Solid Network Adapter Link LEDs,
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.