The Paris metro remains an interesting research location for me as especially during rush hour you can easily experience what congestion does to a network. Even with Opera Mini it's difficult to use the network at such times as you can almost see individual bits trickling in. Anyway, so I was wondering for a long time just how many cells they have in the metro to get an idea of the effort invested and the capacity available.
Recently I had the time and opportunity to dig a bit deeper. One line number 12 I've been looking, each metro station is covered by its own cell. By extending the coverage into the tunnels, handovers for voice calls and uninterrupted web browsing works most of the time. In other words, once cell has to absorb the load generated by all people currently waiting for a metro plus the traffic generated by the people in the metro trains currently in the stations or the tunnels.
That's an important first indication of the capacity deployed but a second parameter to come to a conclusion is more difficult to obtain: The number of carriers deployed. Unfortunately, that can't be seen with publicly available methods such as using AT commands that give you the current cell-id whenever it changes. So that part remains a mystery for the moment.
But capacity seems to be sufficient for voice calls, as all call attempts I made on the trip resulted in a connection. So no blocking here. The GPRS / EDGE side of the network is another story though. While speeds are good during non rush-hour times, the PS side is totally overloaded when people go and come from work. So either there are not enough timeslots available or some of them are taken for voice calls. Again, difficult to tell.
But one thing is good to know: With one base station per metro station, it is much easier for network operators to increase capacity if they wanted to compared to a situation in which a single cell was used for much longer parts of the underground network. But please don't bother adding a GSM TRX or two, just add 3G to the mix as other cities have already done to fix the issue for good.