link type has been selected. Also, the test standards have evolved sufficiently that the requirements for a particular test, such as ISO Class D, may no longer be the same as what is loaded in the software of the tester. Check the website of the manufacturer of the tester for new tester software two or three times per year. Most wiremap failures occur at cable terminations, either at the RJ-45 (plug or jack), or at an intermediate crossconnect or patch panel. Faults at the RJ-45 can usually be seen by checking the wire colors carefully against T568A or T568B pinout colors, or by checking the RJ-45 plug for wires that did not seat fully to the end of the connector when it was crimped. Another source of RJ-45 related problems is how well the connector was crimped. Propagation Delay and Delay Skew
TIA/EIA-568-B permits up to 498 ns of propagation delay for the Permanent Link and up to 555 ns of propagation delay for the Channel Link, for all Categories. It is unlikely that this parameter could fail without other parameters failing as well. Failing propagation delay suggests an inappropriate or bad cable in the link. Check the overall length of the cable. Inspect the cable closely to see if the correct type of cable was installed. If this is the only parameter that failed, it will probably be necessary to replace the cable link. Delay Skew
Delay skew occurs as a result of different wire pairs within a cable being insulated with different materials. This could occur if there is an industry supply problem for a favored insulating material. In this case the critical RX and TX pairs may be coated with the favored insulative materials and the pairs not utilized in TIA/EIA-568-A or TIA/EIA-568-B standards are coated using sub-optimal insulative materials. TIA/EIA-568-B permits up to 44 ns of delay skew for the Permanent Link and up to 50 ns of delay skew for the Channel Link, for all Categories. Both of these numbers are quite generous. It is fairly difficult to fail delay skew if good materials were used in the link. A delay skew failure is possible if wire pairs in a single cable have different insulative material on some pairs. A failure is also possible if various lengths of twisted wire pairs were used as a patch cable or jumper at a connection point. Varying the lengths of pairs at any point along the link probably indicates bad workmanship, as individual pairs should never be used for networking applications. This situation should cause other parameters to fail too. Inspect the connection points in the link, and if the workmanship appears reasonable, there may be little choice but to replace the entire cable run. Test a sample of the new cable before installing it to be sure that the materials are not causing the problem.
Content 3.5 Isolating Physical Layer Problems 3.5.4 Cabling incorrect Check for wires that were not fully seated in the crimping process. Also check to see if the correct type of RJ-45 was used, stranded or solid wire pins. This is difficult once the end has been crimped. Using the wrong style of pin may cause intermittent connections after a period of time, though the cable usually works immediately after it is made. If a non-proprietary terminator with a non-standard pinning is used, the connection may look the same but the internal wiring could be off. Check on the standard used. There have been notable cases where large campus sites have been wired by two different groups of contractors. If the cabling process is not coordinated correctly different contractors may use different wiring standards in their cabling. This would result in incompatible wiring schemes in the two halves of what was meant to be a single network. Cabling length is also a major issue. Cabling guidelines are constructed to take into account factors such as propagation delay and signal attenuation, which are related to the length of the cable used. Other issues such as grounding and ability to exclude internal and external noise from interfering with data transmission must also be taken into account. UTP is not the correct cable type to be laid near strong sources of external EMI, such as elevator cavities, power lines, radio, microwave, television signal, and transmission stations. In these types of environments where redirection of the data cables cannot occur, then either shielded CAT5 or optic fiber must be considered as a replacement. More dangerous is where there is a different electrical potential at each end of cabling runs. The terminating device may become live and capable of discharging significant voltage to other attached equipment, or even personnel. In this case, fiber-optic cable is to be used instead of UTP or STP, because it does not carry an electronic signal. It uses nonelectrically conductive material that transmits modulated light signals instead.
Content 3.5 Isolating Physical Layer Problems 3.5.5 Interface configuration Prior to examining the interface configurations on network devices, it is important to discount the physical causes first: When examining interfaces on a Layer 2 switching device: When examining a Layer 3 device: Console Not Responding
Console problems occur when the router becomes unresponsive to input at the console port. If the console is not responsive, it means that a high priority process prevents the console driver from responding to input. If traffic is still flowing through the device, try disconnecting network interfaces and see if the router starts responding. Many times the router thinks it is doing something too important to service exec sessions. Another possibility is that one of the device interfaces may be down. There are multiple events that could cause this, such as a wrong configuration command, or a hardware failure of the interface or the cable. If some interfaces appear down when using the show interfaces command, try to find out what caused it.
Content 3.5 Isolating Physical Layer Problems 3.5.6 Operational statistics A variety of end-station and network device commands can be used to gather operational statistics on a network or on specific component links and devices. These commands have been discussed in section 3.3 and also in 3.4.8 which details CPU statistics over time. Operational statistics can also be gained from