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12.3.9 The Network System
The Network System (also called the Switching System) consists of five functional entities.
•
The Mobile Services Switching Centre (MSC). The MSC switches calls between mobile users
and between mobile users and the Public Switching Telecommunication Network (PSTN).
•
The Home Location Register (HLR). The HLR contains data on all mobile subscribers
registered in its Public Land Mobile Network (PLMN).
•
The Visitor Location Register (VLR). The VLR contains data on all subscribers visiting its
MSC service area. The HLR and VLR, together with the MSC, provide the call-routing and
roaming capabilities of the GSM network.
•
The Authentication Centre (AuC). In every subscriber’s SIM card, a secret key for
authentication and encryption over the radio channel is stored. The AuC is a protected
database that stores a copy of the secret key.
•
The Equipment Identity Register (EIR). Each Mobile Station is identified by a code called
the International Mobile Equipment Identity (IMEI). The EIR contains a list of all valid
mobile equipment on the network. If a unit is reported stolen, its IMEI will be marked as
invalid and cannot be used on the network anymore.
As the Mobile Station moves between location areas and MSC service areas, data is transferred
between the HLR and the VLR. By doing this the GSM network will always know where the
Mobile Station is located.
12.4 How does GSM data work?
When a user wishes to make a data call (for more information see paragraph: Data Calls), they
simply dial the number they wish to send information to. The GSM network connects the user’s
call to its GSM Interworking Unit (GIWU), which then completes the call to the remote
terminal. In effect the user is dialling the GIWU, and the GIWU is dialling the remote terminal.
The GIWU acts as a gateway, translating between the GSM set of protocols, and the protocols
used by different types of networks. This allows a GSM network to connect to a range of different
network types, such as PSTN, ISDN or X.25 network.
Once a connection between the mobile user and the remote terminal has been established, data
can then be transmitted. Data can be sent over the GSM network using one of two modes, these
are known as transparent mode and non transparent mode (for more information see paragraphs:
Transparent Mode and Non Transparent Mode).
12.4.1 Data calls
To be able to handle data calls, each MSC must have a dedicated Data Transmission Interface
(DTI)/GSM Interworking Unit (GIWU). The picture on the next side explains how it works.
