09-02-2012, 03:41 PM
TECHNICAL AWARENESS OF GSM MOBILE
Principle of Mobile Communication
Multiple Access methodology
The technique of dynamically sharing the finite limited radio spectrum by multiple users is called Multiple Access Technique.
Generally there are three different types of multiple access technologies. They are
• Frequency Division Multiple Access (FDMA)
• Time Division Multiple Access (TDMA)
• Code Division multiple Access (CDMA)
Frequency Division Multiple Access (FDMA):
FDMA is a familiar method of allocating bandwidth, where a base station is allowed to transmit on one or more number of preassigned carrier frequencies and a mobile unit transmits on corresponding reverse channels.
Time Division Multiple Access (TDMA)[b]
In a TDMA system each channel is split up into time segments, and a transmitter is given exclusive use of one or more channels only during a particular time period.
[b]Duplexing and Multiple Access Techniques in use:
No Name of System Multiple Access Duplexing
1 GSM FDMA-TDMA FDD
2 CDMA CDMA FDD
FREQUENCY ALLOCATION
Two frequency bands have been allocated for the GSM system:
• The band 890-915 MHz and 1710-1785 MHz has been allocated for the uplink direction (transmitting from the mobile station to the base station).
• The band 935-960 MHz and 1805-1880 MHz has been allocated for the downlink direction (transmitting from the base station to the mobile station).
ARCHITECTURE OF THE GSM NETWORK
The architecture of the GSM network is presented in figure
Subsystems and network elements in GSM
The GSM network is called Public Land Mobile Network (PLMN). It is organised in three subsystems:
• Base Station Subsystem (BSS)
• Network Switching Subsystem (NSS)
• Network Management Subsystem (NMS)
The three subsystems, different network elements, and their respective tasks are presented in the following.
1. Network Switching Subsystem (NSS)
The Network Switching Subsystem (NSS) contains the network elements MSC, GMSC, VLR, HLR, AC and EIR. The Network Switching Subsystem (NSS)
The main functions of NSS are:
Call control
This identifies the subscriber, establishes a call, and clears the connection after the conversation is over.
Charging
This collects the charging information about a call (the numbers of the caller and the called subscriber, the time and type of the transaction, etc.) and transfers it to the Billing Centre.
Mobility management
This maintains information about the subscriber's location.
Signalling
This applies to interfaces with the BSS and PSTN.
Subscriber data handling
This is the permanent data storage in the HLR and temporary storage of relevant data in the VLR.
Mobile services Switching Centre (MSC)
Mobile-services Switching Center (MSC) performs the switching functions for all mobile stations located in the geographic area covered by its assigned BSSs. Functions performed include interfacing with the Public Switched Telephone Network (PSTN) as well as with the other MSCs and other system entities, such as the HLR, in the PLMN.
Functions of the MSC include:
• Call handling that copes with mobile nature of subscribers (e.g., paging)
• Management of required logical radio-link channel during calls
• Management of MSC-BSS signalling protocol
• Handling location registration and ensuring interworking between Mobile Station and VLR
• Control of inter-BSS and inter-MSC handovers
• Acting as a gateway MSC to interrogate the HLR
• Exchange of signalling information with other system entities
• Standard functions of a local exchange switch in the fixed network (example: charging)
Gateway Mobile services Switching Centre (GMSC)
The GMSC is responsible for the same tasks as the MSC, except for paging. It is needed in case of mobile terminated calls. In fixed networks, a call is established to the local exchange, to which the telephone is connected to. But in GSM, the MSC, which is serving the MS, changes with the subscriber’s mobility. Therefore, in a mobile terminated call, the call is set up to a well defined exchange in the subscriber’s home PLMN. This exchange is called GMSC. The GMSC than interacts with a database called Home Location Register, which holds the information about the MSC, which is currently serving the MS. The process of requesting location information from the HLR is called HLR Interrogation. Given the information about the serving MSC, the GMSC then continues the call establishment process. In many real life implementations, the MSC functionality and the GMSC functionality are implemented in the same equipment, which is then just called MSC. Many operators use GMSCs for breakout to external networks such as PSTNs.
Home Location Register (HLR)
The Home Location Register (HLR) contains the identities of mobile subscribers (called International Mobile Subscriber Identities or IMSIs), their service parameters, and their location information.
In summary, the HLR contains:
• Identity of mobile subscriber
• ISDN directory number of mobile station
• Subscription information on teleservices and bearer services
• Service restrictions (if any)
• Supplementary services
• Location information for call routing
Visitor Location Register (VLR)
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area (i.e., cells) controlled by that VLR.
In summary, the VLR contains:
• Identity of mobile subscriber
• Any temporary mobile subscriber identity
• ISDN directory number of mobile
• A directory number to route calls to a roaming station
• Location area where the mobile station is registered
• Copy of (part of) the subscriber data from the HLR
Equipment Identity Register (EIR)
The Equipment Identity Register (EIR) is accessed during the equipment validation procedure when a mobile station accesses the system. It contains the identity of mobile station equipment (called International Mobile Station Equipment Identity or IMEI) which may be valid, suspect, or known to be fraudulent.
This contains:
• White or Valid list - List of valid MS equipment identities
• Grey or Monitored list - List of suspected mobiles under observation
• Black or prohibited list - List of mobiles for which service is barred.
Authentication Center (AUC)
The Authentication Center (AUC):
• Contains subscriber authentication data called Authentication Keys (Ki)
• Generates security related parameters needed to authorize service using Ki
• Generates unique data pattern called a Cipher Key (Kc) needed for encrypting user speech and data
2. Base Station Subsystem (BSS)
The Base Station Subsystem is responsible for managing the radio network, and it is controlled by an MSC. Typically, one MSC contains several BSSs. A BSS itself may cover a considerably large geographical area consisting of many cells (a cell refers to an area covered by one or more frequency resources). The BSS consists of the following elements:
• BSC Base Station Controller
• BTS Base Transceiver Station
• TRAU Transcoder and Rate Adaptation Unit (often referred to as TC (Transcoder))
Radio path control
In the GSM network, the Base Station Subsystem (BSS) is the part of the network taking care of radio resources, that is, radio channel allocation and quality of the radio connection.
3.OPERATION AND MAINTENANCE CENTER (OMC)
The Operations and Maintenance Center (OMC) is the centralized maintenance and diagnostic heart of the Base Station System (BSS). It allows the network provider to operate,administer, and monitor the functioning of the BSS.
4.OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include:
Billing Center:
Each MSC writes call accounting records to local disk memory. The Billing Center periodically polls the disk records of each MSC to collect the billing data for the PLMN.
Service Center:
The Service Center interfaces with the MSCs to provide special services, such as the Short Message Service (SMS), to mobile subscribers in the PLMN.
The Billing Center and Service Center are not a basic part of the GSM system.
Principle of Mobile Communication
Multiple Access methodology
The technique of dynamically sharing the finite limited radio spectrum by multiple users is called Multiple Access Technique.
Generally there are three different types of multiple access technologies. They are
• Frequency Division Multiple Access (FDMA)
• Time Division Multiple Access (TDMA)
• Code Division multiple Access (CDMA)
Frequency Division Multiple Access (FDMA):
FDMA is a familiar method of allocating bandwidth, where a base station is allowed to transmit on one or more number of preassigned carrier frequencies and a mobile unit transmits on corresponding reverse channels.
Time Division Multiple Access (TDMA)[b]
In a TDMA system each channel is split up into time segments, and a transmitter is given exclusive use of one or more channels only during a particular time period.
[b]Duplexing and Multiple Access Techniques in use:
No Name of System Multiple Access Duplexing
1 GSM FDMA-TDMA FDD
2 CDMA CDMA FDD
FREQUENCY ALLOCATION
Two frequency bands have been allocated for the GSM system:
• The band 890-915 MHz and 1710-1785 MHz has been allocated for the uplink direction (transmitting from the mobile station to the base station).
• The band 935-960 MHz and 1805-1880 MHz has been allocated for the downlink direction (transmitting from the base station to the mobile station).
ARCHITECTURE OF THE GSM NETWORK
The architecture of the GSM network is presented in figure
Subsystems and network elements in GSM
The GSM network is called Public Land Mobile Network (PLMN). It is organised in three subsystems:
• Base Station Subsystem (BSS)
• Network Switching Subsystem (NSS)
• Network Management Subsystem (NMS)
The three subsystems, different network elements, and their respective tasks are presented in the following.
1. Network Switching Subsystem (NSS)
The Network Switching Subsystem (NSS) contains the network elements MSC, GMSC, VLR, HLR, AC and EIR. The Network Switching Subsystem (NSS)
The main functions of NSS are:
Call control
This identifies the subscriber, establishes a call, and clears the connection after the conversation is over.
Charging
This collects the charging information about a call (the numbers of the caller and the called subscriber, the time and type of the transaction, etc.) and transfers it to the Billing Centre.
Mobility management
This maintains information about the subscriber's location.
Signalling
This applies to interfaces with the BSS and PSTN.
Subscriber data handling
This is the permanent data storage in the HLR and temporary storage of relevant data in the VLR.
Mobile services Switching Centre (MSC)
Mobile-services Switching Center (MSC) performs the switching functions for all mobile stations located in the geographic area covered by its assigned BSSs. Functions performed include interfacing with the Public Switched Telephone Network (PSTN) as well as with the other MSCs and other system entities, such as the HLR, in the PLMN.
Functions of the MSC include:
• Call handling that copes with mobile nature of subscribers (e.g., paging)
• Management of required logical radio-link channel during calls
• Management of MSC-BSS signalling protocol
• Handling location registration and ensuring interworking between Mobile Station and VLR
• Control of inter-BSS and inter-MSC handovers
• Acting as a gateway MSC to interrogate the HLR
• Exchange of signalling information with other system entities
• Standard functions of a local exchange switch in the fixed network (example: charging)
Gateway Mobile services Switching Centre (GMSC)
The GMSC is responsible for the same tasks as the MSC, except for paging. It is needed in case of mobile terminated calls. In fixed networks, a call is established to the local exchange, to which the telephone is connected to. But in GSM, the MSC, which is serving the MS, changes with the subscriber’s mobility. Therefore, in a mobile terminated call, the call is set up to a well defined exchange in the subscriber’s home PLMN. This exchange is called GMSC. The GMSC than interacts with a database called Home Location Register, which holds the information about the MSC, which is currently serving the MS. The process of requesting location information from the HLR is called HLR Interrogation. Given the information about the serving MSC, the GMSC then continues the call establishment process. In many real life implementations, the MSC functionality and the GMSC functionality are implemented in the same equipment, which is then just called MSC. Many operators use GMSCs for breakout to external networks such as PSTNs.
Home Location Register (HLR)
The Home Location Register (HLR) contains the identities of mobile subscribers (called International Mobile Subscriber Identities or IMSIs), their service parameters, and their location information.
In summary, the HLR contains:
• Identity of mobile subscriber
• ISDN directory number of mobile station
• Subscription information on teleservices and bearer services
• Service restrictions (if any)
• Supplementary services
• Location information for call routing
Visitor Location Register (VLR)
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area (i.e., cells) controlled by that VLR.
In summary, the VLR contains:
• Identity of mobile subscriber
• Any temporary mobile subscriber identity
• ISDN directory number of mobile
• A directory number to route calls to a roaming station
• Location area where the mobile station is registered
• Copy of (part of) the subscriber data from the HLR
Equipment Identity Register (EIR)
The Equipment Identity Register (EIR) is accessed during the equipment validation procedure when a mobile station accesses the system. It contains the identity of mobile station equipment (called International Mobile Station Equipment Identity or IMEI) which may be valid, suspect, or known to be fraudulent.
This contains:
• White or Valid list - List of valid MS equipment identities
• Grey or Monitored list - List of suspected mobiles under observation
• Black or prohibited list - List of mobiles for which service is barred.
Authentication Center (AUC)
The Authentication Center (AUC):
• Contains subscriber authentication data called Authentication Keys (Ki)
• Generates security related parameters needed to authorize service using Ki
• Generates unique data pattern called a Cipher Key (Kc) needed for encrypting user speech and data
2. Base Station Subsystem (BSS)
The Base Station Subsystem is responsible for managing the radio network, and it is controlled by an MSC. Typically, one MSC contains several BSSs. A BSS itself may cover a considerably large geographical area consisting of many cells (a cell refers to an area covered by one or more frequency resources). The BSS consists of the following elements:
• BSC Base Station Controller
• BTS Base Transceiver Station
• TRAU Transcoder and Rate Adaptation Unit (often referred to as TC (Transcoder))
Radio path control
In the GSM network, the Base Station Subsystem (BSS) is the part of the network taking care of radio resources, that is, radio channel allocation and quality of the radio connection.
3.OPERATION AND MAINTENANCE CENTER (OMC)
The Operations and Maintenance Center (OMC) is the centralized maintenance and diagnostic heart of the Base Station System (BSS). It allows the network provider to operate,administer, and monitor the functioning of the BSS.
4.OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include:
Billing Center:
Each MSC writes call accounting records to local disk memory. The Billing Center periodically polls the disk records of each MSC to collect the billing data for the PLMN.
Service Center:
The Service Center interfaces with the MSCs to provide special services, such as the Short Message Service (SMS), to mobile subscribers in the PLMN.
The Billing Center and Service Center are not a basic part of the GSM system.