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| 3.4 |  | Identifying Physical Layer Problems |
| 3.4.3 |  | Cabling faults – fiber and coax |

All fiber links are crossed over. The connectors are always the same on stations and infrastructure equipment, so the TX output is connected to the RX input through careful attention to the cable polarity. Check fiber for swapped RX/TX connections when polarized or small form factor multi-fiber connectors are not used. Someone may have reconnected the cable incorrectly after disconnecting for some reason. Figure shows the coaxial cable and Figure shows the fiber-optic cable.

If a spare fiber-optic cable did not have dust covers on it, then it should be considered suspect even if it tested good yesterday. Clean all fiber-optic cables before attaching them as routine procedure.

If a fiber power test fails, inspect and clean all connections on the link. While walking the cable run route, watch for excessively tight bends and over tightened cable ties that cause micro bends.

Test fiber for power level and link indicator LEDs.

Coaxial cable
Coaxial cabling runs form a bus topology. Each station NIC acts as an intermediary connector for the run, linking devices in the LAN in one continuous run. Thin Ethernet (10BASE2) can contain 30 hosts in a LAN segment. Breaks in the cable can be difficult to locate because all stations form one continuous segment, with no micro-segmentation occurring.

Problems with coaxial cable often occur at the connectors. When the center conductor on the coaxial cable end is not straight and of the correct length, a good connection will not be achieved.

When the center conductor is not straight or is not the correct length, cut the coaxial cable behind the connector end, and strip the insulation back. Make sure that the newly exposed center conductor is straight. Before replacing the new cable connector end, check the general condition of the cable. Make sure that the new cable conductor end is securely crimped to the cable. The center connector should extend 3.2 mm (1/8 inch) beyond the end of the connector.

Check that the coaxial cable end is securely screwed onto the F-connector at the back of the cable access router. Hand-tighten the connector, making sure that it is finger tight; then give it a 1/6 turn.

The connection of a host NIC to 10BASE2 cabling is usually via a BNC connector. Some buildings use a special type of connector in place of the more common BNC to try to prevent problems arising when users disconnect or reconnect their equipment. Without the special connector, a person unplugging their equipment could cause a break in the transmission medium inhibiting communication between other users. The end of each segment of cable must be terminated with a 50 Ohm BNC terminator.

Thick Ethernet (10BASE5) cabling runs use thick coaxial cable to form a bus topology. A maximum of 100 transceivers can be used for host connection to the bus. Thick Ethernet cabling is difficult to install because of its weight and large diameter of 0.5 inch. It is not acceptable to bend 10BASE5 cabling, there are also constraints on how tightly the cabling run can be curved. Thick Ethernet was primarily used for LAN backbone connections and it was uncommon to find it used to connect end-stations to the LAN.