some info about GRPS - NAT, multicard, international prepaid card, USSD - Unstructured Supplementary Service Data

Robert Michel openmoko at
Mon Dec 18 13:46:58 CET 2006


By using GPRS I was concerned that someone does a DOS
against my MS¹and I have to pay for this high prized
traffic. But it seems that in Germany 3 of 4 GSM provider
use NAT.

But this means no server on the Neo1973 without a trick,
and no Neo1973 to Neo1973 communication without a server
that helps.
One Idea would be to use a NSLU2 at home or a vserver,
prizes starts at 3 Euro/month (Additional IP 1Euro/month):

The vserver will have the handycape that it is not possible
to use iptables (and some other NIC near services like ping,
traceroute...) I think routing via a  virtual network device
could help to use a cheap vserver with a firewall.

So NAT of the GSM provider would protect us to pay traffic
that we don't use and an own server (at home, or vserver)
would allow us to be open for other Neo1973 user.

Then a hint "multicard/twincard" some GSM provider offers
a second (and third) SIM card with the same number for a
single payment of 25 Euro:
With the serial port hack of a NSLU2 it would be possible
to use our old GSM mobile at home and use there GPRS 24/24

I have no experiances with multicards and if the phones
ring simultanious and if SMS are fetchted by only one
device or more.
With a multi-SIM 2 or more on the Neo1973, a second GSM mobil
could forward incomming SMS.

Cheaper Roaming for incomming calls *with* Caller ID
A *second* Neo1973 at home could forward incomming calls via
VoIP to an international prepaid card:
(Free incomming calls in many contries, call fee by VoIP 0.25
To get the information of the caller number, the multicard
could signalisate the incomming caller number and the Neo could
switch the 1. SIM of and fetch the call forwarded by Asterisk
with an international prepaid card
(2. Sim login, call asterisk, asterisk reject, asterisk forwarded
call to 2. Sim)

You see 2 or better 4 SIM card slots inside the Neo1973 would
be comfortable - nice to have :)

Many Prepaidcard uses USSD communication to get the prepaid account
information, but also to use roaming abroad. 

I think that USSD is not a question of the GSM chip, it can be
implemented with the extended AT-commands. But with the freedom
to program what we like - new, better solutions with USSD would
be possible. Because it is possible to ask about the level of
the prepaid account for free, an accurate cost information after
every call (maybe during a call) is possible to show automaticaly.


¹ MS mobile station

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