1 GSM Network Infrastructure

The following figure depicts a typical GSM network (called, Public Land Mobile Network or PLMN) infrastructure.
Note: The solid lines are for user traffic plus control signalling, if any. The dotted lines represent
control/management signalling/messaging only.

AUC Authentication Center
BSC Base Station Controller
BSS Base Station Subsystem
BTS Base Transceiver System (Antenna System + RadioBase Station)
EIR Equipment Identification Register (for IMEI verification)
IMEI International Mobile Equipment Identity
FNR Flexible Numbering Register (for number portability)
GMSC Gateway MSC
HLR Home Location Register
ISDN Integrated Services Digital Network
IWF Interworking Function
ILR Interworking Location Register (for roaming between AMPS and GSM system)
IWMSC Interworking MSC
MS Mobile Station
MSC Mobile Switching Center
NSS Network Switching Subsystem
OSS Operation and Support System
PDN Public Data Network
PSTN Public Switched Telephone Network
SMS Short Message Service
VLR Visitor Location Register


The GSM divides the infrastructure into the following three parts.
•  Network Switching Subsystems (NSS)
•  Base Station Subsystem (BSS)
•  Network Management Subsystem (NMS)
If we count the Mobile Station (MS) or cell-phone is the 4
th
element.
Any telecommunications network requires some kind of NMS. A part of NMS is generic for any telecom system. The billing
and messaging are two examples. The core of the NSSis the MSC (Mobile Switching Center) which is basically a PSTN
switch with mobility management related enhancement/add-on. The BSS is entirely new (compared to PSTN)that are
required for wireless access and mobility. The following sections of this document provide an overviewof the network
elements and their functions. The role of these elements will be clearer as we learn more.

2 Network Switching Subsystem (NSS) 

Fundamentally, the network and switching subsystems(NSS) is responsible for call connection, supervision and release
operations between calling and called stations, where one or both of them are mobile stations (MS). Other functions include:
•  Handling short messages and packet data (email, faxand a variety of notifications)
•  Providing ‘bearer’ channel for data communications
•  Maintaining database of its own users as well as visitors
•  Variety of authentication and encryption
•  Gateway to PSTN, other mobile networks and data networks including the Internet
Further Reading: http://en.wikipedia.org/wiki/Network_and_Switching_Subsystem
Cellular Mobile Systems and Services (TCOM1010)  2009-April
Day-02_GSM Network Architecture.doc  Monzur Kabir, Ph.D., P.Eng Page 4 of 7
2.1 Home Location Register 
The home location register (HLR) is a database usedfor storing and managing subscriptions. Generally  a PLMN (Public
Land Mobile Network) consists of several HLRs. The first two digits of the mobile directory number (e.g. 0171 2620757) are
the number of the HLR where the mobile subscriber is stored. The data includes permanent data on subscribers (such as
subscriber's service profile) as well as dynamic data (such as current location and activity status). When an individual buys a
subscription from one of the GSM operators, he or she is registered in the HLR of that operator.
Data Elements (Subscriber) Examples:
•  Mobile Station’s Identities:
o  IMSI (International Mobile Subscriber Identity) (the primary Key),
o  Current TMSI (Temporary IMSI)
o  IMEI (International Mobile Equipment Identity)
•  Mobile Station’s Telephone number
o  MSISDN (Mobile Stations ISDN number)
o  Current MSRN (Mobile Station Roaming Number), if assigned
•  Name and address of the subscriber
•  Current service subscription profile
•  Current location (MSC/VRL address)
•  Authentication and encryption keys
o  Individual Subscriber Authentication Key (KI
)
•  Mobile Country Code (MCC) and MNC (Mobile Network Code)
•  List of MSC/VLR that beltion Registeongs to this HLR
2.2 Mobile Switching Center and Visitor Locar 
The mobile switching center (MSC) performs the telephony switching function. A mobile station must be attached to a single
MSC at a time (either homed or visitor), if it is currently active (not switched off). The visitor location register (VLR) is a
database attached to an MSC to contain information about its currently associated mobile stations (notjust for visitors).
Note: A basic switch (that is a PSTN/ISDN switch) already has a database for its telephone connections. However, it is not
designed to include visitors since a visitor has telephone number that does not belong to this switch.That is why a separate
VLR is needed. An MSC, with the help of the HLR, allocates a visitor a ‘local’ telephone number (the MSRN), which is not
currently allocated to anyone. This allocation is temporary (like visitor ID card). The VLR stores theMSRN as mobile
station’s telephone number (along with other information). However, VLR also stores some information like ‘security triple’
(authentication and encryption information) for each mobile station that are currently attached to theMSC. A VLR stores
such information not only for its visitors but alsofor the homed mobile stations. From this perspective VLR is for homed
mobile stations as well.
Data
•  Information of currently attached mobile stations
o  IMSI/TMSI numbers
o  MSISDN/MSRN numbers
o  Security triple (authentication and encryption information)
o  Location Area Identity (where the mobile station iscurrently located)
•  List of base stations that belong to this MSC/VLR (by their BSIC or Base Station Identity Code)
•  List of location areas that belong to this MSC/VLR (by their LAI or Location Area Identity code)
2.3 Authentication Center
The authentication center (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. The GSM has standard encryption and authentication algorithm which are used to dynamically compute challenge
keys and encryptions keys for a call.
Cellular Mobile Systems and Services (TCOM1010)  2009-April 
2.4 Equipment Identity Register
The equipment identity register (EIR) is a databasethat contains information about the identity of mobile equipment that
prevents calls from stolen, unauthorized, or defective mobile stations. The AUC and EIR can be implemented as stand-alone
nodes or as a combined AUC/EIR node.
2.5 Gateway MSC 
The Gateway MSC (GMSC) is an MSC that connects the PLMN (Public Land Mobile Network) to a PSTN/ISDN.
2.6 GSM Interworking Unit/Function
The GSM Interworking (IW) function or Unit (GIWF/U)is for data communication (such as the Internet access) support.
Though the basic function of MSC is voice traffic switching the MSC has additional capability to forward data between the
mobile station and GIWF/U
2.7 Message Service Gateway
The NMS (network Management subsystem) includes a message center. This includes Short Message Service (SMS),
Multimedia Message Service (MMS), Fax, Voice Mail, Email and a variety of notifications. The MSC requires special
capability to forward those messages between the message center and the mobile station.
2.8 Flexible Numbering Register
The local number portability (LNP) service is an advance intelligent network (AIN) service of telecommunications network.
This service allows a person to move his residence to a new city/province and still retain his/her oldtelephone number. The
local telephone service provider/switch will recognize the old telephone number, and no new number will be assigned. A
cell-phone with LNP service can do the same and Flexible Numbering Register (RNR) takes care of that.

3 Base Station Subsystem (BSS) 

All radio-related functions between mobile stationsand network are performed in the base station subsystem (BSS).
The BSS consists of:
•  One base station controller (BSC) and
•  All base transceiver stations (BTS) under the BSC
Further reading: http://en.wikipedia.org/wiki/Base_Station_Subsystem
3.1 Base Transceiver Station
A Base Station Transceiver (BTS) is a radio transceivers station that communicates with the mobile stations. Its backend is
connected to the BSC. More detail about BTS will becovered later. A BTS is usually placed at the center of a cell. Its
transmitting power defines the size of a cell. There are more on this later.
3.2 Base Station Controller
A Base Station Controller (BSC) is a high-capacity switch with radio communication and mobility control capabilities. The
functions of a BSC include radio channel allocation, location update, handover, timing advance, power  control and paging.
Database (Subscriber)
•  Associated IMSI (Primary key)
•  Paging Groups
Database (Network)
•  MSC Address
•  List of Paging Groups
•  List of LAI
•  List of BSIC
•  List of Cell ID (CI)
3.3 Transcoder/Rate Adaptation Unit
The Transcoder/Rate Adaptation Unit (TRAU) is the data rate conversion unit. The PSTN/ISDN switch is aswitch for 64
kbps voice. Current technology permits to decrease the bit-rate (in GSM radio interface it is 13 kbps for full rate and 6.5 kbps
for half rate). Since MSC is basically a PSTN/ISDN  switch its bit-rate is still 64 kbps. That is why arate conversion is
required in between the BSC and MSC (see the figurebelow)

4 Network Management Subsystem (NMS) 

Network Management Subsystem (NMS) includes networkmanagement center (NMC), operations and maintenance center
(OMC) and a variety of other functions (see the network infrastructure diagram at the beginning of this document). A
telecommunications network requires some kind of NMS. A part of NMS is generic for any telecom system.
4.1 Operation and Support System 
The operation support system (OSS) is to do a variety of operation and maintenance works such as commissioning and
integrating new network elements to the existing system, software upgrade, collecting network performance statistics,
reconfiguring network dimension and frequency planning.
4.2 Message Center (MXE) 
The MXE, also called Short Message Service Center (SMSC), is a node that provides integrated voice, fax, and data
messaging. Specifically, the MXE handles short message service, cell broadcast, voice mail, fax mail,  e-mail, and
notification.
4.3 Mobile Intelligent Service Node
Like PSTN/ISDN the GSM system supports AIN (Advanced Intelligent Network) services through its mobile  intelligent
services node (MISN). This enables an operator to develop an innovative service and deploy that in itsnetwork