Page 67 - From GMS to LTE

Global System for Mobile Communications (GSM) 53

by the radio network. Before the message can be sent, however, the mobile device needs
to authenticate itself and ciphering is usually activated as well.
Once the connection is secured against eavesdropping, the mobile device is usually
assigned a new TMSI by the network, which it uses for the next connection establish-
ment to identify itself instead of the IMSI. By the use of a constantly changing tempo-
rary ID, the identity of a subscriber is not revealed to listeners during the first phase of
the call, which is not ciphered. Once TMSI reallocation has been performed, the loca-
tion area update message is sent to the network, which acknowledges the correct recep-
tion. After receipt of the acknowledgment, the connection is terminated and the mobile
device returns to idle state.
If the old and new location areas are under the administration of two different MSC/
VLRs, a number of additional steps are necessary. In this case, the new MSC/VLR has
to inform the HLR that the subscriber has roamed into its area of responsibility. The
HLR then deletes the record of the subscriber in the old MSC/VLR. This procedure is
called an inter‐MSC location update. From the mobile point of view, however, there
is no difference compared to a standard location update as the additional messages are
only exchanged in the core network.

1.8.2 The Mobile‐Terminated Call
An incoming call for a mobile subscriber is called a mobile‐terminated call by the GSM
standards. The main difference between a mobile network and a fixed‐line PSTN
network is that the telephone number of the mobile subscriber does not hold any infor-
mation about where the subscriber is located. In the mobile network, it is thus neces-
sary to query the HLR for the current location of the subscriber before the call can be
forwarded to the correct switching center.
Figure 1.42 shows the first part of the message flow for a mobile‐terminated call initi-
ated from a fixed‐line subscriber. From the fixed‐line network, the Gateway‐Mobile
Switching Center (G‐MSC) receives the telephone number (MSISDN) of the called
party via an ISUP IAM message. The subsequent message flow on this interface is as
shown in Figure 1.6 and the fixed‐line network does not have to be aware that the called
party is a mobile subscriber. The G‐MSC in this example is simply a normal MSC with
additional connections to other networks. When the G‐MSC receives the IAM mes-
sage, it sends a Send Routing Information (SRI) message to the HLR to locate the sub-
scriber in the network. The MSC currently responsible for the subscriber is also called
the subscriber’s Visited Mobile Switching Center (V‐MSC).
The HLR then determines the subscriber’s IMSI by using the MSISDN to search
through its database and thus is able to locate the subscriber’s current V‐MSC. The
HLR then sends a Provide Roaming Number (PRN) message to the V‐MSC/VLR to
inform the switching center of the incoming call. In the V‐MSC/VLR, the IMSI of the
subscriber, which is part of the PRN message, is associated with a temporary Mobile
Station Roaming Number (MSRN), which is returned to the HLR. The HLR then trans-
parently returns the MSRN to the G‐MSC.
The G‐MSC uses the MSRN to forward the call to the V‐MSC. This is possible as the
MSRN not only temporarily identifies the subscriber in the V‐MSC/VLR but also
uniquely identifies the V‐MSC to external switches. To forward the call from the
G‐MSC to the V‐MSC, an IAM message is used again, which, instead of the MSISDN,