 Darshan Pandit, Assistant Professor, Department of Computer Science and Engineering from Walton Institute of Technology, Solaapur. So, today we are going to discuss about localization and calling. So, at the end of the session student can differentiate between mobile terminated call MTC and mobile originated call MOC technologies. So, localization and calling comes in two ways that is mobile terminated call and mobile originated call. Mobile terminated call or nothing, but the call originated from landline phone and terminating at your mobile phone and mobile originated call or call originated by your mobile phone. So, in localization and calling GSM performs periodic location update. So, even if a user does not use the mobile station, the GSM system always knows current location of a user because of his phone number which is valid worldwide. So, GSM uses two types of database that is home location register and visitor location register. So, HLR always contains information about current location. So, whenever MS moves into the range of new VLR that is a new location area, HLR sends all user data needed for new VLR. So, changing VLR that is visitor location register with uninterrupted availability of all services is called roaming. So, roaming can take place within network of one service provider. So, service provider are nothing, but BSNL, IDM, Vodafone, Reliance, so this all are service provider. So, roaming can also take place in two service provider in one country, so that is known as national roaming. So, roaming can also take place in different service provider in different countries, so that is known as international roaming. So, in order to locate MS and to address MS, several numbers are needed, so those are mobile station ISDN number, MSISDN, international mobile subscriber identity, IMSI, temporary mobile subscriber identity, TMSI, mobile station roaming number, MSRN, so let us see one by one. So, mobile station international subscriber directory number that is MSISDN is a number used to identify mobile number internationally. So, based on SIM, a mobile station can have many MSISDN as each subscriber assigned with a separate MSISDN to their SIM respectively. So, as an example you can see in the table plus 9-1 is a country code, 2-1-7 is national destination code and 1-2-3-4-5-6-7 is a subscriber number. So, international mobile subscriber identity, IMSI, so here every registered user has an original international mobile subscriber identity, IMSI with a valid IMEI stored in their subscriber identity module that is SIM, so it is an internal unique identification of a subscriber and usually presented as a 15-digit number, so in which first three digit are mobile country code, so you can see in table 404 is a mobile country code followed by mobile network code MNC, so it may be 2 or 3 digit and remaining digits are mobile subscriber identification number MSIN, so it is mostly 9 or 10 digit based on MNC, so next is temporary mobile subscriber identity TMSI, so this TMSI hides the IMSI where GSM uses 4 by TMSI for local subscriber identification, so TMSI is selected by current VLR, so this TMSI temporary, it is a temporary address valid within location area of VLR, so TMSI is stored in VLR and it is not passed to HLR, so next is mobile station roaming number MSRN, so it is another temporary address that hides identity and location of a subscriber in MSRN, so VLR generates this address on request from MSC and the address is also stored in HLR, so using MSRN the incoming calls are channeled to mobile station, the MSRN has same structure as of MSISDN that is country code of the visited network, national destination code, NDC of the visited network and subscriber number in the current mobile network, so just think and write what are the content of mobile station international ISDN number, so the content are like country code CC, national destination code NDC and subscriber number, so let us see what do you mean by MTC, so it is a scenario in which a call is made from landline phone to the mobile station, so where so you can see you are having calling station after that PSTN, PSTN is nothing but public switched telephone network, so it is a switching office to switch call from mobile and landline, so it is connected to a PSTN network, after that GMSC, GMSC is nothing but gateway MSC, HLR is home location register, VLR visitor location register, so you can see MSC, MSC is nothing but mobile switching center to switch the call between BSS, BTS, so we require MSC, so after that BSS base station subsystem and MSC is nothing but mobile station, so here, so in step 1 a user dials phone number of a GSM subscriber, so in step 2 PSTN that is switching office notices and power call to GMSC, so in step 3 GMSC identifies HLR that is to which HLR the mobile station belongs and signals call setup to HLR, so in step 4 and 5, so request MSRN that is mobile station roaming number from VLR, so if mobile station is present in visitor location register that is in new location area then we require MSRN after that in step 6 forward responsible MSC to GMSC, so in which MSC the VLR is present, so that MSC is forwarded to GMSC that is in step 6, so in step 7 forward call to current MSC, so whatever current MSC is there the call is forwarded to current MSC, so in step 8 and 9 get current status of MS that is in which BSS MSC is present we need to identify the exact cell that is in which cell mobile station is present, so in step 10 and 11 paging of MS that is identifying current location of MS, after that in step 2 by line 13, so we need to identify MS to get connected and in step 14 and 15 security check is done that is encryption setup is done, so in step 16 and 17 VLR signals MSC for connection setup, so in this way connection setup is done between calling station and MS. So next is MOC, so this is the scenario of mobile originated call in which mobile station calls a PSTN network that is landline phone, so in this we are having very few steps then compared to MTC, so here in step 1 MS transmit a request for a new connection in step 2 the BSS forward this request to MSC so you can see in the figure, so call is initiated from mobile phone in step 1, so after that BSS forward this request to MSC, so in step 3 and 4 the MSC checks if the user is allowed to set up a call with the requested service, so whether he has allowed to call the landline phone, whether he has subscribed to the service everything is checked in step 3 and 4, so in step 5 to 8 MSC checks the availability of resources through GSM network and in PSTN network, so in step 9 and 10 MSC setups a connection between mobile station and a fixed network. So these are the references which I have used to prepare this video, thank you.