GSM: Network Architecture
The GSM technical specifications define
the different entities that form the GSM network by defining their functions
and interface requirements.
The GSM network can be divided into
four main parts:
The architecture of the GSM network
is presented in figure 1.
The Mobile Station (MS).
The Base Station Subsystem (BSS).
The Network and Switching Subsystem (NSS).
The Operation and Support Subsystem (OSS).
figure 1: Architecture
of the GSM network
A Mobile Station consists of two
The Subscriber Identity Module (SIM): It
is protected by a four-digit Personal Identification Number (PIN). In order
to identify the subscriber to the system, the SIM card contains amongst
others a unique International Mobile Subscriber Identity (IMSI). User mobility
is provided through maping the subscriber to the SIM card rather than the
terminal as we done in past cellular systems.
Mobile equipment/terminal (ME): There
are different types of terminals (MN) distinguished principally by their
power and application:
`fixed' terminals mainly installed in cars. Their maximum allowed output
power is 20W
Base Station Subsystem
portable terminals can also be installed in vehicles. Their maximum allowed
output power is 8W.
handheld terminals; their popularity is owed to their weight and volume,
which is continuously decreasing. According to some specification these
terminals may emit up to 0.8W. However, as technology has evolved
their maximum allowed power ouput is limited to 0.1W.
The BSS provides the interface between
the ME and the NSS. It is in charge of the transmission and reception.
It may be divided into two parts:
Base Station Controller (BSC): It
controls a group of BTSs and manages their radio ressources. A BSC is principally
charge of handoffs, frequency hopping, exchange functions
and power control over each managed BTSs.
Base Transceiver Station (BTS) or Base Station:
it maps to transceivers and antennas used in each cell of the network.
It is usually placed in the center of a cell. Its transmitting power
defines the size of a cell. Each BTS has between 1-16 transceivers
depending on the density of users in the cell.
Its main role is to manage the communications
between the mobile users and other users, such as mobile users, ISDN users,
fixed telephony users, etc. It also includes data bases needed in order
to store information about the subscribers and to manage their mobility.
The different components of the NSS are described below.
Operation and Support Subsystem (OSS)
MSC: the central component of
the NSS. The MSC performs the switching functions of the network. It also
provides connection to other networks.
GMSC: A gateway that interconnects
two networks: the cellular network and the PSTN. It is in
charge of routing calls from the fixed network towards a GSM user. The
GMSC is often implemented in the same machines as the MSC.
HLR: The HLR stores information
of the suscribers belonging to the coverage area of a MSC; it also stores
the current location of these subscribers and the services to which they
have access. The location of the subscriber maps to the SS7 address
of the Visitor Location Register (VLR) associated to the MN.
VLR: contains information from
a subscriber's HLR necessary to provide the subscribed services to visiting
users. When a subscriber enters the covering area of a new MSC, the VLR
associated to this MSC will request information about the new subscriber
to its corresponding HLR. The VLR will then have enough data to assure
the subscribed services without needing to ask the HLR each time a communication
is established. The VLR is always implemented together with a MSC; thus,
the area under control of the MSC is also the area under control of the
Authentication Center (AuC): It
serves security purposes; it provides the parameters needed for authentication
and encryption functions. These parameters allow verification of the subscriber's
Equipment Identity Register (EIR):
stores security-sensitive information about the mobile equipments. It maintains
a list of all valid terminals as identified by their International Mobile
Equipment Identity (IMEI). The EIR allows then to forbid calls from stolen
or unauthorized terminals (e.g, a terminal which does not respect the specifications
concerning the output RF power).
GSM Interworking Unit (GIWU): The
GIWU provides an interface to various networks for data communications.
During these communications, the transmission of speech and data can be
It is connected to components of
the NSS and the BSC, in order to control and monitor the GSM system.
It is also in charge of controlling the traffic load of the BSS.
It must be noted that as the number of BS increases with the scaling
of the subscriber population some of the maintenance tasks are transferred
to the BTS, allowing savings in the cost of ownership of the
A cell, as identified by its Cell
Global Identity (CGI) number, maps to the radio coverage of a BTS. Similarly
an LA as identified by its Location Area Identity (LAI) number ,
is a cluster of cells served by a single MSC/VLR. A group of
LA under the control of the same MSC/VLR defines the MSC/VLR
area. A Public Land Mobile Network (PLMN) is the area served by
In this paragraph, the description
of the GSM network is focused on the differents functions to fulfil by
the network and not on its physical components. In GSM, five main functions
can be defined:
Transmission: of data and signaling. Not
all the components of the GSM network are strongly related with both types
of types of Tx. While the MSC, BTS and BSC, among others, are involved
with data and signaling, components such as HLR, VLR or EIR registers,
are only concerned with signaling.
Radio Resources Management (RRM).
Mobility Management (MM).
Communication Management (CM).
Operation, Administration and Maintenance (OAM).
Radio Resources Management
The role of the RR function is to
establish, maintain and release communication links between mobile stations
and the MSC. The elements that are mainly concerned with the RR function
are the MN and the BTS. However, since the RR component performs connection
management also during cell handoffs, it also affects the
MSC which is the handoff management component.
The RR is also responsible for the
management of frequency resources as well as varying radio
interface conditions. Main component operations are:
Channel assignment, change and release.
Discontinuous transmission and reception.
The user movements may result a change
in the channel/cell, when the quality of the communication is degrading;
this is known as handoff. Handoffs occur between:
Handoffs are mainly controlled
by the MSC. However to avoid unnecessary signalling, the first two
types of handoffs are managed by the respective BSC (thus, the MSC
is only notified of the handoff).
between channels within a cell
between cells controlled by the same BSC
between cells under the same MSC but controlled by different
between cells controlled by different MSCs.
To perform the handoff the mobile
station controls continuously its own signal strengh and the signal strengh
of the neighboring cells. The list of cells that must be monitored by the
mobile station is given by the base station. Power measurements allow to
decide which is the best cell in order to maintain the quality of
the communication link. Two basic algorithms are used for handoffs:
Mobility Management (MM)
The `minimum acceptable performance' algorithm. When
the quality of the transmission degrades, the power level of the
mobile is increased, until the increase of the power level has no
effect on the quality of the signal. Upon this link layer hint, a handoff
The `power budget' algorithm. Here the handoff
pre-empts the power increase, to obtain a good SIR.
The MM component handles:
Location Management: Location
is managed through periodicaly or on-demand. At power-on time, the MH signals
an IMSI attach. On-demand location updates are signalled
when the MN moves to a different PLMN or new location area (LA). The signal
is sent to the new MSC/VLR, which forwards it to the subscriber's
HLR. Upon authorization in the new MSC/VLR, the subscriber's
HLR removes the registration entry of the MN at the old MSC/VLR.
If after the update time interval, the MN has not registered, it is then
deregistered. On power-off, the MN performs an IMSI detach.
security and authentication:
Authentication involves the SIM card and the Authentication Center. A secret
key, stored in the SIM card and the AuC together with a ciphering algorithm
called A3, are used to authenticate the user. The MN and the AuCcompute
a SRES through A3 using the secret key and a nonce
generated by the AuC. If the two computed SRES are the same, the subscriber
is authenticated. The different services to which the subscriber has access
are also checked. Next the a security check is performed in the equipment
identity (IMEI). If the IMEI number of the mobile is authorized in the
EIR, the mobile station is allowed to connect the network. To assure user
confidentiality, the user is registered with a Temporary Mobile
Subscriber Identity (TMSI) after its first location update procedure.
Enciphering is another option to guarantee a very strong security.
Communication Management (CM)
The CM component manages:
Call control (CC): it controls call
setup, management and tear-down in relation to management of type of service.
Call routing is the primary task for this component. To reach a mobile
subscriber, a user dials the Mobile Subscriber ISDN (MSISDN) number which
a country code
a national destination code; this identifies the subscriber's operator
a code mapping to the subscriber's HLR.
The call is then passsed
to the GMSC (if the call is originated from a fixed network) that 'knows'
the HLR corresponding to the particular MSISDN number. The GMSC signals
the HLR for call routing information. The HLR requests this information
from the subscriber's current VLR. This VLR allocates temporarily
a Mobile Station Roaming Number (MSRN) for the call. The MSRN number is
the information returned by the HLR to the GMSC.
It is latter that routes the call through the MSRN number, to the subscriber's
MSC/VLR. In the subscriber's current LA, the mobile is paged.
Supplementary Services management: This involves the MN and the
SMS management: Here the GSM network contacts the
Short Message Service Center through the two following interfaces:
SMS-GMSC for Mobile Terminating Short Messages (SMS-MT/PP). It has the
same role as the GMSC.
SMS-IWMSC for Mobile Originating Short Messages (SMS-MO/PP).
and Maintenance (OAM)
The OAM component allows the operator
to monitor and control the system as well as modify the configuration
of the elements of the system. Not only the OSS is part of the OAM, but
also the BSS and NSS participate in functions such as:
provide the operator with all the information it needs. This information
is forwarded to the OSS to control the network.
perform self-test tasks in addition to the OAM functions.
control of multiple BTSs by the BSS.