MDD stands for Mac Domain Descriptor, which is a technical term for basically describing to the hub how the equipment at the far end of the cable is setup, what it should be listening out for etc. The underlaying technology is referred to as DOCSIS and it is designed to be highly dynamic, it works on the basis that overlaying digital data onto what was designed originally as a means of transmitting analogue cable TV to subscribers using cables that could be 20-30 years old, is somewhat problematic! Now don’t get me wrong, the mathematics and the infrastructure to implement it, is really quite elegant and remarkable, and as such it is designed to be really quite robust and cope with rapidly changing situations.
Your hub, on startup, listens for specific signals and messages on the cable, when it receives them, they tell the hub which frequencies it should be listening on, how many specific channels it should expect to receive and ignore all others, the hub currently will lock onto either 24 or 32 downstream channels and on one of these the MDD is transmitted, the sort of ‘primary channel’, as long as the hub periodically receives these MDD messages it’s happy and knows exactly how to ‘talk’ to the kit at the other end.
But, as I said DOCSIS is dynamic, so what happens if the channel that the MDD is currently on become impaired, a bit noisy maybe? The CMTS, basically the big modem on the other end of the cable, knows this and switches the MDD notifications to another channel, your hub, though doesn’t know this and suddenly finds that it is no longer receiving the MDD messages where it was expecting to find them, it has basically timed out! So what the hub does is to start looking on the other channels to see if the messages are there and eventually finds them and all is well - it does though log a scary looking message but notice that the priority of this is ‘warning’. In DOCSIS modem parlance a ‘warning’ message just means that something has happened, but no worries, I know how to work around it and it won’t cause any problems. In contrast a Sync Timing message will be ‘critical’ which translates as ‘something really bad has happened which I can’t compensate for and your connection really is going to be impacted’.
So don’t loose sleep about the MDD timeout messages, well except if there are a lot of them in close succession, then that is an indication that something is a bit ‘off’ with the underlaying connectivity.
Bottom line the MDD timeout messages are not in themselves anything to worry about, too many of them (and no, there isn’t a hard and fast rule of how many is too many), then that is a symptom that something else isn’t quite right.
And yes, I do know that most of the above is a massive over simplification of the actual way this stuff all works, I’m just trying to get the salient points across. There are some really good Wikipedia articles on how it functions for those who are interested. Be warned, though, the underlaying mathematics does go off into the weeds really quickly.😉
John
