Our thoughts and prayers are with everyone in London.
with regard to telecommunications services, Tim
Richardson writes in The Register:
[snip]
Phone networks have been jammed today following a
series of blasts that hit London's public transport
network this morning.
Mobile networks in particular have been put under
pressure as people use their phones to contact friends
and family following the explosions.
In a statement Vodafone said: "Understandably we are
experiencing significant network congestion but we are
working closely with the emergency services.
"In these circumstances, we would ask all of our
customers in Central London to avoid making unnecessary
or lengthy phone calls.
BT has also reported that its network is intact although
it is witnessing a massive spike in calls.
[snip]
http://www.theregister.co.uk/2005/07/07/london_phone/
- ferg
Mobile networks in particular have been put under
pressure as people use their phones to contact friends
and family following the explosions.
Luckily, I was 10 minutes late leaving home otherwise
I could very well have been on that first train which
was attacked near Aldgate. When the Central Line shut
down, I tried to get a bus, and when all the bus
service into central London was shut down I gave up
and started walking home. I suspected that the rumours
of terrorist attack were true.
All this while I was trying unsuccessfully to use my
mobile to ring the office. Finally, I decided to try
sending a text message and this worked. Text messages
normally are delivered virtually instantaneously and
there is a time stamp indicating when the message was
sent. During the morning and early afternoon of
Thursday, I was receiving text messages that had been
sent between 20 minutes and one hour previous.
Some of the problems on the mobile networks were the
result of a protocol to reserve mobile capabilities for
the emergency services. The police have the authority to
switch cells to emergency service and then people with
specially registered SIM cards in their mobile can
take priority. Presumably, some amount of capacity is
also held in reserve for these people as well.
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.
--Michael Dillon
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.)
Some of the problems on the mobile networks were the result
of a protocol to reserve mobile capabilities for the
emergency services. The police have the authority to switch
cells to emergency service and then people with specially
registered SIM cards in their mobile can take priority.
Presumably, some amount of capacity is also held in reserve
for these people as well.
Requests from the police on specific SIM numbers on certain
mobile networks whilst others applied such that you got no
access to a cell site, others deployed a limit on normal SIM
cards to limit the access down by 50% so that there was some
level of service.
Regards,
Neil.
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.
This was just a guess on my part because the congestion
in this suburban area lasted well into the evening. The
only time I was able to make phonecalls on my mobile was
when I took a bus out of the area. I planned to travel
away from the city to get away from mobile congestion
but the phone started working again before I had gotten
any further from the centre. However I had moved a km or
two from the hospital. Later, I returned home and lost the
ability to use the mobile even as late as 11:30 p.m.
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.
This, I understand. But it doesn't explain why this area
would have suffered such a prolonged problem.
When it gets
really bad the random access channel gets clogged and all mobile-
intiated communication, including SMS, is dead in the water.
I never had a problem sending or receiving SMS other than
the long delays. The people on the other end were near
Aldgate on the edge of central London so even there, SMS
was still functioning.
It was an interesting experience which seems to show that
it is better to have several completely different communications
channels to choose from. In my case I had lost landline and
DSL Internet access due to moving house, and I lost mobile
voice access due to congestion. But SMS still functioned.
I haven't heard of any Internet outages caused by the attacks
although everyone who has travelled on the tube knows that there
are lots of cables in the tunnels. Presumably, there are so
many tunnels with cables that breaks in three places are easily
covered by protection switching.
--Michael Dillon
A hospital using up "emergency mode" GSM capacity doesn't make much
sense to me.
This was just a guess on my part because the congestion
in this suburban area lasted well into the evening.
Could be lots of things. Maybe it was really the hospital, but then simply the people in the waiting area calling all over the place. Or maybe some completely unrelated problem with the cell network in your area.
When it gets
really bad the random access channel gets clogged and all mobile-
intiated communication, including SMS, is dead in the water.
I never had a problem sending or receiving SMS other than
the long delays. The people on the other end were near
Aldgate on the edge of central London so even there, SMS
was still functioning.
Follow the money... At several hundreds of your favorite currency unit per megabyte, I'm not surprised they manage to keep this service running.
Here in the Netherlands we had free airtime for a few hours at the beginning of the new year several times, and it was interesting to see what this did to the networks.
The lower the bandwidth channel, the less likely it is to break.
Cheers,
-- jr 'cf: Morse Code' a