GSM architecture
A GSM network is composed of several functional entities, whose functions and interfaces are defined. Figure 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.
Mobile Station
We can simply call as mobile phone. These mobile devices are referring signals by sim card through inbuilt signal transmitter.The MS consists of the physical equipment, such as — The radio transceiver, Digital signal processors, The SIM card.
Functions,
• Provides the receptor for SMS messages.
• Enabling the user to toggle between the voice and data use.
• Facilitates access to voice messaging systems.
The Base Station Subsystem(BSS)
BSS is a interface layer and it’s contains two parts,
- The Base Transceiver Station(BTS)
- The Base Station Controller (BSC)
The BTS and the BSC communicate across the specified Abis interface, enabling operations between components that are made by different suppliers.The radio components of a BSS may consist of four to seven or nine cells. A BSS may have one or more base stations. The BSS uses the Abis interface between the BTS and the BSC.
The Base Transceiver Station (BTS)
The BTS houses the radio transceivers that define a cell and handles the radio link protocols with the MS. In a large urban area, a large number of BTSs may be deployed. A BTS is usually placed in the center of a cell. Its transmitting power defines the size of a cell. Each BTS has between 1 and 16 transceivers, depending on the density of users in the cell.
Functions ,
Encoding, encrypting, multiplexing, modulating, and feeding the RF signals to the antenna.
• Time and frequency synchronizing.
• Voice through full- or half-rate services.
• Decoding, decrypting, and equalizing received signals.
The Base Station Controller (BSC)
The BSC manages the radio resources for one or more BTSs. It handles radio channel setup, frequency hopping, and handovers. 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 (PSDN). It assigns and releases frequencies and time slots for the MS. The function of the BSC is to allocate the necessary time slots between the BTS and the MSC(Mobile switching center).
functions,
- Control of frequency hopping.
• Performing traffic concentration to reduce the number of lines from the MSC.
• Providing an interface to the Operations and Maintenance Center for the BSS.
• Reallocation of frequencies among BTSs.
• Time and frequency synchronization.
• Power management.
• Time-delay measurements of received signals from the MS.
The Network Switching Subsystem (NSS)
The Network switching system (NSS), the main part of which is the Mobile Switching Center (MSC), performs the switching of calls between the mobile and other fixed or mobile network users, as well as the management of mobile services such as authentication.
The switching system includes the following functional elements:
• Home Location Register (HLR)
• Mobile Services Switching Center (MSC)
• Visitor Location Register (VLR)
• Authentication Center (AUC)
• Equipment Identity Register (EIR)
Home Location Register FR (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 in the form of SIM, then all the information about this subscription is registered in the HLR of that operator. As soon as mobile subscriber leaves its current local area, the information in the HLR is updated.
Mobile Services Switching Center (MSC)
The central component of the Network Subsystem is the MSC. The MSC performs the switching of calls between the mobile and other fixed or mobile network users, as well as the management of mobile services such as registration, authentication, location updating, handovers, and call routing to a roaming subscriber. It also performs such functions as network interfacing, common channel signaling. Every MSC is identified by a unique ID.
Visitor Location Register FR (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.
Equipment Identity Register (EIR)
The Equipment Identity Register (EIR) is a database that contains a list of all valid mobile equipment on the network, where its International Mobile Equipment Identity (IMEI) identifies each MS. An IMEI (International Mobile Equipment Identity)is marked as invalid if it has been reported stolen or is not type approved. Prevents calls from stolen, unauthorized or defective mobile devices.
Authentication Center FR (AUC)
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. The AUC protects network operators from different types of fraud found in today’s cellular world.
