GPRS CID and APN

[Antti Kaijanmäki <antti kaijanmaki net>]

Hi!

I was supposed to take on this matter at the end of May, but there
already was some discussion[1] about GPRS CIDs and APNs and I think that
this is something that should be investigated properly. This stuff is
not as clear to me as I want it to be and it's crucial that I and others
too understand this completely.

First of all APN is always needed with GSM Internet connection. APN are
also used to select between different subscription models of a service
provider. Many (most?) service providers use "internet" as APN, but not
everyone, and there is no way to automatically detect the correct APN.
If a wrong APN is selected modem just connects to the network, but
Internet connection does not work. There is no error messages or what so
ever.


APN can be set by initializing the modem with special command. 

    AT+CGDCONT=<CID>, "IP", "<APN>"

some resources[2] claim that defining settings with AT+CGDCONT could be
dangerous as it might mess up some settings on the phone. Of course we
do not want that. But I have never or heard someone would
have experienced any side effects.


The other suggested method is to use *99# as dial sequence as it should
use the default connection settings set on a mobile phone. This does not
work at least for me. There also might be wrong configuration in mobile
phone or the configuration might be missing. Besides, if *99# just
magically works my job will be rather pointless ;)


Some modems contain preloaded connection settings. Each setting is
identified with Connection ID, CID. I would like to investigate how
different mobile phones and data modems provide these preloaded or
default settings, if any, and if these default CIDs are only related to
data modems.

Unfortunately I have very limited set of devices at my disposal and all
of them are made by Nokia. Therefore I have written some instructions[3]
how you all can help me to understand this field better :)

=====
I propose that NM should always define the APN. This way there would be
no surprises. I also think that user should create configuration for his
devices even though they might use "internet" as an APN or work with
*99#. This way configuring a new device would be unified between
different devices.

I suggest that NM would initialize GSM modems with

    AT+CGDCONT=1, "IP", "<APN>"

and to select the CID defined above the dial sequence would be

    ATD*99***1#


 -- Antti


[1]
http://mail.gnome.org/archives/networkmanager-list/2008-April/msg00256.html
[2] http://www.woggledog.com/index.asp?subject=GPRS
[3] http://live.gnome.org/NetworkManager/MobileBroadband/GprsCid