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A technology blog for The Economist Group IT team

Wednesday, July 27, 2005

Using mobile phones in emergencies

First, the myths. During recent bomb attacks in London (and at other times) the mobile ‘phone network was not shut down. Also, the mobile phones used by terrorists in Madrid to trigger detonation of bombs did so through their alarm function (any timing device would have done).

What could have happened on July 7th in London was that ACCOLC (ACCess Overload Control) was used to restrict access to the mobile network in specific areas. ACCOLC is part of the GSM standard that is used by the majority of mobile phones in Europe. Each handset has at least two ACCOLC classes set in its SIM card. All handsets have class 10 enabled, which allows calls to the emergency services (i.e. 112 or 999 in Europe and 911 in the US). The other class of service that most handsets have is in the range 0-9 and is the same as the last digit of the mobile phone number so that there is an even spread across the classes. Classes 11-14 are reserved for use by staff who may be involved in dealing with emergency incidents. This includes ambulance and fire service personnel as well as the police and armed forces, but also includes local council staff, civil servants and government officials. Other people, such as kidney dialysis patients, can also obtain a high ACCOLC class. Organisations whose staff need to obtain a high ACCOLC class may be able to approve the request themselves or may need to seek approval from the Cabinet Office, depending on the nature of the organisation. Class 15 is reserved for use by network operators (i.e. Vodafone, Orange, T-Mobile, 02 and 3).

ACCOLC is normally invoked by the Police Incident Commander (but can be invoked exceptionally by the Cabinet Office). Network operators implement ACCOLC on a cell by cell basis and in most circumstances this would be limited to one or two cells in the immediate vicinity of an incident. (Under normal circumstances any one handset will be able to “see” more than one cell - Ofcom's sitefinder shows the location of base stations in the UK - but will register with the one with the strongest signal.) When ACCOLC is invoked it can be done so to partially allow access to the network for handsets with a class in the range 0-10. If the network is not saturated in the incident area, class 10 can remain enabled and classes 0-9 can be enabled one at a time for a minute or so. This would mean that all handsets could make calls to the emergency services and that at any one time one in ten handsets would be able to make a non-emergency call. To further restrict traffic, classes 0-10 could be completely blocked from the local cell. In this situation the local cell will not allow call set up (which includes SMS messages) from any handset not equipped with an ACCOLC class of 11 or higher.

Use of ACCOLC is reserved for use as a contingency as all emergency service personnel have their own radios and Cabinet Office guidelines warn that
"It is important to remember that invoking ACCOLC is a two-edged sword: non-registered users, who may nonetheless be important to the emergency in question, may not be so easily contactable once ACCOLC is invoked."
It’s also important to remember that it’s the Police Incident Commander or Cabinet Office and not the network operators who invoke ACCOLC. Another point worth noting is that ACCOLC only restricts call setup and that calls already in progress are not affected. Operators could, however, shut down a cell and bring it back up with ACCOLC enabled, but I’m not sure whether this would happen in practice because of the risk of dropping calls being made by emergency service personnel.

BT implements traffic management on the fixed line network by using "call gapping". Exchanges can be set to allow only a percentage of calls to be routed to specific numbers. Most usually this is employed in situations where a large number of calls are being attempted to a number which does not have the capacity to handle them (i.e. if I were to place an advert in the national press offering a free holiday to the fist 100 callers to my home number). BT has a procedure for notifying them of such "events" in advance so that they can manage capacity for other callers without needing to use call gapping, but unforeseen situations do crop up.

Whilst on the subject of emergency calls, another myth is that if your handset is out of range of a cell run by your network operator that you will still be able to make an emergency call if you are within range of a competitor’s cell. This capability is included in the GSM standard, but is not allowed by any or the five UK operators (let me know what happens in Hong Kong and the US). In the early days of GSM, some handsets even displayed "SOS calls only" in these circumstances. The only practical way round this is to buy a roaming SIM marketed for use overseas (such as those provided by oneroam.co.uk) and to pop it into your handset as the need arises. Calls will be expensive, but at least you’ll be able to make one.
Comments:
There is an article on the BBC today about emergency calls. Apparently there is a e-mail circulating telling people 112 switches to satallite...which of course it doesn't.

Although an orange spokesperson says: "...do not have a signal from your network operator, emergency calls can be made as your phone will automatically detect the next available network."

URL: http://news.bbc.co.uk/1/hi/technology/4724101.stm
 
Yes....I didn't publish that myth, but apparantly some think that if dialling 999 doesn't work, 112 might do. Tosh. Also that Orange spokesperson is wrong on being able to use another network (the only other 1800MHz network in the UK is T-mobile). The BBC article is not clear on whether that comment actually came from Orange, though. I've e-mailed them.
 
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