 Hello friends, myself, Darshan Pandit, assistant professor, department of computer science and engineering from Walton Institute of Technology, Soulapur. So, today we are going to discuss about GSM system architecture. So, at the end of the session student will be able to identify services provided by various subsystem in GSM system architecture. So, system architecture is divided in three parts. That is radio subsystem, network and switching subsystem and operation subsystem that is OSS. So, the diagram shows functional architecture of GSM system. So, radio subsystem consist of MS, BTS, BSS, BSE and NSS that is network switching subsystem consist of MSC, HLR, VLR, GMSC, IWF and OSS consist of EIR, AUC and OMC. So, one by one we will see this subsystem in detail. So, the radio subsystem consist of mobile station, MS and base station subsystem, BSS. The connection between RSS and NSS is established via A interface that is you can see solid lines between RSS and NSS and connection to OSS via O interface that is you can see the dash line. So, the BSS consist of BTS, BSE and MS, BSE is nothing but base trans receiver station, BSE, base station controller and mobile station. So, in this BSS maintains a radio connection between MS and BTS. So, BSS is also responsible for coding and decoding of voice and BSS consist of several BTS. So, base trans receiver station, BTS so it comprises of all radio equipment like antennas required for signal processing, amplifier which are necessary for radio transmission. So, BTS is mainly responsible for forming radio cell. So, BTS is connected to mobile station via UM interface, UM interface is nothing but radio interface. So, BTS connected to BSE via AB's interface. So, BSE basically manages BTS and it reserves radio frequencies. So, BSE also handles the handover from one BTS to another within BSS. So, it performs paging of MS, paging is nothing but getting exact location of mobile station. The BSE also multiplexes radio channels on to fixed network connection at A interface. So, next is mobile station. Mobile station is a equipment and a software needed for communication with a GSM network. So, it consists of SIM that is subscriber identity module SIM card. So, it also consists of IMEI that is international mobile equipment identity. So, it is a unique number given for your mobile phone. Next is PIN that is used for security purpose, personal identity number. So, which is present in your SIM card when you try unlocking PIN and if you fail three times you require PUK that is PIN unblocking key, so which interface is used between MS and BTS. So, UM interface is used between MS and BTS, UM interface is also known as air interface. So, next is network and switching subsystem. So, it is heart of GSM system architecture, it connects wireless network with standard public network. So, NSS also performs handover between different BSS. So, NSS consists of MSC that is mobile service switching center, HLR that is home location register, VLR that is visitor location register. So, mobile service switching center, so it is used to set up connection to other MSC and to BSC via A interface and it also manages several BSE. So, MSC also handles all signaling needed for connection setup, connection release between MS and BSE. So, it is also responsible for handling handover and connection to other MSC, HLR home location register. So, it is used to store user reliant information, so it may be static information or dynamic information. So, static information consists of mobile subscriber identity number and subscribed services. So, whatever user has subscribed to the services that all services comes under static information. Next is international mobile subscriber identity, so it is also comes under static information. So, the dynamic information consists of current location of mobile station. So, where currently the mobile station is situated and mobile subscriber roaming number MSRN. So, this is also changing according to the roaming of mobile station that is why it comes under dynamic information. So, HLR also supports for charging and accounting. So, VLR is associated with each MSC that is in each MSC we are having one VLR. So, mobile station user current location that is associated to MSC it is stored in VLR. So, VLR copies all reliant information for the user from HLR. So, next is operation subsystem OSS, so operation subsystem function for network operation and maintenance. So, in which we are having SS7, signaling system number 7. So, this system is used to send signal to MSC whenever there is any fault in the GSM system. So, OSS consists of three types that is OMC operation and maintenance center, AUC, authentication center, EIR that is equipment identity register. So, OMC consists of traffic monitoring status report of network entity. So, whatever the entities present in the network that is BSS, BTS, BSC, HLR, VLR. So, all status of this network entity is present in OMC, subscriber and security management and it also responsible for accounting and billing. So, OMC monitors and controls all other network entity via O interface. So, next is AUC authentication center, a separate authentication center has been defined to protect user identity and data transmission. So, various algorithms are used for authentication, a keys for encryption situated in a special protected part of HLR. So, the keys are exchanged between sender and receiver for authentication purpose. So, algorithms we may have A3 algorithm or we may have A5 algorithm used for authentication. Next is equipment identity register EIR. So, it is a database for all IMEI numbers. So, in which we are having three type of list, black list, grey list and white list. So, black list IMEI numbers are nothing but stolen devices that is IMEI numbers of stolen devices comes under black list, grey list consists of IMEI numbers which are used for malfunctioning and white list IMEI numbers are nothing but valid IMEI numbers. So, these are the three list present in EIR. So, in which we can identify valid IMEI numbers. So, these are the references which I had used to prepare this video. Thank you.