London incidents

Mon Jul 11 10:19:10 UTC 2005

On 11-jul-2005, at 11:40, Michael.Dillon at btradianz.com wrote:

> I had moved the weekend before and my landline was not
> yet installed. Also, I live near a large hospital. I noticed
> that my mobile didn't function at all even late on Thursday
> unless I left home and travelled a kilometer or two from
> the hospital. Presumably, the cells in this suburban
> location had also been switched to emergency service.

A hospital using up "emergency mode" GSM capacity doesn't make much  
sense to me. You're not supposed to use cell phones in many places in  
hospitals, and the ones that I've seen have an ample supply of fixed  
lines that are cheaper, more reliable and pose less risk of  
interference with the equipment.

It's probably just congestion. Cellular networks don't come close to  
being able to absorb the burstiness of the (potential) usage patterns  
in situations like this. (The bean counters don't like cell towers  
that are idle 99% of the time.) When all the time slots on all the  
sites in range are filled up you can't get through with voice or  
data, but SMS which just uses signalling still works. When it gets  
really bad the random access channel gets clogged and all mobile- 
intiated communication, including SMS, is dead in the water.

(The random access channel is the one not under control of the  
network: handsets use it to signal their desire to communicate. As  
such, it is very prone to collisions and congestion collapse under  
heavy loads.)