What does the link LED actually mean?

[Mark R]

Some equipment seems to give a "link up" indication with the vaguest hint of a connection, some other equipment seems to want to see properly functioning data flow. I presume, that this is up to the manufacturer of the equipment.

I was messing about earlier, and just as an experiment connected a 1000Base-LX port to a 1000Base-SX port, and was surprised to get a "link up" indication. No data actually went across the link, but the indication on both ends was "link up".

I tried a similar experiment with correctly matched SX ports. In fact, I didn't actually need to connect the link with a patch cable - I just needed to point the 2 ports at each other at reasonably short range. Sure, I got a "link up" indication, but the data capacity of the link was nil.

I tried the same experiment on different equipment, and found that only correctly connected ports would give a "link up" indication. Mismatched ports, patch cable not pushed in until it locked, or wrong patch cable (MM instead of SM) resulted in "link down" indications.

 

[imagoon]

That would be hard to answer because there is no standard as you discovered. I always take it as line is "up" IE physical but I don't assume the link is good. I have seen the same thing as you, cracked fiber would link up but only 5% of the packets were clean and thus the link was useless.

 

[drebo]

Typically means that the physical layer is up and working, whether that means electical signals or light. Whether anything above that is working is irrelevant.

 

[Mark R]

Just for interest, I performed another experiment today.

I connected up a pair of SR transceivers with a length of SM fiber. I wasn't expecting much, and I was right; I got "link down" indications. But what was very interesting is that the link actually carried data. It was very bad (10-20% packet loss), so completely useless for any practical purpose, but I was somewhat surprised that data could be transmitted across a "down" link.

Anyway, I swapped out one of the transceivers for another "identical" one, and got a "link up" indication and a completely stable link. So, the SM fiber does work, but it is obviously right on the margin.

 

[spidey07]

Means the receiver is receiving the link pulse. You should not have gotten link light with SX and LX mixed like that.

The reason SX worked with SM fiber is the light is still being driven into the fiber but you're losing a TON of light due to multimode core size being MUCH larger than single mode. So your shining a big flashlight into a straw, you'll get light at the other end of the straw but it's much less light than the flashlight is putting out.