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.
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].
The Base Station Subsystem [BSS]
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.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].
The NMS and the GSM Network
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.