September 11, 2001

5:05 AM

"They just shrug... They don't think that someone is jamming their conversations."A friend of mine bought a cell-phone jammer two months ago and has been happily creating "sanity zones" within which cell phones are rendered inoperable. People have a right to communicate, he conceded. But "I have the right not to participate in that communication process - I really don't need to hear people yelling into a cell phone if I'm standing half a block away."
posted by tranquileye (10 comments total)

Make sure that your friend doesn't bring his jammer to the States; they're illegal here.
posted by Steven Den Beste at 5:06 AM on September 11, 2001

Sorry, I should have read it first. A portable battery-powered device as described can work against AMPS and against TDMA-based systems such as IS-136, used by ATTWS, or GSM, as used by Cingular, but it won't work against IS-95 CDMA, as used by Sprint, Verizon, Qwest and Bell Mobility. AMPS and TDMA can be jammed by jamming the paging channels, so you don't need much power because you're not jamming very many actual frequencies. Seven or eight carefully selected spikes covering maybe 2 MHz total would be plenty. This would not interfere with existing calls but would prevent any new call from being created; good enough. But to jam CDMA you have to actually jam the entire spectrum and you have to use enough power to seriously overwhelm the actual signal (because CDMA is extremely fault tolerant). The CDMA paging channel isn't a separate frequency; it's just a reserved Walsh-code mixed in with the rest of the traffic flow. A portable battery operated device won't be able to do that; you're not going to be able to build a broad-spectrum transmitter that small anyway, and even if you could the battery would only last a minute or two. I figure you'd have to be transmitting about 30 watts overall covering about 70 MHz of spectrum in two disparate bands, and the device itself would probably dissipate about 100 watts of parasitic heat. It's gonna get warm in your pocket, until the batteries run down.

Of course, the device doesn't have to jam constantly. If it jams for about a second, then all CDMA calls in the vicinity would drop (because of the FER heuristic). But that doesn't help the "I'm sick of hearing cell phones ringing" problem, and while it was off it wouldn't prevent anyone from initiating a new call. So it would be more of a revenge weapon -- if it could be built.

This is one of those things which sounds better than it probably really is. It's really tough to jam CDMA.
posted by Steven Den Beste at 5:22 AM on September 11, 2001

Still more reading. The device in question was designed in the UK, where the predominant system is GSM. It could be doing a repeating frequency sweep, which would be enough to disrupt not just paging channels but also traffic channels in GSM. In that case its power output wouldn't be very high and it could indeed be made that small and run off a battery, since it's actually only transmitting narrow band. But the sweep would have to be slow, maybe repeating every ten or fifteen seconds.

They don't use CDMA in the UK -- yet. (They will be in about three years when GSM 3G gets deployed.) The sweep would have to be carefully designed to be quite broad-band in order to disrupt CDMA, and I bet it isn't if it was designed to attack GSM. Also, this wouldn't prevent all incoming calls (i.e. annoying rings) because the paging channel would still work if the sweep wasn't on that particular carrier at that moment, even for GSM. (The sweep wouldn't disrupt the rather loose system timings needed to make GSM work, so the phone wouldn't lose synchronization during the interference. GSM only has to be accurate to a few milliseconds.)

In any case, it remains true that these devices are illegal in the US.
posted by Steven Den Beste at 5:38 AM on September 11, 2001

The device mentioned in the article (the Wave-Shield) is here.
posted by gi_wrighty at 5:47 AM on September 11, 2001

Everytime I read one of Steven's technical posts, I feel like I just saved about a grand in seminar lecture fees. Thanks, man.
posted by Optamystic at 6:18 AM on September 11, 2001

We talked about making my library a cell-free zone. Then we decided it was impossible. While 90% of the phones belong to undergraduates, the rest belong to cardiac surgeons. You can jam, as long as you understand that you're risking peoples lives.
posted by djfiander at 10:20 AM on September 11, 2001

What we need is a device which broadcasts a signal that turns off the annoying ringtones and switches on the vibrator mode which most cell-phones come with. Steven, I charge you with making this vision a reality. Do not let me down.
posted by Hildago at 1:25 PM on September 11, 2001

The FAQ states it will kill CDMA. Here's an elaborate FAQ I found on the WAC1000
http://www.hubgiant.com/faq.html
posted by Nauip at 1:29 PM on September 11, 2001

They're entitled to their opinion but I'd only believe a battery-powered unit could block CDMA for an extended period of time if I actually saw it happen.
posted by Steven Den Beste at 6:09 PM on September 11, 2001

I'd side with Steve. He's probably the internet authority on all things cellular.
posted by gyc at 6:51 PM on September 11, 2001