GSM Network Architecture picture below contains equipment from a typical GSM network. I will try to explain in details about the GSM Network Architecture. Step by step I will explain the main parts of GSM Network Architecture.
GSM Network Architecture divided into three subsystem, The Network Switching Subsystem (NSS), the Base Station Subsystem (BSS) and Network Management Subsystem (NMS)
Network Switching Subsystem (NSS)
The elements of Network Switching Subsystem-
➤MSC (Mobile Services Switching Centre)
➤VLR (Visitor Location Register)
➤ HLR (Home Location Register)
The MSC is responsible for controlling calls in the mobile network. It identifies the origin and destination of a call (either a mobile station or a fixed telephone in both cases), as well as the type of a call. An MSC acting as a bridge between a mobile network and a fixed network is called a Gateway MSC.
An MSC is normally integrated with a VLR, which maintains information related to the subscribers who are currently in the service area of the MSC. The VLR carries out location registrations and updates. The MSC associated with it initiates the paging process. A VLR database is always temporary (in the sense that the data is held as long as the subscriber is within its service area), whereas the HLR maintains a permanent register of the subscribers. In addition to the fixed data, the HLR also maintains a temporary database which contains the current location of its customers. This data is required for routing calls.
In addition, there are two more elements in the NSS: the Authentication Centre (AC) and the Equipment Identity Register (EIR). They are usually implemented as part of HLR and they deal with the security functions.
The Base Transceiver Station (BTS) is a network element maintaining the Air interface. It takes care of Air interface signalling, Air interface ciphering and speech processing. In this context, speech processing refers to all the functions the BTS performs in order to guarantee an error-free connection between the MS and the BTS.
The TransCoder (TC) is a BSS element taking care of speech transcoding, i.e. it is capable of converting speech from one digital coding format to another and vice versa.For transmission over the air interface, the speech signal is compressed by the mobile station to 13Kbits/s (Full Rate) or 5.6Kbits/s (Half Rate). This compression algorithm is known as "Regular Pulse Excitation with Long Term Prediction" (RPE-LTP). However, the standard bit rate for speech in the PSTN is 64Kbits/s Therefore, a converter has to be provided in the network to change the bit rate from one to another. This is called the Transcoder (TC).
The BTS, BSC and TC together form the Base Station Subsystem (BSS) which is a part of the GSM network.
Inside the BSS, all the BTSs and TCs are connected to the BSC(s). The BSC maintains the BTSs. In other words, the BSC is capable of separating (barring) a BTS from the network and collecting alarm information. Transcoders are also maintained by the BSC, i.e. the BSC collects alarms related to the Transcoders.
The operator workstations are connected to the database and communication servers via a Local Area Network (LAN). The database server stores the management information about the network. The communications server takes care of the data communications between the NMS and the equipment in the GSM network known as “Network Elements”. These communications are carried over a Data Communications Network (DCN) w hich connects to the NMS via a router. The DCN is normally implemented using an X.25 Packet Switching Network.
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| GSM Network Elements |
Network Switching Subsystem (NSS)
The elements of Network Switching Subsystem-
➤MSC (Mobile Services Switching Centre)
➤VLR (Visitor Location Register)
➤ HLR (Home Location Register)
The MSC is responsible for controlling calls in the mobile network. It identifies the origin and destination of a call (either a mobile station or a fixed telephone in both cases), as well as the type of a call. An MSC acting as a bridge between a mobile network and a fixed network is called a Gateway MSC.
An MSC is normally integrated with a VLR, which maintains information related to the subscribers who are currently in the service area of the MSC. The VLR carries out location registrations and updates. The MSC associated with it initiates the paging process. A VLR database is always temporary (in the sense that the data is held as long as the subscriber is within its service area), whereas the HLR maintains a permanent register of the subscribers. In addition to the fixed data, the HLR also maintains a temporary database which contains the current location of its customers. This data is required for routing calls.
The main functions of NSS are:
➤Call Control
➤Charging
➤Mobility management
➤Signalling with other networks and the BSS
➤Subscriber data handling
➤Locating the subscriber
Base Station Subsystem (BSS)
The Base Station Subsystem consists of the following elements:
➤BSC Base Station Controller
➤BTS Base Transceiver Station
➤TC Transcoder
The Base Station Controller (BSC) is the central network element of the BSS and it controls the radio network. This means that the main responsibilities of the BSC are: Connection establishment between MS and NSS, Mobility management, Statistical raw data collection, Air and a interface signalling support.
The TransCoder (TC) is a BSS element taking care of speech transcoding, i.e. it is capable of converting speech from one digital coding format to another and vice versa.For transmission over the air interface, the speech signal is compressed by the mobile station to 13Kbits/s (Full Rate) or 5.6Kbits/s (Half Rate). This compression algorithm is known as "Regular Pulse Excitation with Long Term Prediction" (RPE-LTP). However, the standard bit rate for speech in the PSTN is 64Kbits/s Therefore, a converter has to be provided in the network to change the bit rate from one to another. This is called the Transcoder (TC).
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| BSS |
Main function of BSS
➤Radio Network control and management. The BSS assigns, monitors and releases traffic and control connections on the radio interface. If necessary it performs handovers within one cell, between two cells under the same or different BSC and between cells connected to different MSCs.
➤Speech transcoding The Transcoder is responsible for decreasing (towards the MS) respectively increasing (towards the MSC) the data rate for speech only (never for data or signalling) according to the transmission restrictions on the air interface.
➤Air interface signalling and data processing
➤Signalling towards the NSS and air interface
Network Management Subsystem
The Network Management Subsystem (NMS) is the third subsystem of the GSM network. The purpose of the NMS is to monitor various functions and elements of the network. These tasks are carried out by the NMS/2000 which consists of a number of Work Stations, Servers and a Router which connects to a Data Communications Network (DCN).
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| NMS |
The functions of the NMS can be divided into three categories:
➤Fault Management
➤Configuration Management
➤Performance Management
These functions cover the whole of the GSM network elements from the level of individual BTSs, up to MSCs and HLRs.
The main functions of the NMS are:
➤Fault Management to detect and cancel faults as soon as possible
to keep the network running correctly.
➤Configuration Management to have centralised authority for managing hardware and software changes in addition to security operations.
➤Performance Management on the one hand to avoid traffic overload and on the other hand to use the installed hardware in the most economic way.
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