Originally posted by: cmetz
deadseasquirrel, the call channel on a cellular connection is compressed with an algorithm designed for human voice. It is definitely possible to create a device that will couple the analog side of a standard telephone modem to the cell phone, but you will find that you will get a connect speed in the 1200 baud range if you're lucky, and you better have ARQ enabled because you're going to be retransmitting a lot.
If you want to carry data on a cellular system in a rational way, you need to use (and pay for) a data channel. This costs actual money. Not unreasonable amounts of money, mind you, but if $40/mo for an analog is cost prohibitive, then just give up.
I second what spidey07 has to say - everybody I know of REALLY who cares about this problem brings in a POTS phone line outside of their normal phone system and connects it to an analog module. It's highly low-tech, which is exactly what you want when things are on fire.
The IWF (Interworking Function) chassis handles negotiation of modems on voice channels, and I can assure you it is much quicker than 1200 baud. Heck, even IS-95a has a channel limit of 14.4 kbps (limited due to the ASIC channel elements on cell site hardware)
CDMA-2000 hardware of course changed the air interface, but also introduced logical channels as opposed to physical channels which provide the suplemental burst for datat applications, which is what would be used. The Federal government uses it for their phones in either manner (voice to voice, or data to data) to pass encryption for their voice/data calls securely. Even telemtry devices that a utility company would use to "phone home" are simple phone devices, no data plan needed.
I have done many tests, Hyperterm to Hyperterm with each of two PCs tethered to a phone to pass data between the two. A channel is a channel, it doesn't matter in the air interface, if you need more, you get more up to a theoretical limit, but at a VERY minimum you will get 14k, your type of call will be routed on the backhaul appropriately.
At any rate, I was able to finally look up my notes, and you would need a sort of terminal server that, like Goosemaster said, you could tether the device to to allow the termination of PPP.
OP, like the others I do agree that this would be an odd way to maintain a mission critical piece of equipment. Too many factors, not just limited to RF coverage, could affect the actual connection.