Mobile-Terminated Voice Call

Figure 3-12 shows a basic mobile-terminated call from the PSTN. It begins
with the arrival of an IAM at the GMSC. The IAM contains the directory
number of the called subscriber, known as the Mobile Station ISDN Number
(MSISDN). The GMSC uses this information to determine the applicable
HLR for the subscriber and invokes the MAP operation Send Routing
Information (SRI) towards the HLR. The SRI contains the subscriber’s
MSISDN.
The HLR uses the MSISDN to retrieve the subscriber’s IMSI from its
database. Through a previous location update, the HLR knows the
MSC/VLR that serves the subscriber, and it queries that MSC/VLR using
the MAP operation Provide Roaming Number (PRN), which contains the
subscriber’s IMSI. From a pool, the MSC/VLR allocates a temporary number,
known as a Mobile Station Roaming Number (MSRN) for the call and
returns that number to the HLR. The HLR returns the MSRN to the
GMSC.
The MSRN is a number that appears to the PSTN as a dialable number.
Thus, it can be used to route a call through any intervening network
between the GMSC and the visited MSC/VLR. In fact, that is exactly what
the GMSC does. It routes the call to the MSC/VLR by sending an IAM, with
the MSRN as the called party number. Upon receipt of the IAM, the
MSC/VLR recognizes the MSRN and knows the IMSI for which the MSRN was allocated. At this point the MSRN can be returned to the pool for use
with another call.
Next, the MSC requests the BSS to page the subscriber using the Paging
Request message, which indicates the location area in which the subscriber
should be paged. The BSS uses the PCH to page the MS.

Upon receipt of the page, the MS attempts to access the network using a
Channel Request message on the RACH. The BSS responds with an Immediate
Assignment message, instructing the MS to move to an SDCCH. The
MS moves to the SDCCH and, once there, indicates to the network that it
is responding to the page. The BSS passes the response to the MSC.
At this point, the MSC may optionally authenticate the MS (not shown).

It will then proceed to initiate ciphering, which is done in the same manner
as was described previously for a mobile-originated call. Once ciphering is
started, the MSC sends a Setup message to the MS. This is similar to the
Setup message that is sent from an MS for a mobile-originated call, including
information such as the calling party number and the required bearer
capability.

Upon receipt of the Setup message, the MS sends a Call Confirmed message
to the MSC, indicating that it has the information it needs to establish
the call. The Call Confirmed message acts as an instruction to the MSC to
establish apath through to the MS. Therefore, the MSC begins the assignment
procedure, which establishes a circuit between the MSC and the BSS,
and a TCH between the BSS and the MS (rather than an SDCCH). Further
signaling between the MS and the network will now use the FACCH associated
with the TCH to which the MS has been assigned.

Once established on the TCH, the MS starts ringing to alert the user and
informs the network by sending the Alerting message to the MSC. This triggers
the MSC to open a one-way path back to the original caller, generate a
ring-back tone, and send an ACM message back to the originating PSTN
switch via the GMSC.

Once the called user answers, the MS sends a Connect message to the
MSC. This triggers the MSC to send an ANM message back to the originating
switch and to open a two-way path. Finally, it sends Connect
Acknowledge to the MS and conversation begins.