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Spooky cellphone interference
April 30, 2012 6:41 PM   Subscribe

Walking down the street on a cellphone, my reception cuts out and I hear a different voice through the earpiece, very briefly, say something like "... three months..." It sounds like an advertisement. The person on the other end heard nothing, and the call wasn't dropped. What the heck happened?

I'd love to know technical details- how prone are cellphones to interference like this? I thought they were encrypted to make this sort of thing hard to do by accident. What sort of broadcast could it have picked up?

(or am I hearing things?)
posted by BungaDunga to Technology (6 answers total) 3 users marked this as a favorite
 
Where are you, and who is your service provider?
posted by Chocolate Pickle at 7:05 PM on April 30, 2012


Regular and common occurrence. If you want to learn more about it, you could start reading about crosstalk. It is often caused by co-channel interference in wireless networks.

Failing that, it could be plain old electromagnetic interference but that usually will interrupt a call. I'm not sure if you would experience crosstalk or not.
posted by purephase at 7:09 PM on April 30, 2012 [1 favorite]


I'm guessing USA on a CDMA network?

Some phone companies decided it was cheaper to "encrypt" their phones by having lobbyists pass a law that makes it illegal for people to listen to their unencrypted broadcasts.

My lay guess would be a call on the same frequency but in a different (nearbv) cell was reflected off a nearby structure which just happened to have a good structure for acting a bit like a dish, and as you passed through the hotspot, the signal drowned yours. (Or conversely, you passed through an area that blocked your cell more than the nearby one)
posted by -harlequin- at 7:11 PM on April 30, 2012


What sort of broadcast could it have picked up?

A channel handoff.

Cell phones are called that because they work in cells -- a tower covering a small geographic area. As you move, you have to be handed off from cell to cell. You also get channel changes, where the phone sees the signal strength dropping and asks the tower for a channel change. If the tower can, it'll send back "go to channel X" and you switch.

However, sometimes, the handoff isn't exact. Ideally, when your conversation jumps from Tower A to Tower B, it's seamless, but if a tower's clock is off, it may not get the handoff right. So, you get a brief snippet of the *previous* call on that tower and channel, then the new tower gets the handoff and connects to the circuit to that channel, and all is well again. This is announced by the famous phrase "Hey, are you still there? The phone dropped out for a second...."
posted by eriko at 7:13 PM on April 30, 2012 [5 favorites]


It is interesting that you were actually able to hear the words. As you mentioned, signal should've been encrypted and any kind of intereference will increase noise level of your signal only.

It is possible that you have cross channel intereference and same coding at the same exact time. Very unlikely but possible.

In case you're interested, analysis of cross channel interference is this: at any given time in a single cell tower sector, you and only you have frequency X for your cellphone (this analysis is only for frequency division multiplexing, i.e. GSM and LTE). Other people have different frequencies. You and somebody however could very briefly share the same radio frequency. How that could possibly happen:

1. Frequency Hopping and timing. In order to combat interference, your signal hops from one frequency to another. At time t1 you have frequency X. At time t2 you have another frequency Y. You should be the only person with those frequencies but it is possible (due to longer distance travelled, etc) that the frequency X was not received until t2. At which case it will interfere with someone else's signal.

2. Cell towers interference. In one cell tower (or section at least), you are the only one using frequency X. The next cell tower might have the same frequency. If planning is poor, that signal is non-negligible and you can hear someone that is using that frequency in the next cell tower.

3. Guard band interference. As I mention, you will have frequency X and somebody might have frequency X + d. One thing you have to understand about radio signals is that no signal can be perfectly clean. Your frequency X will have small components of X+d. And vice versa. In other words, you're interefering with his signal and he's interefering with your signal all the time. Guard band is an empty frequency band that is used to minimize that effect but sometimes it is not enough. The signal leeches through.

There are certainly other ways cross channel interference can happen. Above are all I can remember right now.
posted by 7life at 9:25 AM on May 1, 2012


Even in the presence of co-channel interference or crosstalk in the RF system, you can only hear another party's conversation in analog (e.g. AMPS) systems, which are no longer in use in the United States.

Co-channel interference in GSM can degrade the call quality or cause it to drop, but it cannot let you hear someone else's call. Each call in a GSM system is transmitted over-the-air after being encrypted with a unique session key known only to the network and the phone for which the call is intended. If your phone receives somebody else's call over the air due to co-channel interference, timing issues, or any other reason, it will attempt to decrypt the call using a different (incorrect) key, which will not produce any intelligible speech.

Co-channel interference is always present by design in systems like CDMA and UMTS/HSPA, since everything transmits on the same frequency. The transmissions are separated at the physical layer by the long code (in which each phone uses one out of something like 2^42 possible codes) and, as in GSM, the voice data is also encrypted with a unique key shared between the network and the phone for which the call is intended. So, like GSM, even if your phone receives and attempts to decode someone else's signal, it will fail to produce any intelligible speech.

There are no production networks using voice over LTE (yet), so we can ignore that.

The gist of it is that there is nothing in the radio portion of the network that can let your phone pick up someone else's call. That said, once the call hits your carrier's wired network, it is decrypted for land-based transmission to the party you are calling (and re-encrypted at the called party's cell site, if you are calling another cell phone). Thus, erroneous conditions that occur in the land-based network between your serving cell site and the other party *can* in some cases allow you to be connected to others' calls. This used to be much more common in the era of mechanical (including computer-controlled mechanical) telephone switching, but can still happen in digital TDM systems.

As others have alluded to, there could be timing problems in PDH-based switching systems (such as those based on T-1, T-3, & etc signaling) which can cause a delay from the time your phone hands off to a new radio channel and the time the wired portion of the network switches your call to the new cell site (tower). Additionally, it is possible (but increasingly rare) for a TDM switch in the wired network to assign your call to the wrong time slot on a T-carrier trunk. This could result in a potentially quite long period of being connected to the wrong call, but modern TDM switches typically scan all the active calls to ensure they are switched correctly; the delay between assigning the call to the wrong time slot and then detecting and correcting the error could account for the brief snippet of someone else's call.

There are other oddball conditions that can happen due to problems in the wired infrastructure on cellular calls, too. A few times over the past ten years, I have been in the middle of a cell phone call when audio from the person I'm talking to stops and a few seconds later I hear a recording, "We're sorry, your call could not be completed as dialed. Please try your call again later. Switch ID {blah blah blah}". This is usually caused when your phone needs to hand off from one cell site to another and there is an unrecoverable (yet detectable, and, in fact, detected) error in switching the call from the old tower to the new tower in the land-based portion of the carrier's network. It happens frequently enough that they want to play an error message instead of simply allowing the call to drop, but seldom enough that they don't have a separate recorded message for it.


posted by Juffo-Wup at 1:32 PM on May 1, 2012


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