By Jonathan Fildes
Science and technology reporter, BBC News
In the aftermath of the 7 July bombings, people were understandably keen to talk on their mobile phones.
The number of attempted calls rose tenfold on the day
Londoners wanted to assure friends, relatives and colleagues that they were OK; keep up to date with the latest news or find out whether anyone they knew had been caught up in any of the four explosions.
Yet, while speaking on a mobile phone is a routine part of modern life, for a crucial eight hours on 7 July it became difficult, and for many, impossible.
In some areas of London, the sheer number of people wanting to make phone calls was enough to bring the mobile networks to their knees.
This meltdown has been highlighted as one of the failings of 7 July and has left technologists wondering how to prevent the same thing happening again.
Immediately following the four explosions, mobile company Vodafone saw a 250% increase in the number of calls and a doubling of the number of text messages sent.
The company told the London Assembly investigation into what happened on the day that it had "never seen that volume of traffic for any event".
Other operators saw similar increases. Overall, the number of attempted calls rose from 30,000 every 15 minutes to 300,000. The nearest comparison, the networks said, was the telecoms flood experienced after 9/11 in the US.
Kristina was on the Piccadilly line train that exploded between Russell Square and King's Cross, but a sporadic mobile network meant she could not let tell her family that she had survived.
"If you can imagine, I did not get home until about 1430 that day. My family were absolutely worried back in Australia," she told the London Assembly.
"My mum thought she was going to have to come over and collect my body. I do believe there has to be some way to keep the mobile service going, keep access going, so we can find out if people are alive."
Mobile networks have a finite ability to handle calls. They are like a motorway that can only hold a certain number of cars before becoming congested.
"There comes a point when there are too many people trying to access a network, it just grinds to a halt," said Dr William Webb, a researcher at regulator Ofcom and fellow of the Royal Academy of Engineering.
The capacity to take calls is determined by the infrastructure of the network.
Four bombs exploded in London
Mobile operators divide the whole country into thousands of individual geographic areas known as "cells".
At the heart of each of these is a base station or mast, which communicates by radio with individual handsets within the cell.
In urban areas like London, base stations are usually built about 200 to 500m apart. A typical outdoor mast can handle about 100 to 150 simultaneous calls.
The number of cells, and therefore the number of potential calls, is based on factors such as geographical location, population density and how the networks believe people will use their phones in the area.
At specific events when the networks know they will be busy, such as at music festivals, the networks can increase the number of cells by adding temporary base stations on the back of trailers.
At other times like New Year, when they know that people will send a large number of text messages, they can also tweak the network settings to get the most out of the system.
"The networks change all their parameters, hours or even days before," said Jean Luc Jezouin of mobile infrastructure company Nortel.
But on 7 July, none of this was possible.
"The issue with unexpected events like an attack is that the networks have no time to engineer for a specific call profile," Mr Jezouin added.
Instead, on the day, the networks had to react to the sudden deluge using a combination of network sleights of hand to boost their capacity.
A simple trick to quickly double the number of calls in a cell is to use what is known as "half rate coding".
This happened in many central London cells on 7 July and involves compressing the call in a way that allows two calls to be carried in the same bandwidth normally used for one.
It causes some loss of quality but both people will still be able to understand one another.
According to David Sutton, network continuity and restoration manager of operator O2, the system works, but only to a point.
"When it is a major incident such as this, it makes a difference but not sufficient to solve the problem," he told the London Assembly.
ACCESS OVERLOAD CONTROL
System allows a network's individual cells to be shut down
Decision made by a police 'gold' command or the Cabinet Office
Emergency services given special sim cards for access
Used on 7 July to close cells around Aldgate tube
After all, when there is a tenfold increase in the number of attempted calls, even a doubling of capacity only begins to scratch the surface.
Other network tweaks used on the day included diverting calls through neighbouring cells and in some instances barring people's phones on a strict rota.
A system known as Access Overload Control (ACCOLC) can also be used to shut down a cell.
Although these tweaks increase the number of people who can make calls, they amount to little more than "tinkering at the edges", said Dr. Webb.
To cope with increased demand on a day like 7 July, there must be a change to the network.
Voice calls should be kept short to reduce network congestion
One quick fix would be to build new cells. But, said Dr Webb, increasing capacity was not a simple case of just buying more base stations.
"When you build a network you trade off capacity against cost," he explained.
And networks were unlikely to put in that extra capacity on the off chance it might be used one day, he said.
"It is not economically justifiable."
So to increase capacity, network providers must find ways of squeezing the most out of existing systems.
They can do this by using new compression techniques for voice data, using the radio frequencies more efficiently or by using technology such as smart antennas.
Rather than flooding a cell with radio frequencies, these antennas focus a beam towards individual users and therefore use the available bandwidth more effectively.
"It is like using a torch rather than a light bulb in a dark room," said Dr Webb.
Smart antennas are still in development but, according to Mr Jezouin of Nortel, they are "still clumsy and expensive".
WHAT YOU CAN DO
Where possible, try to make calls on a fixed landline
Preferably send text messages; try to avoid voice calls
If you must make voice calls, keep them as short as possible
Ask others outside the affected area to cascade information
Consider using a 3G or wi-fi-enabled mobile phone
Other solutions on the horizon include a handset that is able to change between different networks, including local wireless hotspots.
But while some phones on the market can already make calls over wi-fi networks, the technology is still not clever enough to be able to switch between networks seamlessly.
Another hope comes in the form of third-generation networks.
3G phones were not affected to the same degree as normal mobile phones on the day.
This is in part because these networks are less popular and so have fewer people using them. But they also process calls more efficiently and have a higher capacity, built in to cope with video and other bandwidth-intensive data.
A standard 3G network can handle three times the amount of voice calls as a normal network, although still far below the capacity needed on 7 July.
So even the systems being built now could not cope if there was another disaster tomorrow. Mobile operators and consumers would be in the same position as they were a year ago.
If that is the case, the responsibility falls to government to subsidise the mobile operators or for the phone users themselves to use the networks more effectively.
A government spokesperson admitted state subsidy was unlikely. Education was the key, they said.
People must understand that the mobile comfort blanket that surrounds them most of the time is relatively fragile and that the always-on, always-connected culture does not exist in extreme events.
Many people used their mobile phone to take pictures of events
Vodafone advises customers "to avoid making unnecessary or lengthy phone calls and, preferably, send a text message" to cut congestion.
That, said Dr Webb, was the best advice. In the short term, rather than waiting for technology to provide the answer we must adapt our habits.
"It's a bit like asking people to save water. There's no apparent benefit to the individual, but if everyone does it, we all reap the rewards."
The views expressed by Dr William Webb are his own and do not represent the views of Ofcom