7/7 Phone network performance examined and explained


Following the explosions on the morning of 7th July hundreds of thousands of people used their mobile phones to contact their families and work colleagues. Mobile networks are designed to cope with significant peaks in volumes of calls, and a safety margin is also built in so that even major events do not overload the system. Call volumes on 7th July far exceeded normal levels, and the O2 network alone carried 67% more voice traffic in central London than normal, while text message volumes increased by 20% on the day across the whole country.

The O2 network was not damaged by the attacks and we took steps to manage the demand. The large number of calls did mean that there was network congestion in parts of London, but most customers were able to make calls after several attempts.

Actions taken

Let us consider the equivalent situation on a motorway. Normally the traffic flows freely, and vehicles are able to move at will. Occasionally however, an accident, a breakdown or bad weather cause the traffic to slow down and congestion inevitably follows. Mostly this is short-lived and the situation recovers quickly, but sometimes the incident is more serious and the congestion lasts much longer.

When this happens, the Police have a number of options open to them in order not only to reduce congestion at the point of the incident but also to keep further traffic from entering the motorway and exacerbating the problem.

In much the same way, mobile network operators have a number of call traffic management options open to them. For example we can make network changes in the immediate area of the incident which will normally remove localised congestion quite quickly, and we can restrict additional call traffic from entering the system which reduces the load on the network, especially the gateways between the fixed and mobile networks.

These controls were used on the 7th July by all network operators, with the intention of preserving the integrity of the networks while still allowing as much traffic to flow as possible, but the situation was complicated by having four major incidents confined in a fairly small geographical area. This inevitably resulted in some customers experiencing the busy signal when attempting to make a call.

Those people who were unable to make voice calls turned to text messaging and again, inevitably, the text messaging systems became congested, delaying the time it took for a message to be delivered.

Industry working together

Co-operation among the fixed and mobile networks is the key to an effective response to a major incident. The telecoms industry runs a tripartite forum consisting of fixed and mobile network operators, the industry regulator Ofcom and various areas of central government – which comes together on a conference bridge on such occasions, with the objective of maintaining the integrity of the Critical National Infrastructure.

This forum was convened several times during 7th July, and the spirit of co-operation which it encouraged ensured that all operators worked together to minimise the impact on emergency responders as well as the public at large.

Emergency restrictions

Just before noon, the Police requested that O2 should apply a set of network access restrictions known as Access Overload Control or ACCOLC in a 1 kilometre area around Aldgate. This facility makes the network in the immediate area unavailable to the public at large, but permits a pre-agreed list of emergency responders to use the network in order to
save lives and protect the infrastructure.

The facility was removed later in the afternoon, but during that time, the O2 network would have been unavailable for all normal calls (with the exception of emergency calls to 112 and 999) within a kilometre or so of Aldgate station.


Should events be repeated, there are a number of steps which individuals and businesses can take in order to improve the situation.

• Encourage staff and individuals who might be in the affected area to call from their mobiles to a fixed line number rather than calling from fixed lines to mobiles – this is less onerous on the networks and relieves congestion.

• Keep calls as short as possible – once you have established that an individual is safe, clear the call so that others can do the same.

• Arrange a central 0800 number to which members of staff can call which will provide information regarding the incident and advice or instructions to staff.

We would like to thank David Sutton, Network Continuity & Restoration Manager for O2 (UK) Limited for this piece.