[pchelpers] Re: Modems and mobiles

I've had the opportunity to "play" with a couple of modem-emulating
cellphones and even a dedicated cellular modem in the last few months.

All of these had a [expensive in the case of the phones] serial port that
you can attach to a PC. They all accept the standard (& a few very
non-standard!) AT commands recognised by landline (conventional) modems. To
the PC they act pretty much identically like a regular modem. The difference
is how the data travels to the destination - the cellular networks have
dedicated data channels for just this sort of stuff, so I guess the
Modulation/DEModulation doesn't really happen unless you're talking to a
conventional modem at the other end; in this case I would suspect that the
part of the cellular network actually connected to the other modem does the
analogue MO/DEM stuff & the phone communicates with the network digitally.

I've also seen older phones (a Nokia 5110 GSM set) do this too - in this
case the phone had a serial port, but doesn't recognise AT commands, but a
Windows driver does & translates this to the phone's proprietary serial
format to do the work.

Note that it tends to be the higher-end phones that support this sort of
feature, but with the higher speed wireless digital networks starting to
gain momentum I hope this will filter down to the cheaper models. Pretty
cool when it works, but a good connection is a must as mentioned
previously - we had trouble with our only GSM network as we're in a bit of a
shadow, but the newer CDMA network available here in NZ worked better - the
cellsite's just across the road.

Jeremy

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