28-03-2014, 02:48 PM
GSM AERCHITCTURE
INTRODUCTION
A GSM system is basically designed as a combination of three major subsystems: the network subsystem, the radio subsystem, and the operation support subsystem. In order to ensure that network operators will have several sources of cellular infrastructure equipment, GSM decided to specify not only the air interface, but also the main interfaces that identify different parts. There are three dominant interfaces, namely, an interface between MSC and the base Transceiver Station (BTS), and an Um interface between the BTS and MS.
MOBILE STATION
The MS includes radio equipment and the man machine interface (MMI) that a subscribe needs in order to access the services provided by the GSM PLMN. MS can be installed in Vehicles or can be portable or handheld stations. The MS may include provisions for data communication as well as voice. A mobile transmits and receives message to and from the GSM system over the air interface to establish and continue connections through the system .
Different type of MSs can provide different type of data interfaces. To provide a common model for describing these different MS configuration, ”reference configuration” for MS, similar to those defined for ISDN land stations, has been defined.
Each MS is identified by an IMEI that is permanently stored in the mobile unit. Upon request, the MS sends this number over the signaling channel to the MSC. The IMEI can be used to identify mobile units that are reported stolen or operating incorrectly.
Functions of MS
The primary functions of MS are to transmit and receive voice and data over the air interface of the GSM system. MS performs the signal processing function of digitizing, encoding, error protecting, encrypting, and modulating the transmitted signals. It also performs the inverse functions on the received signals from the BS.
In order to transmit voice and data signals, the mobile must be in synchronization with the system so that the messages are the transmitted and received by the mobile at the correct instant. To achieve this, the MS automatically tunes and synchronizes to the frequency and TDMA timeslot specified by the BSC. This message is received over a dedicated timeslot several times within a multiframe period of 51 frames. We shall discuss the details of this in the next chapter.
Power Levels
These are five different categories of mobile telephone units specified by the European GSM system: 20W, 8W, 5W, 2W,
and 0.8W. These correspond to 43-dBm, 39-dBm, 37-dBm, 33-dBm, and 29-dBm power levels. The 20-W and 8-W units (peak power) are either for vehicle-mounted or portable station use.
The MS power is adjustable in 2-dB steps from its nominal value down to 20mW (13 dBm). This is done automatically under remote control from the BTS, which monitors the received power and adjusts the MS transmitter to the minimum power setting necessary for reliable transmission.
BASE STATION SYSTEM
The BSS is a set of BS equipment (such as transceivers and controllers) that is in view by the MSC through a single A interface as being the entity responsible for communicating with MSs in a certain area. The radio equipment of a BSS may be composed of one or more cells. A BSS may consist of one or more BS. The interface between BSC and BTS is designed as an A-bis interface. The BSS includes two types of machines: the BTS in contact with the MSs through the radio interface and the BSC, the latter being in contact with the MSC. The function split is basically between transmission equipment, the BTS, and managing equipment at the BSC. A BTS compares radio transmission and reception devices, up to and including the antennas, and also all the signal processing specific to the radio interface. A single transceiver within BTS supports eight basic radio channels of the same TDM frame. A BSC is a network component in the PLMN that function for control of one or more BTS. It is a functional entity that handles common control functions within a BTS.
Functions of BTS
As stated, the primary responsibility of the BTS is to transmit and receive radio signals from a mobile unit over an air interface. To perform this function completely, the signals are encoded, encrypted, multiplexed, modulated, and then fed to the antenna system at the cell site. Trans-coding to bring 13-kbps speech to a standard data rate of 16 kbps and then combining four of these signals to 64 kbps is essentially a part of BTS, though, it can be done at BSC or at MSC. The voice communication can be either at a full or half rate over logical speech channel. In order to keep the mobile synchronized, BTS transmits frequency and time synchronization signals over frequency correction channel (FCCH and BCCH logical channels. The received signal from the mobile is decoded, decrypted, and equalized for channel impairments.
INTRODUCTION
A GSM system is basically designed as a combination of three major subsystems: the network subsystem, the radio subsystem, and the operation support subsystem. In order to ensure that network operators will have several sources of cellular infrastructure equipment, GSM decided to specify not only the air interface, but also the main interfaces that identify different parts. There are three dominant interfaces, namely, an interface between MSC and the base Transceiver Station (BTS), and an Um interface between the BTS and MS.
MOBILE STATION
The MS includes radio equipment and the man machine interface (MMI) that a subscribe needs in order to access the services provided by the GSM PLMN. MS can be installed in Vehicles or can be portable or handheld stations. The MS may include provisions for data communication as well as voice. A mobile transmits and receives message to and from the GSM system over the air interface to establish and continue connections through the system .
Different type of MSs can provide different type of data interfaces. To provide a common model for describing these different MS configuration, ”reference configuration” for MS, similar to those defined for ISDN land stations, has been defined.
Each MS is identified by an IMEI that is permanently stored in the mobile unit. Upon request, the MS sends this number over the signaling channel to the MSC. The IMEI can be used to identify mobile units that are reported stolen or operating incorrectly.
Functions of MS
The primary functions of MS are to transmit and receive voice and data over the air interface of the GSM system. MS performs the signal processing function of digitizing, encoding, error protecting, encrypting, and modulating the transmitted signals. It also performs the inverse functions on the received signals from the BS.
In order to transmit voice and data signals, the mobile must be in synchronization with the system so that the messages are the transmitted and received by the mobile at the correct instant. To achieve this, the MS automatically tunes and synchronizes to the frequency and TDMA timeslot specified by the BSC. This message is received over a dedicated timeslot several times within a multiframe period of 51 frames. We shall discuss the details of this in the next chapter.
Power Levels
These are five different categories of mobile telephone units specified by the European GSM system: 20W, 8W, 5W, 2W,
and 0.8W. These correspond to 43-dBm, 39-dBm, 37-dBm, 33-dBm, and 29-dBm power levels. The 20-W and 8-W units (peak power) are either for vehicle-mounted or portable station use.
The MS power is adjustable in 2-dB steps from its nominal value down to 20mW (13 dBm). This is done automatically under remote control from the BTS, which monitors the received power and adjusts the MS transmitter to the minimum power setting necessary for reliable transmission.
BASE STATION SYSTEM
The BSS is a set of BS equipment (such as transceivers and controllers) that is in view by the MSC through a single A interface as being the entity responsible for communicating with MSs in a certain area. The radio equipment of a BSS may be composed of one or more cells. A BSS may consist of one or more BS. The interface between BSC and BTS is designed as an A-bis interface. The BSS includes two types of machines: the BTS in contact with the MSs through the radio interface and the BSC, the latter being in contact with the MSC. The function split is basically between transmission equipment, the BTS, and managing equipment at the BSC. A BTS compares radio transmission and reception devices, up to and including the antennas, and also all the signal processing specific to the radio interface. A single transceiver within BTS supports eight basic radio channels of the same TDM frame. A BSC is a network component in the PLMN that function for control of one or more BTS. It is a functional entity that handles common control functions within a BTS.
Functions of BTS
As stated, the primary responsibility of the BTS is to transmit and receive radio signals from a mobile unit over an air interface. To perform this function completely, the signals are encoded, encrypted, multiplexed, modulated, and then fed to the antenna system at the cell site. Trans-coding to bring 13-kbps speech to a standard data rate of 16 kbps and then combining four of these signals to 64 kbps is essentially a part of BTS, though, it can be done at BSC or at MSC. The voice communication can be either at a full or half rate over logical speech channel. In order to keep the mobile synchronized, BTS transmits frequency and time synchronization signals over frequency correction channel (FCCH and BCCH logical channels. The received signal from the mobile is decoded, decrypted, and equalized for channel impairments.