CCNA 1 and 2 Companion Guide, Revised (Cisco Networking Academy Program) part 98 pdf

Installation Process 939 The Trimout Phase In the rough-in phase of cable installation, excess cable was left at both ends of the cable run. These coils of cable, which are used to take up slack and facilitate later changes, are known as service loops. Service loops are discouraged by EIA/TIA stan- dards. It is not uncommon to have 1m (3 ft.) of ends hanging out of a wall jack at the finish of the rough-in stage. In the telecommunications room (TR), where hundreds of cables are terminated, it is not uncommon to have 2m to 3m (6 ft. to 10 ft.) of ends. Although this practice appears to be wasteful, experienced installers know that an excess of cable provides more flexibility in cable routing and provides greater access to cables when toning (testing) individual cables. A common mistake of new installers is to cut the cable short. Remember, excess can always be cut off, but a short cable can- not be extended. If a cable is too short, the only alternative is to pull another cable, and this is a costly alternative in terms of labor and time. If there is 1m (3 ft.) of cable coming out of the wall at the jack location, it is best to cut this back to about 25 cm (9.8 in.). A new label should be applied to the cable about 15 cm (5.9 in.) from the end. The jacket then should be stripped back about 5 cm to 7 cm (2 in. to 2.8 in.), exposing the individual twisted pairs. The completed jack termination should have no more that 1.5 cm (.6 in.) of unjacketed conductor exposed and no more than 1.5 cm (.6 in.) of untwist in the cable pairs. Excess conductor length should be cut off at the final termination (see Figure A-40). Figure A-40 Cutting Cable to Length Lab Activity Terminating Category 5e to a 110-Block In this lab, you learn how to terminate Category 5e cable to a 110-type termi- nation block, as well as how to properly use a 110 punchdown tool and a 110 multipunch tool. appen_a.fm Page 939 Tuesday, May 20, 2003 4:54 PM 940 Appendix A: Structured Cabling The jack is terminated with approximately 15 cm to 20 cm (6 in. to 8 in.) of cable still protruding from the wall. This excess cable is coiled carefully into the wall or wall box when the jack is installed. This excess cable can be used to reterminate the jack at a later date or enable the removal of the faceplate and the addition of another jack to the outlet. At workstation terminations, it is common for the wires in the jack to lose con- tact with the pins because the patch cord to the work area often is pulled, kicked, or stretched by the workstation users. Terminate or Punchdown The termination of communications cables at a TR is referred to as punching down. Cables also are punched down on termination panels mounted on wall fields and at the rear of cross-connect panels. Wires are inserted into the appropriate locations on termination panels, and then the punchdown tool is placed over the wires. Depending on the type of termination hard- ware used, replaceable blades in the termination tool can changed out to accommodate the termination type (see Figure A-41). Figure A-41 Removable Termination Blade As pressure is exerted on the tool, spring tension increases to a point at which a firing- pin type mechanism releases the energy stored in the spring. The wire instantly is forced between two insulation-displacement connections, and excess wire is cut off in the same operation. The connection is referred to as insulation displacement because the insula- tion is pushed out of the way by the contacting points on the terminal. Insulation-displacement connections provide a secure, gas-tight connection, which means that the actual connection is not exposed to the atmosphere because the dis- placed insulation presses tightly against the block. This is necessary to provide long-term corrosion-free connections. Patch panels typically are used for data networks, as are 110-blocks, which also are used for voice applications. appen_a.fm Page 940 Tuesday, May 20, 2003 4:54 PM Installation Process 941 Wire Management Some termination systems come with a wire-management scheme built in. 110-blocks use plastic troughs and spacers between blocks. Troughs can be used both horizontally and vertically. Rack-mount installations incorporate a variety of wire-management features (see Figure A-42). Some use a combination of D-rings and troughs. Figure A-42 Panduit Wire Management When purchasing cable-management systems, consider the following: ■ The system should protect the cable from pinching and should maintain the max- imum bend radius. ■ The system is scalable, so when more cables are needed, it can handle them. ■ The system is flexible, so cables can come into it from all directions. ■ The system offers a smooth transition to horizontal pathways so that cable is not damaged or exceeds maximum bend radius. ■ The system is durable, so it will last as long as the cables and equipment mounted on it. Careful Labeling Labeling is another important part of a structured cabling system. If cables are not labeled clearly on both ends, there can be confusion. TIA/EIA-606 specifies that each hardware termination unit must have some kind of unique identifier. This identifier must be marked on each termination hardware unit or on its label. When identifiers are used at the work area, station terminations must have a label on the faceplate, the housing, or the connector itself. Most Requests For Proposals and specifications require that labels be computer generated so that they are permanent, legible, and more pro- fessional in appearance. appen_a.fm Page 941 Tuesday, May 20, 2003 4:54 PM 942 Appendix A: Structured Cabling Use labels that will remain understandable to someone who might work on the system many years in the future. Many network administrators incorporate room numbers in the label information. They assign letters to each cable that leads to a room. Some label- ing systems, particularly those in very large networks, also incorporate color-coding. To ensure that the labels do not rub off or get cut off (the end) later, mark the cable several times, approximately 60 cm (23.6 in.) apart, at the free end. After the cable is run, repeat the procedure at the box or spool end. To keep all cables tied securely together, use electrical tape. Bind the cable ends with the end of a pull string. Ensure that the pull string does not come loose by tying some half-hitch knots around the cables with the pull string before taping the ends. Do not skimp on the tape. If the string or cables pull out later, it could cost time and money. After pulling the cable along the selected route, bring it into the TR. Allow enough cable for the ends to reach all the way to each jack location, plus enough excess or slack to reach the floor and extend another 60 cm to 90 cm (23.6 in. to 35.4 in.). Go back to the spools of cable at the central point or TR. Use the labels on each spool as a reference, and then mark each cable with the appropriate room number and letter. Do not cut the cables unless they have a label. If each of these steps is followed, the networking media used for the horizontal cabling run should be labeled at both ends. Finish Phase The finish phase is the point at which installers test and, in some cases, certify their work. Testing makes certain that all the wires route to their appointed destinations. Certification is a statement of the quality of the wiring and connection. Important aspects of the finish phase include these: ■ Cable testing ■ Time domain reflectometer (TDR) ■ Cable certification and documentation ■ Cutting over Diagnostic tools are important in determining existing and potential problems or flaws in a network cabling installation. Cable Testing Cable testers are used to test cables for opens, shorts, split pairs, and other wiring prob- lems. After the cable installer has terminated a cable, the cable should be plugged into the cable tester to verify that the termination was done correctly. If a wire accidentally appen_a.fm Page 942 Tuesday, May 20, 2003 4:54 PM Finish Phase 943 was mapped to the wrong pin, the cable tester will indicate the wiring mistake. Similarly, it can test for problems with the cable, such as shorts or opens. A cable tester should be a part of every cable installer’s toolbox. After the cable has been tested for continuity using these cable testers, the cables can be certified by using certification meters. Testing is the most important step in the finish phase of cable installation. Testing veri- fies that all wires are working so that the customer does not find that there are problems later. It is better to catch a problem before it becomes a major issue. Tests relating to cable function are found in TIA/EIA-568-B.1. Common things to test for include the following (see Figure A-43): ■ Opens—Wires in cables fail to make a continuous path from end to end. This is usually the result or improper termination or breakage. Occasionally it is because of faulty cable. ■ Shorts—Wires in cables touch each other, shorting the circuit. ■ Split pairs—Wires are mixed among pairs. ■ Wire-mapping errors—Wires in a multipair cable do not terminate at the appro- priate contacts in the connector at the far end. Figure A-43 Wiring Faults Caused by Improper Termination 1 2 3 6 5 4 7 8 1 2 3 6 5 4 7 8 1 2 3 6 5 4 7 8 1 2 3 6 5 4 7 8 1 2 3 6 5 4 7 8 1 2 3 6 5 4 7 8 Split 1 2 3 6 5 4 7 8 1 2 3 6 5 4 7 8 Wire-Mapping Errors Correct Reversed Pair G/W G/W G/W G/W O/W O/W O/W O/W B/W B/W B/W B/W B/W = Brown/White O/W = Orange/White G/W = Green/White appen_a.fm Page 943 Wednesday, May 21, 2003 8:43 AM 944 Appendix A: Structured Cabling In most cases, simple functional testing for opens, shorts, split pairs, and wire-mapping errors are done from one end of the cable only. Testing for Shorts A short is formed when the two wires in a pair touch each other, providing an undesired shortcut in the flow of signal (see Figure A-44). This shortcut is a completion of the circuit before the voltage reaches the intended target. Figure A-44 Wire Short To determine whether there is a short, measure the continuity or resistance between the wires. No continuity should be measured between them, and there should be an infinite amount of resistance between them. Make these measurements with an ohm- meter using a low-resistance scale. If a higher-resistance scale is used, the installer runs the risk of inadvertently measuring the installer’s own body resistance when the wires are held to the probes. Some installers find it useful to create a small test fixture to avoid this problem. Many test probes can be fitted with slip-on alligator clips. They can hold one of the wires that so both leads are not touched at the same time. Testing for Reversals A reversal occurs when the tip (or ring) side of a pair is terminated on the ring (or tip) position at the opposite end of the wire (see Figure A-45). Figure A-45 Reversal appen_a.fm Page 944 Tuesday, May 20, 2003 4:54 PM Finish Phase 945 To repair a reversed pair in a cable, the RJ-45 connector must be removed and the cable end with the pair reversal must be terminated again. Testing for Split Pairs Split pairs happen when wires are mixed among pairs (see Figure A-46). One way to test for splits is with an ohmmeter. First, test the pairs for shorts. If none is found, place a short across each pair. When it is tested with an ohmmeter, finding a short is the anticipated result. If an open is found, something is wrong. The pair is either split or open. A tone generator then can be used to determine which is the case. Higher-end testers detect split pairs by measuring crosstalk between pairs. Figure A-46 Split Pairs A simple cable tester can be used to check for split pairs as well. This type of tester uses LEDs that immediately notify the installer if there is a problem with polarity or continuity. To repair a split, one or both of the connectors must be removed and the cable end must be terminated again. Time Domain Reflectometer A time domain reflectometer (TDR) works by sending a pulse down the wire and then monitoring the electronic echoes that occur on the cable because of cable problems. TDRs determine whether there is a cable fault and, if so, whether it is an open or short; they also determine the distance from the meter to the fault. The signal is reflected back when it reaches the end of the cable, as well as anytime it encounters a defect in the cable along the way. The speed at which the signal travels is known as the nominal velocity of propagation. This is a known quantity for different cable types. When set, appen_a.fm Page 945 Tuesday, May 20, 2003 4:54 PM 946 Appendix A: Structured Cabling the tester knows how fast the signal travels and can measure the length of the cable by measuring the amount of time that it takes for the signal to be sent and reflected back. A TDR readout typically is calibrated in feet or meters. This is an extremely efficient means of locating cable problems, although the instrument must be adjusted properly and used with skill. Cable Certification and Documentation Testing is not the same as certification. Testing is for functionality—that is, it determines whether the wire can carry the signal from end to end. Certification, or performance testing, is a statement about cable performance. It answers these questions: How well does the signal travel down the cable? Is the signal free from interference? Is the signal of adequate strength at the other end of the cable? Certification Tester Certification goes beyond functionality testing. Performance testing also must be done. Structured cabling systems that adhere to installation standards are required to be certi- fied. Certification meters perform all of the required performance tests to adhere to the ANSI/TIA/EIA-568-B standards (see Figure A-47). Meters have an autotest function, so all required tests are performed with the touch of a single button. These tests include near-end crosstalk (NEXT), wire map, impedance, length, DC loop resistance, propaga- tion delay, return loss, delay skew, attenuation, and attenuation-to-crosstalk ratio. These meters hold multiple test results in memory. Test results are downloaded to a computer so that a test report can be generated and presented to the customer. In addition to cer- tification, these meters include diagnostic features that not only identify problems, but also actually show how far these problems are from the end of the cable being tested. Figure A-47 Fluke Networks 4000 Cable Certification Meter appen_a.fm Page 946 Tuesday, May 20, 2003 4:54 PM Finish Phase 947 Performance testing usually takes place at a designated test frequency. The frequency is selected to exercise the cable at a speed that will be part of its intended operation. For example, Category 5e cable is tested at 100 MHz, and Category 6 cable is tested at 250 MHz. Performance testing is described in various addenda to TIA/EIA-568-B. Modern testing hardware and software can provide both text and graphic output. This allows ready comparisons as well as analysis at a glance. The cable certification process forms a baseline measurement for the cabling system. When the contract is established, the certification standard to which the resulting job must conform usually is included as part of the contract. The installation must meet or exceed the specifications for the wire grade that is being used. Detailed documentation showing that the cabling has reached these standards is submitted to the customer. The certification procedure is an important step in completing a cabling job. It enables the installer to say unequivocally that at a certain day and time, the cables performed to certain specifications. Any later change in cable performance must be attributable to some cause, and it will be easier to figure out what that cause is if there is hard, fast evidence of the cables’ condition at an earlier point. Different grades of cable require different minimum test results to be acceptable. Generally, the higher the cable cate- gory is, the tighter the manufacturing tolerances are, the higher the quality is, and the better the performance is. Certification Tests To pass certification, cables must meet the minimum test results for their grade. Cables must meet or exceed these specifications. Actual test results that outperform the mini- mum often are encountered. The difference between the actual test results and the minimum test results is known as headroom. If the results show lots of headroom, less cable maintenance should be needed in the future, and the network should be more tolerant of poor-grade patch cords and equipment cables. The commonly used specifications include these: ■ Specified frequency range—Each cable is tested at a frequency range that it is most likely to be used in daily service. The higher the grade is, the higher this range is. ■ Attenuation—The amount of signal that a cable will absorb is a measure of its attenuation. The lower the attenuation is, the more perfect the conductors are and the higher quality the cable is. ■ Near-end crosstalk (NEXT)—Near-end crosstalk occurs when signals from one pair interfere with another at the near end of the cable. Crosstalk can affect the capability of the cable to carry data. The amount of NEXT that a cable must be capable of tolerating is specified for each grade. appen_a.fm Page 947 Tuesday, May 20, 2003 4:54 PM 948 Appendix A: Structured Cabling ■ Power Sum NEXT—In cables in which all the conductors are used (such as Giga- bit Ethernet), the signals on one cable interfere with several pairs, not just one. Calculating the effect of these disturbances requires that the interactions of all pairs in the cable be taken into account. The power sum NEXT equation mea- surement does this. ■ ACR—The attenuation-to-crosstalk ratio (ACR) is an indication of how much stronger the received signal is when compared to the NEXT or noise on the same cable. Sometimes this measurement is referred to as the signal-to-noise ratio (SNR). Be aware that SNR takes into account external interference as well. ■ Power sum ACR—When all of the pairs in a cable are used, the interaction between the pairs becomes more complicated. This is because more wires are involved, meaning that there are more mutual interactions. The power sum equations help take this greater mutual disturbance into account. ■ Equal-level far-end crosstalk (ELFEXT)—Equal-level far-end crosstalk is a calcu- lated measurement of the amount of crosstalk that occurs at the far end of the wire. If this characteristic is high, it means that the cable is not carrying the sig- nals well and that the ACR (signal-to-noise) ratio is not well controlled. ■ Power sum ELFEXT—As with the other power sum measurements, interaction among multiple pairs in the same cable increase the complexity of equal-level far- end crosstalk characteristics. The power-sum version of the measurements takes this into account. ■ Return loss—Some of the signal traveling down a wire bounces off imperfections such as impedance mismatches in the wire. It can be reflected back toward the sender and can form a source of interference. This is called return loss. ■ Propagation delay—The electrical properties of the cable can affect the speed at which signals travel through it. The value of this delay must be known to perform certain measurements, such as time domain reflectometry. Propagation delay for cable usually is specified as a maximum allowable amount of delay, in nanoseconds. ■ Delay skew—Because each pair in a cable has a different number of twists, signals that enter the cable at the same time are bound to be slightly out-of-sync when they get to the far end. This lagging and leading of signals on adjacent pairs is called delay skew. This problem can be heightened by sloppy termination, in which the cables are asymmetric with respect to the connector pins. Finally, if there is a difference in propagation delay between the wires in a cable pair, it could affect the signal because of delay skew. appen_a.fm Page 948 Tuesday, May 20, 2003 4:54 PM . Faults Caused by Improper Termination 1 2 3 6 5 4 7 8 1 2 3 6 5 4 7 8 1 2 3 6 5 4 7 8 1 2 3 6 5 4 7 8 1 2 3 6 5 4 7 8 1 2 3 6 5 4 7 8 Split 1 2 3 6 5 4 7 8 1 2 3 6 5 4 7 8 Wire-Mapping Errors Correct. Category 5e to a 11 0-Block In this lab, you learn how to terminate Category 5e cable to a 11 0-type termi- nation block, as well as how to properly use a 11 0 punchdown tool and a 11 0 multipunch. Proposals and specifications require that labels be computer generated so that they are permanent, legible, and more pro- fessional in appearance. appen_a.fm Page 9 41 Tuesday, May 20 , 20 03 4:54 PM 942