GSM architecture : Network Subsystem (NSS) (6/6)

GSM architecture : Network Subsystem (NSS)

The Equipment Identity Register (EIR) can be implemented as an option by the network
operator. The EIR permits the detection of stolen terminal equipment used in GSM networks
by checking the IMEI (International Mobile Equipment Identity) against the data stored in the
EIR. This check is carried out independently of the SIM card, and only applies to the mobile
station in question.

All the components which control and forward the call, and are responsible for security and
mobility management, that is the MSC, HLR, VLR, AUC and EIR, form the Network
Subsystem (NSS).

GSM architecture : Network Subsystem (NSS) (5/6)

GSM architecture : Network Subsystem (NSS)

The Authentication Center (AuC) contains the customer data necessary to protect
connections against unauthorised access, and is mostly integral to the HLR. The AUC
checks the information stored in the Subscriber Identity Module, that is the SIM card, for
correspondence with its own register. If the data proves to be identical, the authentication of
the subscriber is successful, and he is given permission to enter the network. If the SIM card
is stolen, authorisation to access the network is disabled very easily via the AUC.
Additionally, the AUC provides necessary information to cipher the air interface.

GSM architecture : Network Subsystem (NSS) (4/6)

GSM architecture : Network Subsystem (NSS)

For GSM customer data acquisition, there is a register, the so-called Home Location Register
(HLR), in which each network operator registers the customer data necessary for dealing
with traffic. The HLR supplies these data to all VLRs in which the GSM customers involved
are to be found at any given moment. Inversely, the VLR in question informs the HLR of the
location area of the customer, and is thus able to give routing information when calls come
in. The HLR data contain information on access rights with regard to roaming, service rights
with regard to voice, fax and data services, and additional subscribed services.

GSM architecture : Network Subsystem (NSS) (3/6)

GSM architecture : Network Subsystem (NSS)

One of these registers is the Visitor Location Register (VLR), which is usually to be found in
the MSC, but is a functional unit in its own right. It is designed as a dynamic subscriber file
with dedicated geographical areas of responsibility, the so-called Location Areas. The VLR
acquires the data of all GSM customers in its areas and is always well informed of their
whereabouts. It assists the MSC in the acquisition of charge-relevant data with subscriber
information. The bills are prepared from these data in the Billing Center. But where does the
VLR get the GSM customer data from?

GSM architecture : Network Subsystem (NSS) (2/6)

GSM architecture : Network Subsystem (NSS)

The MSC is responsible for the following functions:
- It forwards incoming and outgoing calls.
- It makes a connection to other MSCs in the same mobile radio network and makes
connections with other mobile radio networks and to fixed networks.
- It monitors and controls the calls.
- It is responsible for call data acquisition and the forwarding of signalling information to
connected registers or data bases.
- In order to monitor, route and control mobile telephone calls in GSM networks, several
registers are connected to the MSC.

GSM architecture : Network Subsystem (NSS) (1/6)

GSM architecture : Network Subsystem (NSS)

The Base Station Subsystem forwards the signals to the Network Subsystem (NSS) where
speech and circuit-switched data are controlled and forwarded to other networks if
necessary. The NSS provides data relevant to security and mobility.

The speech signals processed by the transcoder reach the Mobile Services Switching Center
(MSC) via the A interface. The MSC serves as a digital exchange for the forwarding of
messages, connecting mobile subscribers with each other or with subscribers in other
networks such as the Public Switched Telephone Network, the ISDN network, or data
networks.

GSM architecture : Base Station Subsystem (BSS) (3/3)

GSM architecture : Base Station Subsystem (BSS)

The transcoder is the third element in the BSS and is needed to convert 64 kbps original
speech into a 16 kbps signal of speech description parameters to ensure a spectrum-efficient
modulation on the air interface. BTS, BSC and TC together form the Base Station Subsystem
(BSS).