Page 68 - From GMS to LTE

54 From GSM to LTE-Advanced Pro and 5G

V-MSC HLR G-MSC PSTN

IAM (MSISDN)
SRI (MSISDN)
PRN (IMSI)
PRN Ack. (MSRN)
SRI Ack. (MSRN)

IAM (MSRN)

Call establishment as
shown in Figure 1.43

Address Complete Message (ACM)
ACM

Answer Message (ANM)
ANM

…
Figure 1.42 Mobile‐terminated call establishment, part 1.

contains the MSRN to identify the subscriber. This has been done as it is possible, and
even likely, that there are transit switching centers between the G‐MSC and V‐MSC,
which are thus able to forward the call without querying the HLR themselves.
As the MSRN is internationally unique instead of only in the subscriber’s home net-
work, this procedure can still be used if the subscriber is roaming in a foreign network.
The presented procedure, therefore, works for both national and international roaming.
As the MSRN is saved in the billing record for the connection, it is also possible to
invoice the terminating subscriber for forwarding the call to a foreign network and to
transfer a certain amount of the revenue to the foreign network operator.
In the V‐MSC/VLR, the MSRN is used to find the subscriber’s IMSI and thus the
complete subscriber record in the VLR. This is possible because the relationship
between the IMSI and MSRN was saved when the HLR first requested the MSRN. After
the subscriber’s record has been found in the VLR database, the V‐MSC continues the
process and searches for the subscriber in the last reported location area, which was
saved in the VLR record of the subscriber. The MSC then sends a Paging message to the
responsible BSC. The BSC in turn sends a Paging message via each cell of the location
area on the PCH. If no answer is received, then the message is repeated after a few
seconds.
After the mobile device has answered the Paging message, an authentication and
ciphering procedure has to be executed to secure the connection in a similar way as
previously presented for a location update. Only then is the mobile device informed
about the details of the incoming call with a Setup message. The Setup message con-
tains, for example, the telephone number of the caller if the Calling Line Identification