by Thomas Wailgum

Minneapolis Bridge Collapse: Why Cellular Service Goes Down During Disasters

Aug 03, 20075 mins
MobileSecuritySmall and Medium Business

It happens all the time when calamity strikes. Here's what you should know to prepare for the next one.

The tragedy that unfolded in Minneapolis on Wednesday, August 1, 2007 is still being assessed, and rescuers continue to search for more of the missing. The horrifying bridge collapse, however, is yet another recent example of the power and limitations of wireless devices and networks during a disaster situation.

Emergency responders rely largely on wireless communications to coordinate operations at the scene, find those who are injured and rescue them from the wreckage. And ordinary citizens use mobile devices to alert loved ones to their status or whereabouts.

But what also usually happens near accident scenes like the one in Minneapolis is a disruption in cellular service because there’s too much radio and network congestion. Many news outlets reported that cell phone service in the greater Minneapolis area went down, citing the fact that cell phone towers and antennae were overloaded by the sheer number of users trying to place calls.

An image of a cellular tower

One story reported that “Jay Reeves, 39, was one of the first people on the scene after the collapse. He tried calling 911, but all the lines were jammed.”

A separate first-person account of Wednesday’s events in Minneapolis detailed the cellular disruptions and related frustrations of many. “While I was out, I got a dozen or so SMSes but only one or two calls. [Only] every tenth call I tried to make went through, and half of the successful ones had problems like not hearing the other end, dropping or unusable quality,” wrote Charlie Demerjian, a contributor to The Inquirer website who lives just five blocks from the bridge.

“Why? Simple overload. The infrastructure could not handle it,” he continued. “Friends as far away as 10-15 miles could not place calls, the entire network was teetering on going down.”

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This tragic incident isn’t the first time such a wireless infrastructure failure has occurred–and it likely won’t be the last. Recent events such as Hurricane Katrina, the London subway bombings, the 2003 electrical blackout in the Northeast United States, and the 9/11 attacks on New York City and Washington, D.C., all led to mobile phone-service outages.

So why does this happen?

According to David Crowe, a 20-year veteran of the cell phone industry and founder of Cellular Networking Perspectives consultancy, the cause is a combination of simple overload on the cellular systems and the randomness of the events. “The problem with major tragedies is that they are completely unpredictable and often occur where large amounts of cell phone capacity are not normally required,” he says.

A fascinating article on cell phone service overload can be found on Crowe’s website.

In the article, Crowe points out that wireless systems are assigned limited frequencies, and “the amount of frequency that can be used in one place is also constrained by neighboring cells needing to use part of the same block and the amount of radio equipment installed in the cellsite. With analog and TDMA/GSM cellular systems, the number of users who can be supported in a single cellsite is a simple function of the number of transceivers installed, but with CDMA it is more complex.”

“No matter what the technology,” he writes, “cellsite capacity is carefully engineered to ensure that the undesirable tone known as ‘fast busy’ (two beeps per second versus one beep for normal busy) is rarely heard, even during the busiest times of the day. This tone indicates a lack of resources for the call, usually radio capacity, although sometimes it reflects the lack of a connection back to the main switch or other network overload or failure situations.”

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An event such as the Minneapolis bridge collapse can generate as much as 10 times more cellular traffic than normal levels, Crowe says. For example, Crowe, who is based in Canada, notes that the Dawson College shootings in Montreal last September led to 11 times the amount of normal traffic. “It is impossible for cellular carriers to have this much extra capacity in place,” he writes. “It is not just that this would increase the cost of cell phone communications several-fold, something that consumers would not tolerate, but there simply is not enough frequency available in many locations, particularly in urban areas.”

What’s interesting to note is just how much companies rely on wireless communication services for their disaster recovery and business continuity plans. “I would suggest that [CIOs] ensure that their wireless communication is split between at least two carriers,” Crowe advises. “[CIOs] should all have long-distance cards as well, so that if their cell phone isn’t working perhaps they can find another phone.”

Crowe points out that the Minneapolis bridge tragedy should remind businesses and CIOs that it’s wise for disaster-recovery teams to play out all kinds of different scenarios. For example, he says, what would they do if a bridge took out their sites and their cellular communications functionality? What would be their most critical communications needs, and how could they be accomplished?

“But in many situations,” Crowe says, “there is no choice but cellular communications.”