What are the LEDs on RJ45 connector for?

I just noticed (in fact a non-technical colleague of mine told me) that there are 2 LEDs on a RJ45 connector. I always thought there was only 1.

Could somebody tell me what they're for? I'm pretty sure that the one on the right that flashes all the time indicates traffic but what about the left one?

Thanks,

Solution 1:

One of the LEDs is typically an "activity" indicator, so it will flash whenever data is being transmitted.

The other LED is usually a "line speed" indicator and is typically orange for 10Mbit and green for 100Mbit line speeds. There may be another convention for gigabit speeds or full/half duplex but they will depend on the manufacturer.

Solution 2:

RJ45 connectors are used in telephony, and the connectors used for Ethernet are more correctly referred to using the generic name 8P8C, although calling them RJ45 connectors is much more common.

They also do not always have LEDs. When there are two one commonly flashes when there is 100mbit connectivity and one for 1Gbit, or one for connection and one for data, or different combinations of those. It depends on the manufacturer.