Compaq 247320-003 - Deskpro 4000 - 5166 Model 2500/CDS Reference Manual page 71

The rear panel has two network status lights for the RJ-45
connector. The yellow link light will illuminate when the
network driver is properly loaded and the system is physically
connected to an active network.
During 10Mbs operation, the link light indicates that the
system is receiving link pulses from the network.
During 100Mbs operation, the link light indicates scrambler
lock and the receipt of valid idle codes.
The green activity light illuminates when the computer detects
network activity. When the system is connected to a highly used
network, the activity light will remain on almost constantly.
If Remote Wakeup is enabled, the link and activity lights will
remain functional even when power to the computer is turned
off.