North American Network Operators Group

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Re: London incidents

  • From: Iljitsch van Beijnum
  • Date: Mon Jul 11 06:23:40 2005

On 11-jul-2005, at 11:40, [email protected] 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.)