TDR Tutorial and Riser Bond TDR Product Review
Applications
LAN
Time domain Reflectometry is one of the easiest and most accurate ways to qualify copper cabling for data communications. As Local Area Networks proliferate, the TDR has become the standard method for verifying and maintaining LAN cabling systems. The cabling of today's LAN system involves a complex topology of cable and connectors, taps, and terminators. A system this complex presents many possibilities for cable defects such as opens, shorts, mis-terminations, frays, knots, kinks, and wet transceiver conditions. Any of these defects can cause system failure and costly downtime.
Cable manufacturers go to great lengths to manufacture Category 5 UTP cable, making sure of the exact number of twists per foot and that the twists between pairs do not overlap. This insures maximum bandwidth (minimum attenuation per foot) and minimum crosstalk between pairs.
The following list of poor installation practices will almost guarantee system errors and early system saturation: locating the wire closet next to an air conditioning fan motor or compressor can cause excessive random noise; installing 120 meters of cable instead of the specified maximum of 90 meters can cause excessive signal loss and therefore no noise margin; striping he cable back a foot or more from both ends of the cable (the wall plate and the patch panel) and re-twisting the pairs can cause increased near end crosstalk; and using one or more of the unused cable pairs for the intercompany telephone system can cause excessive crosstalk.
A system with only 10 to 20% traffic may appear to work perfectly. As traffic and collisions increase, the system will get slow and finally crash. The system can saturate from retries long before it saturates from traffic. There are two ways around this problem: first is follow the design guidelines and do not cheat on installation procedures; and second is to test the system after installation and find and repair the weak links.
A TDR is very useful in testing both the backbone and the segments. Obviously the TDR can test the cable lengths. A TDR with a 2 nsec pulse, such as Model 1220 or Model 1205, can also look at the quality of the cable within the first critical feet of the wall plate and patch panel . Look for impedance changes caused by excessive cable stripping and re-twisting. A person with minimal training and the right equipment can very quickly and easily document a complete system and find those weak links.
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