LEDs are also included on cable modems and DSL modems, which are commonly used in small or home office implementations for Internet connectivity. The number of LEDs and their functionality depends on the device. For example, one cable modem might have four LEDs: one indicating that the modem is online, a Send indicator, a Receive indicator, and one labeled Message. In contrast, a DSL modem might have six LEDs. One shows that the device is powered, and one flashes to indicate that the device is operating normally. Then there is a link light for both the local network and the DSL connection, and another LED for each interface that flashes to indicate activity on those links.
The usefulness of LEDs in troubleshooting scenarios cannot be overstated. LEDs provide an instant, visual indicator about the state of a network link. In some cases, as with collision lights, they can even alert you to problems on the network. Understanding how to interpret information provided by LEDs is important for the real world.
Imagine a scenario in which a user who is working at workstation A calls and tells you she is unable to access the Internet. The Internet connection could be down, but by connecting to the Internet yourself, you determine that it is working correctly; therefore, it is safe to assume that the problem is at the user's end rather than with the Internet connectivity. Next, you decide to visit the user's workstation to see whether you can ping the Internet router. Before you begin the ping test, you look at the back of the system and see that the link LED on the NIC is not lit. At this point, you can be fairly sure that the ping test will not work because without the link light, there is no connectivity between the NIC and the switch.
Now you have narrowed the problem to one of a few sources. Either the NIC or the cable is faulty, the switch to which the user is connected is not functioning, or the port on the switch to which the user is connected is faulty.
The easiest way to test whether the cable is the problem is to borrow a known working cable from workstation B or C and swap it with the cable connecting workstation A to the hub, switch, or wall port. When you try this, if the link light does not come on, you can deduce that the NIC is faulty. If the light does come on, you can deduce that either the port on the switch or a cable is faulty. The next step is to swap the cable out or try the original cable in another switch port.
Whatever the actual problem, link lights play an important role in the troubleshooting process. They give you an easy method of seeing what steps do and don't work.