Hundreds of cell phone users in Minneapolis on Wednesday evening complained that they were unable to make or receive phone calls during the aftermath of the I-35 bridge collapse. But many people said they could still contact loved ones via text messaging.
This is not unusual, experts say. People in London and New York City after terrorist attacks in those cities reported similar issues. So did some students during the massacre at Virginia Tech earlier this year. While some of these issues can be blamed on damage to infrastructure, as was the case after the September 11 attacks and during Hurricane Katrina, more frequently cell phone networks are crippled for the simple reason that there isn't enough capacity on the network.
While I don't know for sure what happened to the cell phone network in Minneapolis on Wednesday night, my guess is that the network was overloaded. Cellular networks are designed to handle a certain amount of call volume in each cell or region. And operators have built their networks to serve the average usage volume.
But during a disaster when just about anyone with a cell phone is trying to either contact 911 for help, or loved ones to tell them they are alright, the network is oversubscribed and it isn't able to handle all the phone calls.
As a result, people aren't able to make or receive calls, and they often hear recordings saying, "The network is busy, try again later." Experts point out the same thing can happen to the regular phone network.
"Communications networks in general, regardless of whether they are wireless or wireline, have a finite amount of capacity," said Charles Golvin, an analyst with Forrester Research. "And for example, there have been many times in California, during an earthquake when the regular phone network is tied up."
So why do text messages get through when phone calls can't? For one, SMS text messages are very short, so they require very little capacity when they are transferred over the network. The second reason is that text messaging works by allowing messages to be stored and sent through the network.
If there is a delay in connecting to the network, the phone will store the message in its memory and it will continue attempting to send the message until it gets through. By contrast, voice is a delay-sensitive application. If a sustained connection can't be made, the person on the other end won't be able to understand what you are saying. And so the call cannot be completed.
While it's quite common for cell phone networks to get overloaded during serious emergencies, there isn't much that can be done to fix the problem. The main reason is that it just isn't economically viable for carriers to build their networks to handle a tenfold increase in capacity in every inch of their footprint.
"People have to remember that this is a commercial service," Golvin said. "It was never designed to be an emergency network. And it just doesn't make business sense for carriers to try to build it that way."
In most communities, emergency responders have established their own dedicated wireless networks. So while you may not be able to call your husband on his cell phone, firefighters and police are able to contact one another to respond to the emergency.
A nationwide public safety network should be coming in the next few years, if all goes well in the upcoming Federal Communications Commission's 700MHz spectrum auction scheduled for next year. Some spectrum in this block, which is being vacated by analog TV channels, has been set aside to build a nationwide first-responder wireless network.
But even when this network is built, it will likely have little impact on the availability of commercial services during emergencies.
Citywide Wi-Fi networks could help alleviate the problem, said principal analyst Craig Mathias from Farpoint Group. But it would only be helpful if everyone had phones that were able to switch between the citywide Wi-Fi network and a commercial cellular network. Proliferation of such services and phones isn't likely to happen anytime soon. Even though the technology is available today, there are business issues and cost issues that prevent it from becoming a reality.
The best thing that people can do when an emergency strikes is to only use their cell phones if it's absolutely necessary. Secondly, if you need to communicate with someone, try sending a text message instead of making a call. The chances of it getting to a recipient are much greater.
I'll be discussing this topic today on CNBC's On The Money at 4:00 p.m. PDT. So check it out, if you want to hear more. I'd also love to hear your comments on the message board below.