The link is the system that makes it possible for two machines to communicate over a media (cable, radio). There are a number of different standards but on this page we will concentrate on one major link protocol, Ethernet that over the last decades became the absolutely most used link protocol.

In an ordinary PC you always find an Ethernet connection with a RJ45 plug. There is mostly attached two LED:s beside the connection, one to see that there is a "link" and one that is flashing when data pass over the connection. If the to LED:s has no lights you probably have a cable error.

As mentioned under the Ethernet chapter the category 5 or 6 cable used for Ethernet should contain 4 pairs that should be straight connected between the PC and a switch or hub. If you connect between two PC:s, printers routers or similar end equipment you should use a special crossover cable. The same goes for connection between two switches. Most modern switches has sensors that adjust this automatically. Switches sends data on wire 3 and 6 and expect data to be received on wire 1 and 2. Vice versa for end equipment.

Other problems with Ethernet is mismatching speed or duplex configurations. It is possible to run 10Mbit, 100Mbit or 1 and 10 Gigabit on Ethernet. Gbit and over uses all 8 wires for the connections while 10 and 100 uses only 4 wires as mentioned before. Duplex is the ability to send and receive data on the same time on different wires. In the first Ethernet media (shared koax) it was not possible with full duplex. Most equipment have sensors that automatically adjust speed and duplex. However, with different brands of equipment it is very common that the auto configuration mismatch. A way to avoid this is to configure the switch only for one speed and full duplex. The Gigabit standard however, implies negotiating and works best that way.

Is everything correct the link lamp should light in both ends of the wire and the data lamp should flash irregular. Those to signals could be integrated in one common LED with two colors.

For wireless networks you can look in the Control panel in your PC to see if you have link on the wireless adapter card. You got link when the signal from the access point is strong enough.

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