Cell phone madness?

July 26, 2006 3:15 PM

Is it possible to do this large group cell phone activity easily and affordably?

For a corporate activity I would like to purchase 100 cheap or disposable cell phones that would make about 10 calls to a speed dialed number over the course of a 10 or 15 minute period.

This activity would take place in a hotel ballroom. The way it would work is that 50 of the phones would have a speed dialed number that was one of the 50 other phones. On a signal the group with the speed dial would call their assigned number. They would then try to find the person on the other end of the phone based on description only. When they have found their person, they will answer some assigned questions. The group with speed dial would then switch cell phones with another member of the speed dial group. They would then repeat the process, One member of the receiving group would have a special piece of information that they would share with each person they meet and they would ask them to share the information with everyone they meet. The activity would end when that information had been this disseminated throughout the group.

Can this be done for under $750? How much might it cost maximum or minimum? What could go wrong technically or organizationally? What else might I do with a large group and cell phones?

posted by Xurando to Technology (8 answers total)

What you're saying is that you want the phones for $7.50 each. Do you really think that is possible?

The only way I can see that this could happen for anything like the amount of money you're willing to spend is if you convinced some cell company to do it as a publicity stunt. But for that there'd have to be a pretty compelling case of it being good publicity for the cell company.

The other potential problem I would see with this is cell system capacity. It might not be possible to have 100 traffic channels open at once in the same exact location. That's not typical usage and cell systems aren't built out to handle that kind of extraordinary loading.

To make it work, it might be necessary to bring in a microcell, which again would require direct participation of the cell company.
posted by Steven C. Den Beste at 3:35 PM on July 26, 2006

I see no reason why these would have to be cell phones. Wouldn't 2-way radios (the kind used in large manufacturing plants and construction sites) work equally well? As long as they have some kind of "dialing number" and it's not just a walkie-talkie it should suit your purposes. This frees you from having to actually activate 100 cell plans and then cancel them a few hours later. But you still need the hardware. I'd suggest calling around and see if anyone rents these things. Check into the kinds of places that supply production companies on location, construction sites, etc.
posted by Rhomboid at 3:45 PM on July 26, 2006

You might be able to find a cellphone provider interested in sponsoring this event.
posted by aubilenon at 4:02 PM on July 26, 2006

I think you can get sets of decent two-way radios for ~$20 a pair these days. Bulk discount might take you the rest of the way. Set pairs of radios to the same frequency, and you've got your speed-dialed connection - turn the radio on, and it talks only to the other radio on the frequency. However, at the low end, they're more likely to offer only 25 preset unique frequencies, instead of the 50 that you seek.
posted by -harlequin- at 4:54 PM on July 26, 2006

Could you use keychain digital recording devices instead of cell phones?
posted by sexymofo at 7:25 PM on July 26, 2006

I'm with Steven: suddenly dumping 100 calls onto the network is [i]not[/i] going to work.

Nitpick: the 22-channel radios you see cheaply these days are comprised of 14 license-free FRS frequencies, and 8 GMRS frequencies, which require a license. (Also note that the "privacy codes" are just PL (CTCSS) tones superimposed on the signal; two people with different codes on the same channel will hear each other if they're both talking at the same time.)

I like the creativity of this idea, but I'm not sure how it'll work out.
posted by fogster at 9:06 PM on July 26, 2006

I don't remember if GSM carriers are spaced 225 KHz or 250 KHz apart. So here's a GSM system scenario: Begin with a 30 MHz license, the biggest available for GSM. That's 15 MHz uplink, 15 MHz downlink.

Usually they divide that into 5 groups, because of the reuse problem. Any given cell is permitted to use 3 MHz each way. Each carrier has 200 KHz bandwidth and since it's TDMA they carry up to 8 calls each. If they're spaced 225 KHz apart, then there are 13 carriers. One of those is the paging channel; it doesn't carry calls. So you have 12*8 = 96 potential calls in each sector, which has to serve the entire sector (which can be several square miles) and even if you used it all it still wouldn't support your experiment.

Going with a CDMA system, the numbers are a bit more friendly. Carrier frequencies are 1.2288 MHz bandwidth and spaced 1.5 MHz apart. The whole 15 MHz can be used by every cell sector, so you've got up to 10 carrier frequencies. With 100 calls all in the same exact area, there will be more noise floor than usual; I suspect you'd be lucky to get 20 calls per carrier; 15 is more likely. So the total capacity for the sector is about 150 simultaneous calls, and you're going to try to use two thirds of them.

Problem is, if they've built out that sector with 10 full carrier frequencies, it's because other people are using that sector heavily. Unless you do your thing at 4 in the morning, it won't be the case that two-thirds of the sector capacity is free just when you need it.

The only way either system would even have a chance of doing what you want is if the cell system brought in a microcell just for you. That's what they do at the Superbowl, for example; there will actually be several cells operating at once at that location. (Easier to do with CDMA than with TDMA...) But it isn't cheap, because each cell needs a high speed data link (many megabits per second) back to the central office, and those can't be set up at a moment's notice.

Even if someone gave you those hundred phones, I just don't think that what you want to do is possible.
posted by Steven C. Den Beste at 9:57 PM on July 26, 2006

I blew it: IS-95 carriers are 1.25 MHz apart, not 1.5 MHz. In a 30 MHz system, there are 11 carriers, not 10. So it's something like 165 calls.

It's difficult to really say for sure; CDMA has "soft capacity", unlike TDMA systems which have "hard capacity". The number of calls a given carrier can support can vary a lot depending on about a thousand different factors. It's impossible to predict. Sometimes it can be more than 30 calls per carrier; sometimes it's a whole lot less. Unfortunately, the situation you're describing is not a good one for capacity for reasons much too complicated to go into.
posted by Steven C. Den Beste at 10:01 PM on July 26, 2006