Re: Does SMS messaging keep going when cellular voice is down? [Telecom]

Sep 07, 2011 3 Replies

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Michael D. Sullivan 15 years ago

In Message-ID , Bill Horne asked:

I just read an email that claims cell phone text messaging (SMS) keeps
>going when cellular voice service is down. Please tell me if this is >true or not.

Yes and no. SMS keeps going when the cellular voice traffic is so heavy that the system seems like it is down. SMS uses the signaling or paging channels and does not require a dedicated connection, and moreover does not use much "bandwidth," so thousands of text messages occupy the same network capacity that would support only one voice call. But when the voice service is down because a tower is out of commission, there is no signaling channel and thus no SMS.

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John C. Fowler 15 years ago

It depends why the voice service is down. If the voice service is down because the only cell tower in range just got hit by a giant meteorite, then obviously, your SMS isn't going to work, either. If you can't get through by voice because some big event is happening and everyone is trying to call everyone at once, overloading the system, then SMS could very well get through when voice does not. SMS is carried at the signalling level and uses much less bandwidth than voice, so it's easy to prioritize.

If "voice service is down" means "calls start OK but keep getting dropped quickly", then it's very likely SMS will make it through.

John C. Fowler, snipped-for-privacy@yahoo.com

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Greg Monti 15 years ago

It depends what the underlying failure mode is.

When all voice channels are busy (during a crisis) so that it appears

*to you* that the cellular voice network is "down," SMS messaging continues to work because all users time-share tiny bursts of time on a data channel. If the SMS channel is busy at the moment you press Send, your phone places the message into a queue and sends it when the data channel is not busy. This could be a fraction of a second later, or ten seconds later, or a minute later. It does not matter to you since your message did eventually get sent.

If the cell network itself is down, or if all cell towers within reasonable receiving range of your handset are down simultaneously, then neither voice nor SMS will work.

Greg Monti, New York, NY snipped-for-privacy@mindspring.com

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Joseph Singer 15 years ago

It's only sort of true. What's really going on here is that if you try to make voice calls when the network is heavily loaded with people attempting to make calls your call attempt may fail due to lack of voice circuits to handle your call. SMS i.e. text messages are more likely to succeed in being sent and delivered partly because they don't use the voice circuits or use an infinitesimal amount of the mobile infrastructure to work. Of course it will only work if the infrastructure to process it is live and not cut off because either the base station has lost power or the back hall[sic Mod.] from the base station to the rest of the network has been damaged.

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