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To access the contents, click the chapter and section titles.
Wireless Networking Handbook
The authors of TSB 67 chose two levels of accuracy for testing links: Level I for low accuracy and Level II for high accuracy. These two accuracy levels take into consideration the test configurations you implement for testing the Basic and Channel Links. For instance, Channel Link testing almost always requires the use of an RJ45 interface attached directly to your tester. The problem is the RJ45 interface offers unpredictable crosstalk and affects the accuracy of crosstalk measurements. This type of test, therefore, would only need Level I testing. On the other hand, Basic Link testing enables you to interface the tester to the cable via a connector having much lower crosstalk, such as a DB-9 or DB-25 connector. Thus, with Basic Link testing, it is possible to run the more accurate Level II tests. After installing Category 5 cabling, test the installation by performing the following tests that TSB-67 recommends. WiremapThe wiremap test ensures a link has proper connectivity by testing for continuity and other installation mistakes, such as the connection of wires to the wrong connector pin. For example, if you dont wire an RJ45 connector exactly according to a standard, such as EIA/TIA 568As T568A or T568B wiring scheme, then you might produce split pairs. A split pair occurs when you attach the connector in a way that a wire pair consists of one lead from one twisted pair and another lead from a different twisted pair, creating a pair of wires that are untwisted. The split pair might result in an excessive amount of external noise interference and crosstalk, which will cause transmission errors. Most cable testers perform wiremap tests to detect this type of cable problem. Link LengthLink length measurements identify whether a cable meets the length limitations. Cable testers utilize a Time-Division Reflectometer (TDR), which measures the length of a cable. The operation of a TDR is shown in figure 10.8. The TDR emits a pulse at one end of the cable, which travels to the opposite end of the cable, and then reflects back to the TDR. The TDR measures the propagation time and calculates the cable length based on an average wave propagation rate.
Several products on the market run TDR tests on metallic or optical-fiber cable. Tektronix TS100 Option 01 Metallic TDR, for example, tests LocalTalk, Type 1 and 3, Category 3, 4, and 5, and thin and thick coax cables. This test set finds shorts, opens, and breaks in the cable. The Tektronix TFP2A Fibermaster OTDR tests single-mode and multimode fiber-optic cables. AttenuationAttenuation tests ensure the cabling will offer acceptable attenuation over the entire operating frequency range. If too much attenuation is present, digital signals sent throughout the cable will experience rounding, resulting in transmission errors. Cable testers examine attenuation by measuring the effects of sending a series of signals that step through the cables operating frequency bandwidth. For Category 5 testing, most cable testers cover bandwidth of 1 MHz100 MHz by taking readings in 1 MHz increments, certifying whether the cable meets specifications in the part of the frequency spectrum where the signal mostly resides. The Microtest Pentascanner is an example of a cable tester that measures attenuation on Category 3, 4, and 5 cable. Near-End Crosstalk (NEXT)Crosstalk is the crossing of current from one wire to a nearby wire, causing transmission errors. Near-end Crosstalk (NEXT) is a specific case where signals at one end of the link interfere with weaker signals coming back from the recipient. The amount of NEXT varies erratically as you sweep through the operating bandwidth of a cable. For an accurate measurement, cable testers record NEXT by stepping though the cables operating frequency range at very small increments. For Category 5 cable, TSB-67 recommends a maximum step size of 0.15 MHz for lower frequencies and 0.25 MHz for higher frequencies within the 1100 MHz frequency range. This requires a fast instrument to take the hundreds of samples necessary. Flukes DSP-100 handheld cable tester is an example of an incredibly fast NEXT tester. The DSP-100 utilizes digital signal processing to increase its speed and allow samples to be taken at close 100 KHz intervals. The DSP-100 performs all tests required by TSB-67 for a 4-pair cable in under 20 seconds. The DSP-100 not only identifies the presence of crosstalk, but also locates its source. If any defects are found through unit testing, correct the problems of each unit before integrating them together with other components. Performing Integration TestingThe concept of integration testing, as shown in figure 10.9, is to test the network as you connect the components together. In this figure, integration testing would verify that components A and B work together okay. Then, after component C is installed, integration testing would verify that all three components work together acceptably.
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