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Solid Network Adapter Link LEDs, Faulty Cable?
Last updated: 5/27/01

Q. Why would the Link (or LNK) LEDs be on solid, if there is a faulty cable connecting two Ethernet devices?

A.  Solid Link LEDs usually indicate a good network connection between two network adapters connected by a crossover cable or between a PC and hub or switch connected by straight-thru cable, but not always...

10BASET and 100BASE-TX Ethernet interfaces have two transmit pins (+ and -) and two receive pins (+ and -). The rest of the pins are unused. Transmit + pins must be connected to receive + pins, etc.

Solid LINK (or LNK) LEDs on two 10/100 Ethernet devices (network interface card--NIC, hub, switch, etc.) that are connected together indicates that the two transmit pins are connected to the correct receive pins.  It does not, however, guarantee that the cable is made properly, is made with the correct cable and connectors, and that will reliably transmit data.  For example, the Ethernet standard specifies that the transmit pins be connected to corresponding receive pins with wires from the same twisted pair.  It is certainly possible to connect a set of pins using one wire from one pair and another wire from a different pair. The reason it is possible to get solid LINK LEDs and unreliable data transfers is that link determination is made with a link integrity test pulse which is transmitted at a much slower rate than the actual Ethernet signals that transfer data.  Broken, disconnected, improperly terminated (coax), or miswired cables are responsible for over 70% of all LAN problems.

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