GSM -> Architecture
Architecture of the GSM network
A GSM network is composed of several functional entities, whose functions and
interfaces are defined.
Figure 1 shows the layout of a generic GSM
network. The GSM network can be divided into three broad parts.
The Mobile Station is carried by the subscriber, the Base Station Subsystem
controls the radio link with the Mobile Station. The Network
Subsystem, the main part of which is the Mobile services Switching Center,
performs the switching of calls between the mobile and other fixed or mobile
network users, as well as management of mobile services, such as authentication.
Not shown is the Operations and Maintenance center, which oversees the
proper operation and setup of the network. The Mobile Station and
the Base Station Subsystem communicate across the Um interface, also known as
the air interface or radio link. The Base Station Subsystem
communicates with the Mobile service Switching Center across the A interface.
,---------------------------------------------------------------------------,
| Um A |
| Interface A-bis Interface |
| | Interface | ,--------------------, |
| | ,-----, ,-----, | |
| | ,----------|----------, | | | VLR | | HLR | | ,-------, |
| ,-----, | ,-----, | | `-----' `-----' | | Other | |
| | SIM | | | | BTS | | ,-----, | | | ,-----, | | MSCs | |
| `--,--' | `-----'---| | | | | |--------------'-------' |
| | , | : | | BSC |-----------| MSC | | |
| ,-----, /| | ,-----,---| | | | | |--------------,------, |
| | MS |' | ,---| BTS | | `-----' | | | `-----' | / PSTN / \ |
| `-----' |/ | `-----' | | ,-----, ,-----, | \ ISDN / |
| ' `----------|----------' | | | EIR | | AC | | `------' |
| | `-----' `-----' | |
| | Base Station Subsystem `--------------------' |
| Network Subsystem |
`----------------------------------------------------------------------------'
SIM Subscriber Identity Module HLR Home Location Register
MS Mobile Station VLR Vistor Location Register
BTS Base Transceiver Station EIR Equipment Identity Register
BSC Base Station Controller AC Authentication Center
MSC Mobile services Switching Center PSTN Public Switched Telecomm Network
VLR Visitor Location Register ISDN Integrated Services Digital Network
FIGURE 1
Mobile Station
The mobile station (MS) consists of the physical equipment, such as the radio
transceiver, display and digital signal processors, and a smart card called the
Subscriber Identity Module (SIM). The SIM provides personal
mobility, so that the user can have access to all subscribed services
irrespective of both the location of the terminal and the use of a specific
terminal. By inserting the SIM card into another GSM cellular
phone, the user is able to receive calls at that phone, make calls from that
phone, or receive other subscribed services.
The mobile equipment is uniquely identified by the International Mobile
Equipment Identity (IMEI). The SIM card contains the
International Mobile Subscriber Identity (IMSI), identifying the subscriber, a
secret key for authentication, and other user information. The
IMEI and the IMSI are independent, thereby providing personal mobility.
The SIM card may be protected against unauthorized use by a password or
personal identity number.
Base Station Subsystem
The Base Station Subsystem is composed of two parts, the Base Transceiver
Station (BTS) and the Base Station Controller (BSC). These
communicate across the specified Abis interface, allowing (as in the rest of
the system) operation between components made by different suppliers.
The Base Transceiver Station houses the radio tranceivers that define a cell
and handles the radiolink protocols with the Mobile Station. In
a large urban area, there will potentially be a large number of BTSs deployed.
The requirements for a BTS are ruggedness, reliability, portability, and
minimum cost.
The Base Station Controller manages the radio resources for one or more BTSs.
It handles radiochannel setup, frequency hopping, and handovers, as
described below. The BSC is the connection between the mobile and
the Mobile service Switching Center (MSC). The BSC also
translates the 13 kbps voice channel used over the radio link to the standard 64
kbps channel used by the Public Switched Telephone Network or ISDN.
Network Subsystem
The central component of the Network Subsystem is the Mobile services Switching
Center (MSC). It acts like a normal switching node of the PSTN or
ISDN, and in addition provides all the functionality needed to handle a mobile
subscriber, such as registration, authentication, location updating, handovers,
and call routing to a roaming subscriber. These services are
provided in conjuction with several functional entities, which together form the
Network Subsystem. The MSC provides the connection to the public
fixed network (PSTN or ISDN), and signalling between functional entities uses
the ITUT Signalling System Number 7 (SS7), used in ISDN and widely used in
current public networks.
The Home Location Register (HLR) and Visitor Location Register (VLR),
together with the MSC, provide the callrouting and (possibly international)
roaming capabilities of GSM. The HLR contains all the
administrative information of each subscriber registered in the corresponding
GSM network, along with the current location of the mobile. The
current location of the mobile is in the form of a Mobile Station Roaming Number
(MSRN) which is a regular ISDN number used to route a call to the MSC where the
mobile is currently located. There is logically one HLR per GSM
network, although it may be implemented as a distributed database.
The Visitor Location Register contains selected administrative information
from the HLR, necessary for call control and provision of the subscribed
services, for each mobile currently located in the geographical area controlled
by the VLR. Although each functional entity can be implemented as
an independent unit, most manufacturers of switching equipment implement one VLR
together with one MSC, so that the geographical area controlled by the MSC
corresponds to that controlled by the VLR, simplifying the signalling required.
Note that the MSC contains no information about particular mobile stations
- this information is stored in the location registers.
The other two registers are used for authentication and security purposes.
The Equipment Identity Register (EIR) is a database that contains a list of
all valid mobile equipment on the network, where each mobile station is
identified by its International Mobile Equipment Identity (IMEI). An
IMEI is marked as invalid if it has been reported stolen or is not type
approved. The Authentication Center is a protected database that
stores a copy of the secret key stored in each subscriber's SIM card, which is
used for authentication and ciphering of the radio channel.
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