Location Update

When an MS is first turned on, it must first “camp on” a suitable cell. This
largely involves scanning the air interface to select a cell with a suitably
strong received signal strength and then decoding the information broadcast
by the BTS on the BCCH. Generally, the MS will camp on the cell with
the strongest signal strength, provided that cell belongs to the home PLMN
(HPLMN) and provided that the cell is not barred. The MS then registers
with the network, which involves a process known as location updating, as
shown in Figure 3-10.

The sequence begins with a channel request issued by the MS on the
RACH. This includes an establishment cause, such as location updating, voice call establishment, and emergency call establishment. In the example
of Figure 3-10, the cause is location updating.

The BSS allocates an SDCCH for the MS to use. It instructs the MS to
move to the SDCCH by sending an Immediate Assignment message on the AGCH. The MS then moves to the SDCCH and sends the Location Updating
Request. This contains a set of information including the location area
identity (as received by the MS on the BCCH) and the mobile identity. The
mobile identity is usually either the International Mobile Subscriber Identity
(IMSI) or the Temporary Mobile Subscriber Identity (TMSI). This is sent
through the BSS to the MSC using a generic message known as Complete
Layer 3 Info. This message is included as part of an SCCP Connection
Request. Hence, it uses connection-oriented SCCP.

If the subscriber attempts to register with TMSI and the TMSI is
unknown in the MSC/VLR, then the MSC/VLR may request the MS to
send the IMSI (not shown in the figure). Equally, the MSC/VLR may
request the MS to send the IMEI so that it can be checked (also not shown
in the figure).

Upon receipt of the location updating request, the MSC/VLR may
attempt to authenticate the subscriber. If the MSC/VLR does not already
have authentication information for the subscriber, then it requests that
information from the HLR, using the Mobile Application Part (MAP) operation
Send Authentication Info. The HLR/AuC sends a MAP return result
(RR) with up to five authentication vectors, known as triplets. Each triplet
contains a random number (RAND) and a signed response (SRES).

The MSC sends an Authentication Request to the MS. This contains only
the RAND. The MS performs the same calculations as were performed in
the HLR/AuC and sends an Authentication Response containing an SRES
parameter. The MSC/VLR checks to make sure that the SRES received
from the MS matches that received from the HLR/AuC. If a match is made,
then the MS is considered authenticated.

At this point, the MSC/VLR uses the MAP operation Update Location to
inform the HLR of the subscriber’s location. The message to the HLR
includes the subscriber’s IMSI and the SS7 Global Title Address (GTA) of
the MSC and VLR. The HLR immediately sends a MAP Cancel Location
message to the VLR (if any) where the subscriber had previously been registered.
That VLR deletes any stored data related to the subscriber and
issues a return result to the HLR.

The HLR uses the MAP operation Insert Subscriber Data to the VLR to
inform the VLR about a range of data regarding the subscriber in question,
including information regarding supplementary services. The VLR
acknowledges receipt of the information. The HLR then issues a return
result to the MAP Update Location.

Upon receipt of that return result, the MSC/VLR sends the DTAP message
Location Updating Accept to the MS. It then clears the SCCP connection to the BSS. This causes the BSS to release the MS from the SDCCH by
sending a Channel Release message to the MS.

A number of optional messages have been excluded in Figure 3-10. For a
complete understanding of all the options, the reader is referred to GSM
specification 04.08. A number of messages shown in Figure 3-10 (Channel
Request, Immediate Assignment, Channel Release) are common to many
traffic scenarios. For the sake of brevity, they are not shown in the following
call examples. 74