This function is the first mobility management (MM) action performed by a mobile station
(MS). If the mobile subscriber has inserted a subscriber identification module (SIM) for
2G, or user services identity module (U-SIM) for 3G into the mobile station, or has turned
on the mobile station, the SGSN is informed of the activated state of the MS. The HLR
receives information on the new MS location and subscription data is retrieved from the
HLR. The attach function is used to establish a mobility management context (MM context)
for an MS in the SGSN. After a successful attach, subscriber data is available in
the SGSN location register (SLR). Mobility management allocates a packet temporary
mobile subscriber identity (P-TMSI) and the MS derives a temporary logical link identifier
(TLLI) from the P-TMSI (only for 2G).
Attach procedure without a change of SGSN
The attach procedure without a change of SGSN since detach is shown in Fig. 2.1:
– (1) The MS sends an attach request to the SGSN, which in turn attaches the relevant
international mobile subscriber identifier (IMSI)/P-TMSI to the data record in the
– The SGSN network element sends an update location message to the HLR to inform
the HLR of the new location of the MS so that it can update the subscriber database.
If no MM context exists for the MS anywhere in the network, packet-switched authentication
– (2) The SGSN sends an attach accept to the MS.
Attach procedure with a change of SGSN
If the MS initiates an attach in an SGSN that differs from the SGSN that served the MS
previously, MM requests the identification data (IMSI, authentication triplets for 2G or
quintets for 3G) from the previous SGSN. If interworking with the previous SGSN fails,
MM starts the identification procedure towards the MS to get the IMSI required to interwork
with the HLR.
The attach function is also executed if an MM context already exists, in order to make
an MS available for mobile terminating requests. This procedure is known as re-attach.
Attach procedure on PDP context setup
To save MM resources, certain MSs do not automatically attach to the SGSN after activation,
but wait until the MS sets up a PDP context.
Combined GPRS/IMSI attach
Upon receipt of a combined GPRS/IMSI attach request, MM triggers a location update
towards the corresponding VLR using the Gs interface after the packet mobility management
procedure has been completed successfully. The network operation mode indicates
to the MS whether the Gs interface is supported, i.e. the MS triggers the combined
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