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Using SMS in an Emergency is No Guarantee of Establishing Successful Communication

Some agencies are advising the public to use text messaging (SMS) in a large-scale emergency. The rationale is that the voice lines will be overloaded, but the little data packets for text messaging will get through. Conceptually this sounds nice. In fact, I used to advise this in the past, too. But based on my understanding of mobile standards and the results of some unplanned tests, I’m not so sure it’s good advice anymore.

You’re likely using CDMA or GSM mobile networks wherever you are in North America. CDMA theoretically has no capacity limits. However, in practice, capacity per cell is limited by the computer power present at each base station. GSM has an upper capacity limit based on the channels available and the time slots available per channel. In other words, whichever network you’re on, there is a real and finite capacity cap.

From what I know, there is no dedicated data channel or dedicated data capacity on either GSM or CDMA. Text message data packets compete with voice calls to get through. True, it’s easier to tap Send to try sending your SMS through than to redial and Send a voice call, but your chances of making a successful connection remains the same each time.

Following a major emergency, everyone who has a cell phone jumps on the network and tries to call family or other loved ones. The network is jammed. Most people are unable to connect their calls. If, at this point, you try to send text messages through, you are still competing with all the other callers out there – and text messagers.

Now, if everyone sent text messages rather than calling, everyone would probably get through. One SMS message imposes a negligble load on the system. Unfortunately, voice is a much richer, faster way to communicate and you can’t blame people for trying to make voice calls after an emergency. So the reality is that the cell networks will be jammed for voice and for data.

I’ve had two unplanned test experiences with this. The first was during the initial hours of the 2003 Blackout in NE North America. The second was in the first few minutes after midnight New Year’s.

Right after the 2003 Blackout hit, the cell networks were, predictably, flooded and overloaded. I worked for a cell carrier at the time, so everyone had a Blackberry already. Some of us had the older Berries running on the Mobitex network, and some of us had the newer Berries that ran on the GSM network. Mobitex was RIM’s proprietary network for Berry data. Guess what happened? Nobody could make any voice calls successfully without trying at least a dozen or more times. Those of us on Mobitex Berries could PIN-to-PIN message one another freely, but those of us on GSM Berries were out of luck.

There is no dedicated data network like Mobitex anymore. All Blackberry data is now carried by the GSM & CDMA networks, and data packets are treated no differently than voice packets.

Right after New Year’s, everyone wants to wish their friends and family a Happy New Year. My friend and I, independently, figured we’d send text message greetings out and outsmart all the people trying to make voice calls out. Unfortunately, we both found our phones saying the same thing: they were unable to send the messages.

When the cell networks are jammed, it doesn’t matter whether you’re trying to connect by voice or data. Capacity is shared, and capacity is limited. You simply cannot rely on text messaging to enable timely, efficient mobile communication following a major emergency. The agencies who advise you to do so either do not know any better, or figure giving you one extra option – no matter how flawed it may be – is better than nothing.

If you find yourself in a pickle after a major emergency and can’t get a cellular voice call through, by all means try text messaging. You do have a higher chance of getting that one message through in a given timeframe and that is simply because you can retry an SMS faster than you can retry making a voice connection – but know that you are not guaranteed to be able to get through just because you’re trying to send data.

Contrary to what some agencies are advising, do not count on being able to establish timely, effective communication using SMS.