GSM network is logically divided in two parts Access network (AN) and Core Network (CN). Access network deals with communication of Mobile handset with the network over radio interface and Core networks deals with routing of the call within same network or to another network like PSTN. Show
Network Architecture for GSM is hierarchical i.e. 1 BSC (Base station controller) handles multiple BTS (Base Transceiver Station), 1 MSC (Mobile-service switching centre) handles multiple BSC. Various entities and their functionality is described in following section
Radio Frequency BandsFrequency bands used by GSM are as follows T-GSM 380 band380,2 MHz to 389,8 MHz: Uplink390,2 MHz to 399,8 MHz DownlinkT-GSM 410 band410,2 MHz to 419,8 MHz: Uplink 420,2 MHz to 429,8 MHz DownlinkGSM 450 band450,4 – 457,6 MHz: Uplink 460,4 – 467,6 MHz: DownlinkGSM 480 band478,8 – 486 MHz: Uplink 488,8 – 496 MHz: DownlinkGSM 710 band728 – 746 MHz: Downlink 698 – 716 MHz: UplinkGSM 750 band777 – 793 MHz: Uplink 747 – 763 MHz: DownlinkT-GSM 810 band806 – 821 MHz: Uplink 851 – 866 MHz: DownlinkGSM 850 band824 – 849 MHz: Uplink 869 – 894 MHz: DownlinkPrimary GSM 900 band890 915 MHz: Uplink 935 960 MHz: DownlinkE-GSM 900 band880 915 MHz: Uplink 925 960 MHz: DownlinkR-GSM 900 band876 915 MHz: Uplink 921 960 MHz: DownlinkDCS 1800 band1710-1785 MHz: Uplink 1805-1880 MHz: DownlinkPCS 1900 band1850-1910 MHz: Uplink 1930-1990 MHz: Downlink RF channel spacing in GSM is 200 kHz, allowing for 41 (T-GSM 380), 41 (T-GSM 410), 35 (GSM 450), 35 (GSM 480), 89 (GSM 710), 74 (GSM 750), 74 (T-GSM 810), 124 (GSM 850), 194 (GSM 900), 374 (DCS 1 800) and 299 (PCS 1900) radio frequency channels, thus leaving a guard band of 200 kHz at each end of the sub-bands. A GSM network comprises of many functional units. These functions and interfaces are explained in this chapter. The GSM network can be broadly divided into −
GSM - The Mobile StationThe MS consists of the physical equipment, such as the radio transceiver, display and digital signal processors, and the SIM card. It provides the air interface to the user in GSM networks. As such, other services are also provided, which include −
The MS also provides the receptor for SMS messages, enabling the user to toggle between the voice and data use. Moreover, the mobile facilitates access to voice messaging systems. The MS also provides access to the various data services available in a GSM network. These data services include −
We will discuss more about GMS services in GSM - User Services. What is SIM?The SIM provides personal mobility so that the user can have access to all subscribed services irrespective of both the location of the terminal and the use of a specific terminal. You need to insert the SIM card into another GSM cellular phone to receive calls at that phone, make calls from that phone, or receive other subscribed services. GSM - The Base Station Subsystem (BSS)The BSS is composed of two parts −
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. A separate high-speed line (T1 or E1) is then connected from the BSS to the Mobile MSC. 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. The BTS corresponds to the transceivers and antennas used in each cell of the network. 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. Each BTS serves as a single cell. It also includes the following functions −
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 is the connection between the mobile and the MSC. 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) or ISDN. It assigns and releases frequencies and time slots for the MS. The BSC also handles intercell handover. It controls the power transmission of the BSS and MS in its area. The function of the BSC is to allocate the necessary time slots between the BTS and the MSC. It is a switching device that handles the radio resources. The additional functions include−
GSM - 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)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. 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 toll ticketing, network interfacing, common channel signaling, and others. Every MSC is identified by a unique ID. Visitor Location Register (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. Authentication Center (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. 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 is marked as invalid if it has been reported stolen or is not type approved. GSM - The Operation Support Subsystem (OSS)The operations and maintenance center (OMC) is connected to all equipment in the switching system and to the BSC. The implementation of OMC is called the operation and support system (OSS). Here are some of the OMC functions−
The operation and Maintenance functions are based on the concepts of the Telecommunication Management Network (TMN), which is standardized in the ITU-T series M.30. Following is the figure, which shows how OMC system covers all the GSM elements. The OSS is the functional entity from which the network operator monitors and controls the system. The purpose of OSS is to offer the customer cost-effective support for centralized, regional, and local operational and maintenance activities that are required for a GSM network. An important function of OSS is to provide a network overview and support the maintenance activities of different operation and maintenance organizations. A simple pictorial view of the GSM architecture is given below − The additional components of the GSM architecture comprise of databases and messaging systems functions −
The following diagram shows the GSM network along with the added elements − The MS and the BSS communicate across the Um interface. It is also known as the air interface or the radio link. The BSS communicates with the Network Service Switching (NSS) center across the A interface. GSM network areasIn a GSM network, the following areas are defined −
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