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| 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.

- Do not install long cables. Make all runs as short as possible, certainly no longer than is permitted by the media access protocol being used. For example, never install UTP runs longer than 100 meters.
- Whether building cables onsite or buying pre-made cables, be sure to test them with a reliable cable tester before use, especially if already in troubleshooting mode.

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.