Page 27 - From GMS to LTE

Global System for Mobile Communications (GSM) 13

Figure 1.10 Interfaces and nodes
in an IP‐based NSS architecture. BSS BSS
A-interface

PSTN Nb
MGW MGW
Mc
MSC-S Nc MSC-S

S-GW
VLR VLR
C and D-interface

HLR

1.6.1 The Mobile Switching Center (MSC), Server and Gateway
The MSC is the central element of a mobile telecommunication network, which is also
called a Public Land Mobile Network (PLMN) in the standards. In a classic circuit‐
switched network, all connections between subscribers are managed by the MSC and
are always routed over the switching matrix even if two subscribers who have estab-
lished a connection communicate over the same radio cell.
The management activities to establish and maintain a connection are part of the Call
Control (CC) protocol, which is generally responsible for the following tasks:

Registration of mobile subscribers: When the mobile device, also referred to as MS, is
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switched on, it registers to the network and is then reachable by all other subscribers
of the network.
Call establishment and call routing between two subscribers.
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Forwarding of SMS messages.
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As subscribers can roam freely in the network, the MSC is also responsible for the
Mobility Management (MM) of subscribers. This activity comprises the following tasks:

Authentication of subscribers at connection establishment is necessary because a
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subscriber cannot be identified as in the fixed network by the pair of copper cables
over which the signal arrives. Authentication of subscribers and the authentication
center (AuC) are further discussed in Section 1.6.4.
If no active connection exists between the network and the mobile device, the MS has
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to report a change of location to the network to be reachable for incoming calls and
SMS messages. This procedure is called location update and is further described in
Section 1.8.1.
If the subscriber changes their location while a connection is established with the
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network, the MSC is part of the process that ensures that the connection is not inter-
rupted and is rerouted to the next cell. This procedure is called handover and is
described in more detail in Section 1.8.3.