Secure telephones

[Thomas Shaddack]

On Sun, 18 Jul 2004, Bill Stewart wrote:

> If you're trying to build a usable cellphone,
> you've got much more stringent design criteria than a deskphone.

I am painfully aware of it.

> You've got packaging requirements that force you into
> serious industrial design if you want something pocket-sized
> with good battery life, plus you've got to implement all the
> cellular interface features.

Or use the off-the-shelf modules for industrial applications that already 
do it, and add some glue logic.

> If you're willing to build a backpack-phone, that's a lot simpler,
> because you can use a laptop with a
> [pick-your-favorite-cellular-data-standard] card
> and either a wired headset or a Bluetooth frob for a BT headset.

Check the Gumstix and the Enfora Enabler specs. The first is the 
equivalent of a grossly stripped-down laptop (80x20x6 mm, few mA sleep, 50 
mA command-wait, 250mA full power w/o Bluetooth), the second one is the 
equivalent of a comm card (GSM/GPRS, 50x30x3 mm, tri-band 5mA standby). 

The laptop approach is good for prototyping, though.

> I'm not aware of any cellular data cards in PDA-usable format
> (unless you've got a PDA big enough for PCMCIA),
> but you could take a GSM etc. phone with a wired interface to a PDA.

I'd try the Enfora module in that case. RS232 for data and control, and 
analog I/O for voice.

The PDA approach definitely has its merit.