The wonders of modern technology didn't help much in the wake of the terrorist attacks in London when panicked friends and relatives did what people always do in situations like this: Reach for their telephones.
Not all of their calls got through.
The cellphone and landline phone traffic became so heavy Thursday that British wireless powerhouse Vodafone Group PLC, with nearly 15 million subscribers, posted a note on its website asking "all of our customers in Central London to avoid making unnecessary or lengthy phone calls."
Why does this happen? Volume. All networks — landlines, cellphones and wireless data nets — can become overloaded simply because they weren't designed to handle a major surge in calls at once.
"Different systems in different locations will have different limits, so there is no hard and fast design number," explained Bernard Becker, a network analyst for an applications service provider to the financial services industry.
Most phone companies use Mother's Day as a yardstick in measuring their peak usage, one analyst says. But a terrorist attack could multiply the demand by a factor of 10, as the attacks on the World Trade Center did in 2001.
When each network reaches capacity, Mr. Becker added, callers would be turned away by a recorded voice or what telephone people call the "fast-busy signal."
"Systems don't really crash, they just slow down," he said.
A phone network is like a city street, he added. "Everyone's allowed to drive down it, but not everyone in town can drive on it at the same time."
All telephone providers work on the same principle, said David Gilbert, a systems engineer for EiCatalyst, which installs networks for businesses. They estimate the normal volume of calls they expect their subscribers to make, and then purchase a little bit more capacity than that from the networks they use. Systems are usually designed for 15 per cent of the subscriber capacity, he said, and businesses at 50 per cent — meaning that at any one moment, 15 per cent of a company's residential subscribers can make a call, or half the business subscribers.
"This is called oversubscription, and it's where communication companies make their money," he said.
Two exceptions to the rule, however, involve more modern communications technology: Data-enabled cellphones, such as the Blackberry, and voice over Internet (VoIP) calls.
Cellphone networks became overloaded after the terrorist attacks on the World Trade Center, but people who had Research In Motion's Blackberries got e-mail messages through, because the messages use a channel called GPRS, which does not compete with voice (the GSM channel).
Since then, other companies have added data network channels for their subscribers (Bell Mobility added the 1X network to its CDMA voice channel), which do not compete with voice calls. Data networks are more efficient, because e-mail is handled in a different way — transmission can be delayed up to several seconds, which is acceptable for data, but unacceptable for voice communication.
The same does not apply to short message services (SMS). The SMS system is carried on the same channel as voice, so SMS is as subject to network congestion as voice calls.
For similar reasons, VoIP services can typically handle many more callers than cellphone networks. Subscribers to high-speed Internet lines have an enormous amount of bandwidth for VoIP calls, which use peer-to-peer connections that are somewhat similar to the ones some people use for swapping music files, Mr. Gilbert said.
"Generally, your Internet link is some number of megabits," he said. "Your VoIP needs are some number of kilobits. Voice calls are a drop in the proverbial bucket compared to normal day-to-day ... [data] traffic.
"The Internet is an incredible tool when used in a peer-to-peer way."
But, network experts say, there are still some limitations to VoIP, since the Internet is only part of a call when you're not dialling directly to another VoIP phone. At some point the VoIP caller must connect to the standard phone network to reach landline or wireless phones, and then the call becomes subject to the same restrictions landline and cellphone users face.
Nevertheless, most telephone companies are trying to "build ahead of demand," Bob Berner, chief technology officer of Rogers Communications Inc., said. Most networks are intelligent, and can automatically re-route a call in case a part of the network collapses.
But there is still one thing few companies can deal with, he said. "If everybody tries to use the phone at once, then there's big trouble."
