GSM tutorial network architecture
an overview or tutorial of the network architecture and hardware used in
the GSM system
The architecture of the GSM system with its hardware can
broadly be grouped into three main areas: the mobile station, the base station
subsystem, and the network subsystem. Each area performs its own functions and
when used together they enable the full operational capability of the system to
be realised.
Mobile station
Mobile stations (MS), mobile equipment (ME) or as they are
most widely known, cell or mobile phones are the section of a GSM cellular
network that the user sees and operates. In recent years their size has fallen
dramatically while the level of functionality has greatly increased. A further
advantage is that the time between charges has significantly increased.
There are a number of elements to the cell phone, although
the two main elements are the main hardware and the SIM.
The hardware itself contains the main elements of the mobile
phone including the display, case, battery, and the electronics used to generate
the signal, and process the data receiver and to be transmitted. It also
contains a number known as the International Mobile Equipment Identity (IMEI).
This is installed in the phone at manufacture and "cannot" be changed. It is
accessed by the network during registration to check whether the equipment has
been reported as stolen.
The SIM or Subscriber Identity Module contains the
information that provides the identity of the user to the network. It contains
are variety of information including a number known as the International Mobile
Subscriber Identity (IMSI).
Base station subsystem
The Base Station Subsystem (BSS) section of the GSM network
is fundamentally associated with communicating with the mobiles on the network.
It consists of two elements, namely the Base Transceiver Station (BTS) and the
Base Station Controller (BSC).
The BTS used in a GSM network comprises the radio transmitter
receivers, and their associated antennas that transmit and receive to directly
communicate with the mobiles. The BTS is the defining element for each cell. The
BTS communicates with the mobiles and the interface between the two is known as
the Um interface with its associated protocols.
The BSC forms the next stage back into the GSM network. It
controls a group of BTSs, and is often co-located with one of the BTSs in its
group. It manages the radio resources and controls items such as handover within
the group of BTSs, allocates channels and the like. It communicates with the
BTSs over what is termed the Abis interface.
Network subsystem
The network subsystem contains a variety of different
elements, and is often termed the core network. It provides the main control and
interfacing for the whole mobile network. It includes elements including the
MSC, HLR, VLR, Auc and more as described below:
The main element within the core network is the Mobile
switching Services Centre (MSC). The MSC acts like a normal switching node
within a PSTN or ISDN, but also provides additional functionality to enable the
requirements of a mobile user to be supported. These include registration,
authentication, call location, inter-MSC handovers and call routing to a mobile
subscriber. It also provides an interface to the PSTN so that calls can be
routed from the mobile network to a phone connected to a landline. Interfaces to
other MSCs are provided to enable calls to be made to mobiles on different
networks.
To enable the MSC to perform its functions it requires data
from a number of databases. One is known as the Home Location Register (HLR). It
contains all the administrative information about each subscriber along with
their last known location.
When a user switches on their phone, the phone registers with
the network and from this it is possible to determine which BTS it communicates
with so that incoming calls can be routed appropriately. Even when the phone is
not active (but switched on) it re-registers periodically to ensure that the
network (HLR) is aware of its latest position.
There is one HLR per network, although it may be distributed
across various sub-centres to for operational reasons.
Another of the databases is known as the Visitor Location
Register (VLR). This contains selected information from the HLR that enables the
selected services for the individual subscriber to be provided.
The VLR can be implemented as a separate entity, but it is
commonly realised as an integral part of the MSC, rather than a separate entity.
In this way access is made faster and more convenient.
The third register is the Equipment Identity Register (EIR).
The EIR is the entity that decides whether a given mobile equipment may be
allowed onto the network. Each mobile equipment has a number known as the
International Mobile Equipment Identity. This number, as mentioned above, is
installed in the equipment and is checked by the network during registration.
Dependent upon the information held in the EIR, the mobile may be allocated one
of three states allowed onto the network, barred access, or monitored in case
its problems.
The final register is the Authentication Centre (AuC). The
AuC is a protected database that contains the secret key also contained in the
user's SIM card. It is used for authentication and for ciphering on the radio
channel.
Another element in the network is the Gateway Mobile
Switching Centre (GMSC). The GMSC is the point to which a ME terminating call is
initially routed, without any knowledge of the MS's location. The GMSC is thus
in charge of obtaining the MSRN (Mobile Station Roaming Number) from the HLR
based on the MSISDN (Mobile Station ISDN number, the "directory number" of a MS)
and routing the call to the correct visited MSC. The "MSC" part of the term GMSC
is misleading, since the gateway operation does not require any linking to an
MSC.
The SMS-G or SMS gateway is the term that is used to
collectively describe the two Short Message Services Gateways defined in the GSM
standards. The two gateways handle messages directed in different directions.
The SMS-GMSC (Short Message Service Gateway Mobile Switching Centre) is for
short messages being sent to an ME. The SMS-IWMSC (Short Message Service
Inter-Working Mobile Switching Centre) is used for short messages originated
with a mobile on that network. The SMS-GMSC role is similar to that of the GMSC,
whereas the SMS-IWMSC provides a fixed access point to the Short Message Service
Centre.
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