TMSI's and Identity theft?

Dieter Spaar spaar at
Sat Jan 10 08:15:49 UTC 2009

On Sat, 10 Jan 2009 12:38:54 +0900, "Harald Welte" <laforge at> wrote:

> The IMSI ATTACH/DETACH procedure is an optional procedure that the BTS can
> demand by setting some flag in SYSTEM INFORMATION on the BCCH.
> If enabled, the MS will use a special IMSI ATTACH flag in the location update,
> and it has to send an IMSI DETACH message before it is switched off. Not sure
> how useful that really is... but I definitely want support for it in OpenBSC
> (optionally).  Probably time for a config file ;)

As I understand it, the IMSI Attach/Detach, which is signaled by the
ATT flag in SYS_INFO 3 is used to keep track if a phone is actually
available. (IMSI Attach is used when the phone is turned on and
IMSI Detach when it is turned off). This way the network does not
need to page the phone if it knows that it is turned off and
can immediately signal to the caller that the phone is not
available. If I remember, the ATT flag is not set in the
BS11_Init sample code.

Best regards,
Dieter Spaar, Germany                           spaar at

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