1000BASE-T Part 4 – Link Down Detection
In the previous three parts, we learned about all the interesting things that go on in the PHY with scrambling, descrambling, synchronization, auto negotiation, FEC encoding, and so on. This is all essential knowledge that we need to have to understand how the PHY can detect that a link has gone down, or is performing so badly that it doesn’t make sense to keep the link up.
What Does IEEE 802.3 1000BASE-T Say?
The function in 1000BASE-T that is responsible for monitoring the status of the link is called link monitor and is defined in 40.4.2.5. The standard does not define much on what goes on in link monitor, though. Below is an excerpt from the standard:
Link Monitor determines the status of the underlying receive channel and communicates it via the variable
link_status. Failure of the underlying receive channel typically causes the PMA’s clients to suspend normal
operation.
The Link Monitor function shall comply with the state diagram of Figure 40–17.
The state diagram (redrawn by me) is shown below:

While 1000BASE-T leaves what the PHY monitors in link monitor to the implementer, there are still some interesting variables and timers that you should be Continue reading




