiber cable, and patch cord management on installation. Optical fiber cable connecting hardware should be designed to provide flexibility for mounting on walls, racks, or other types and distribution frames, and standard and mounting hardware. You should insist that a minimum of 1 meter of two-fiber cable be accessible for termination purposes. Testing is recommended to ensure correct polarity and acceptable link performance. Clause 2 of 568B.1 provides recommended optical fiber link performance testing criteria. ConnectionsTelecommunication outlet and connector boxes should be securely mounted at planned locations. The telecommunications outlet box or connector box should provide cable management means to assure a minimum bend radius of 25mm and should have slack storage capability. The fiber types should be identified:
Small Form Factor Connectors (SFF)Figure 6.14 shows a popular connector on the market today. Figure 6.14. Small form factor (SFF) fiber-optic connectors are available today. Some advantages of SFF connectors include compact size, modular compatibility with the eight-position modular copper interface, and adaptability to high-density eight-network electronics. Qualified SFF duplex and multi-fiber mode connector designs can be used in the main cross connect, intermediate cross connect, horizontal cross connect, and consolidation points and work areas. A TIA fiber-optic connected inter-mateability standard shall describe each SFF design. This design should satisfy the requirements specified in Annex A of the 568 B-B.3 standard. Centralized optical fiber cabling provides users with flexibility in designing optical fiber cabling systems for centralized electronics typically in single tenant buildings. |
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