GPRS - fairly off topic
Bruce M Simpson
bms at spc.org
Fri Sep 6 10:05:14 BST 2002
Here's what little I know about GPRS.
I've used it with one of the early Motorola Timeport models. The client
software simply initiates a PPP session to the phone at 115.2Kb 8-N-1.
The phone pretends to be a Hayes-like modem, with some hacks to the AT
command set to support GPRS specific commands. Running the vendor provided
client software and using a tool such as those available from sysinternals.com
to tap the serial driver traffic will show you this.
Depending on the phone configuration, it will either assign you the real
IP address that it's been assigned by the GPRS network, or it will assign
you a dummy RFC1918 IP. The phone acts as a NATting bridge; it handles
conversion of PPP-encapsulated IP datagrams to GPRS PDUs. Getting the phone
'flexed' by Motorola is necessary to run things like IPSEC properly, because
this NAT causes no end of problems.
'Flexing', it seems, refers to re-programming the part of the phone's flash
which is responsible for those settings, and it's something you can't do from
the SIM. Even if you know Motorola's vendor-specific file names on the SIM
card, they don't cover it. People more familiar with the esoterica of phone
unlocking will know what I mean here.
It simply acts as a bridge of sorts after that. I must emphasize there doesn't
appear to be a way to send GPRS PDUs at the link layer directly from your PC,
they appear to have locked it down fairly well.
Hope this helps.
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