Having stayed a bit outside of Barcelona for a couple of days before the Mobile World Congress back in February, I ran across a phenomenon again that I call the "Dormant Cell" issue. While a "dormant" cell seems to be up and running for both the mobile devices and the RAN control center, it is somehow locked up internally and is not functioning properly. In my case outside Barcelona it was a UMTS cell and the signalling to establish an Internet connection worked just fine. However, subsequent data transfers did not work at all. A kilometer or so away things worked again as the mobile selected a different base station.
One would think operators would detect such cells quite quickly and reset them. However, it seems that in practice cells sometimes do not send alarms to the operation center so the personnel is oblivious to the issue. In my case the situation remained that way over three days. Now try to report such a behavior to the operator's first line support staff…
To catch such issues, some operators run statistical counter analysis like for example to compare daily data transmission volume per cell. If suddenly a cell shows abnormally low values the analysis program generates an alarm so the network operation center can take a closer look. Unfortunately, not everyone seems to do that or at least not very often, like the operator of the network I used in Spain.
Agreed, it's quite a number crunching effort when you have several thousands of cells in your network and requires a big database to compare current counter values against those measured in the past. But benefits of such analysis go far beyond just finding dormant cells. This way, it's also possible, to give just one example, to follow rising use of certain cells in your network and to predict when it will run out of capacity. That way the network can be upgraded before the cell runs into saturation. The principle does not only apply for Internet access but also for voice calls.
I picked the Barcelona incident to start the post but quite frankly, it is not the first time I've seen such behavior and not only in a single country. Looks like this is a general phenomenon experienced not only with a single RAN vendor.
We refer to them as “sleeping cells” and find them exactly as you describe. If the voice or data traffic suddenly drops, an email is triggered identifying the cell and traffic change. Operations will then investigate. One problem that can happen is if the drop in traffic is due to a hardware fault and is offline for an extended period (say a bushfire burns out the shelter), the zero traffic profile can then become the norm and the statistical comparison email is not triggered and no-one investigates. I’d say this is pretty un-common as hardware faults are tracked pretty well.
The same process triggers on sudden increases in traffic as well. Handy for identifying special events that may have been missed by planning.