3. The GSM Network

GSM provides recommendations, not requirements. The GSM specifications
define the functions and interface requirements in detail but do not address the
hardware. The reason for this is to limit the designers as little as possible but still
to make it possible for the operators to buy equipment from different suppliers.
The GSM network is divided into three major systems: the switching system (SS),
the base station system (BSS), and the operation and support system (OSS).
The Switching System
The switching system (SS) is responsible for performing call processing and
subscriber-related functions. The switching system includes the following
functional units:

• home location register (HLR)—The HLR is a database used for
storage and management of subscriptions. The HLR is considered the
most important database, as it stores permanent data about
subscribers, including a subscriber's service profile, location
information, and activity status. When an individual buys a
subscription from one of the PCS operators, he or she is registered in
the HLR of that operator.
• mobile services switching center (MSC)—The MSC performs the
telephony switching functions of the system. It controls calls to and
from other telephone and data systems. It also performs such functions
as toll ticketing, network interfacing, common channel signaling, and
others.
• visitor location register (VLR)—The VLR is a database that
contains temporary information about subscribers that is needed by
the MSC in order to service visiting subscribers. The VLR is always
integrated with the MSC. When a mobile station roams into a new MSC
area, the VLR connected to that MSC will request data about the
mobile station from the HLR. Later, if the mobile station makes a call,
the VLR will have the information needed for call setup without having
to interrogate the HLR each time.
• authentication center (AUC)—A unit called the AUC provides
authentication and encryption parameters that verify the user's identity
and ensure the confidentiality of each call. The AUC protects network
operators from different types of fraud found in today's cellular world.
• equipment identity register (EIR)—The EIR is a database that
contains information about the identity of mobile equipment that
prevents calls from stolen, unauthorized, or defective mobile stations.
The AUC and EIR are implemented as stand-alone nodes or as a
combined AUC/EIR node.
The Base Station System (BSS)
All radio-related functions are performed in the BSS, which consists of base
station controllers (BSCs) and the base transceiver stations (BTSs).

• BSC—The BSC provides all the control functions and physical links
between the MSC and BTS. It is a high-capacity switch that provides
functions such as handover, cell configuration data, and control of
radio frequency (RF) power levels in base transceiver stations. A
number of BSCs are served by an MSC.
• BTS—The BTS handles the radio interface to the mobile station. The
BTS is the radio equipment (transceivers and antennas) needed to
service each cell in the network. A group of BTSs are controlled by a
BSC.
The Operation and Support System
The operations and maintenance center (OMC) is connected to all equipment in
the switching system and to the BSC. The implementation of OMC is called the
operation and support system (OSS). The OSS is the functional entity from which
the network operator monitors and controls the system. The purpose of OSS is to
offer the customer cost-effective support for centralized, regional, and local
operational and maintenance activities that are required for a GSM network. An
important function of OSS is to provide a network overview and support the
maintenance activities of different operation and maintenance organizations.
Additional Functional Elements
Other functional elements shown in Figure 2 are as follows:
• message center (MXE)—The MXE is a node that provides
integrated voice, fax, and data messaging. Specifically, the MXE
handles short message service, cell broadcast, voice mail, fax mail, email,
and notification.
• mobile service node (MSN)—The MSN is the node that handles the
mobile intelligent network (IN) services.
• gateway mobile services switching center (GMSC)—A gateway
is a node used to interconnect two networks. The gateway is often
implemented in an MSC. The MSC is then referred to as the GMSC.
• GSM interworking unit (GIWU)—The GIWU consists of both
hardware and software that provides an interface to various networks
for data communications. Through the GIWU, users can alternate
between speech and data during the same call. The GIWU hardware
equipment is physically located at the MSC/VLR.