Today at BoingBoing, Maggie Koerth-Baker has a fascinating Q&A with communications engineer and entrepreneur Brough Turner about how mobile-phone networks respond to sudden spikes in call volume, as occurred April 15 in the aftermath of the Boston Marathon bombings. Mobile phones are everywhere, but beyond spotting the odd cell tower here and there, few of us understand how they connect up to the rest of the world, and the Turner Q&A does a nice job of breaking down the complex infrastructure in simple terms.
Things have changed a lot since the days when you could see a wire running from your house to a telephone pole—and from there into a visible, tangible network—but many of the same kinds of bottlenecks still exist. In a situation where cell networks are flooded, Turner explains, it’s often easier to communicate with friends or family by text messaging, or SMS (short message service), than via a voice call. Besides, sometimes “I’m OK” is all you need to say. As Turner says:
“Yes. It's much better. The SMS messages have a relatively light footprint, first of all. The second thing is that they're asynchronous. If they can't get through this instant, they keep trying. If it gets over the radio to the cell site, it will get through. Even if it's delayed for 30 seconds or something. With voice you're either connected or you're not, and when you are that means that the traffic channel is tied up until you're done talking. More likely, it means you never get connected because traffic channels are already saturated.”
Turner also notes that cell companies transmit SMS messages on a so-called control channel set aside for network operations rather than on one of the channels designated for voice traffic. The control channel is the wireless channel used to set up and disengage a call, so a text message might make it through even when a network’s voice channels are too overloaded to handle additional calls. It’s a bit like being able to drive on the shoulder when the freeway is jammed with traffic.
Initially I wondered if this text benefit applied to iPhones, which use Apple’s iMessage system rather than regular SMS. It appears that it does: according to Apple, iPhones will revert to SMS or MMS (multimedia messaging service) when iMessage is not available. But iPhone users must ensure that the “Send as SMS” setting on their device has not been disabled.