I've seen it mentioned here, at least a couple of times, that when you can't get a cell signal that text messaging might work. Could someone clarify this for me?

I know that in emergencies, such as in NOLA after Katrina, the cells were jammed with traffic. While voice calls couldn't be completed, text messages were going through because they require very little bandwidth as compared to a voice transmission.

It seems to me that if one cannot communicate with the nearest cell tower for whatever reason (distance, obstruction, it's just plain broke, whatever), your text message isn't going through either.

To make a voice call, you need to have a usable signal continuously for the time it takes to dial, get the other person to answer, say what you have to say, and possibly get a reply.

To send a text message, even if you have no signal at the moment, you can type the message and have it waiting in the "outbox". Then you can wander around waving the phone in the air or whatever. If the phone sees a usable signal, even for a moment, it can send the message.

Likewise, since text messages are short and can be queued up to send in the gaps between voice calls, they should be helpful for getting through during times of very high traffic.

Now, *in general* a low-bandwidth text message can be sent with a much weaker signal and more interference than a high-bandwidth voice transmission. I don't know to what degree actual cell phones take advantage of this fact.

Finally, be aware that I personally don't have a lot of experience in using cell phones with poor signals as I live in an area where the coverage is virtually continuous.

And let's not forget, the store-and-forward capability exists from the network to the recipient as well. If the recipient's phone is off or otherwise not reachable by the network, the system will try to deliver the text message the next time the recipient's phone connects to the network. It's not totally foolproof since different networks will expire a message after checking if the recipient is on the network a certain number of times or after holding onto the message for a certain length of time.

Ah... I can see where queuing would be real handy. I knew the text messages were squeeking by in unused intervals of time. Didn't stop to think it was because they were queued up in the phone itself, thanks.

Cellphone novice here. In ten years of lugging the things around I think I've made/gotten about 4 calls total (it's for emergencies only, but my landline use isn't all that frequent either).

Low-bandwidth is not the only advantage of text messages, although that's a huge one. As Thseng pointed out, store-and-forward let's a busy network queue up messages and they can squirt through when the network is available.

The other reason why text messages often sail through while voice calls have overloaded the nearest cell tower is because (at least with GSM) text messages are transmitted on a signalling channel, not the main voice channel. The singalling channel carries info like telling you that a voicemail has arrived, so it would never be jammed with voice traffic.

I have no idea if bandwidth-intensive uses like the newer video messages or ring tones are sent through the signalling channel, though. If they did, then thousands of people sending videos of their destroyed homes or whatever at the same time would easily block the ability to send text messages, too. I don't remember, but I don't think video messages were really around yet during 9/11. That would be the kind of situation where a lot of people could've been trying to send video messages at the same time in a small location. The London train/bus bombings was definitely in the age of video messages, but I never read anything about video messages clogging the system, though, although I read that voice calls were jammed. Maybe video messages are not really something to worry about during a disaster. Still, I'm sure they eat up your phone batteries to send and play back.

Another big plus for SMS that I wasn't aware of. Thanks.

We should also remember that text messaging isn't a reliable protocol. Just because it indicates it was sent, doesn't mean it got there.

-john

I upgraded my wife's and my cell phones after I heard about SMS working and voice not getting through during Katrina. <img src="/images/graemlins/cool.gif" alt="" />

I believe SMS does this no matter the carrier. The reason you can't make a call in a disaster is the voice channels are full. There may be as little as 1 channel on a tower, or up to 8. If you have to talk via voice. Please, pass your message quickly.

I did after September 11, 2001. That was the only day so far we couldn't make a call, kept getting a fast busy. I called my wife from my office phone before I left to pick her up and told her to walk east on a specific street and I would circle around the building where she worked and drive alonside that sidewalk until I found her in the mess. It was espically bad for us because she worked in a tall building for a law firm which quietly called all their lawyers and told them to evacuate but didn't allow staff to leave until the lawyers were all safely out of the building.