Localization and calling in GSM
GSM has an additional feature of worldwide localization of user’s .GSM System always know where a user currently is, and the same phone number is valid worldwide.
- GSM performs periodic location updates even if a user does not use the mobile station.
- The HLR always contains information about the current location
- The VLR currently responsible for the MS informs the HLR about location changes.
- As soon as an MS moves into the range of a new location area (new VLR) the HLR sends all user data needed to the new VLR.
- Changing VLRs with uninterrupted availability of all services is also called roaming.
- Following are some numbers which are needed to locate an MS and to address.
- Mobile station international ISDN number (MSISDN): This number consists of the country code (CC), the national destination code (NDC) (i.e., the address of the network provider, e.g., 179), and the subscriber number (SN).
- International mobile subscriber identity (IMSI): IMSI consists of a mobile country code (MCC) the mobile network code (MNC) (i.e., the code of the network provider), and finally the mobile subscriber identification number (MSIN).
- Temporary mobile subscriber identity (TMSI): TMSI is selected by the current VLR and is only valid temporarily and within the location area of the VLR
- Mobile station7 roaming number (MSRN): MSRN contains the current visitor country code (VCC), the visitor national destination code (VNDC), the identification of the current MSC together with the subscriber number.
- MOBILE TERMINATED CALL:
- A situation in which a station calls a mobile station.
- Basic steps needed to connect the calling station with the mobile user are:
- Step 1: a user dials the phone number of a GSM subscriber.
- Step 2: The fixed network (PSTN) notices that the number belongs to a user in the GSM network and forwards the call setup to the Gateway MSC.
- Step 3: The GMSC identifies the HLR for the subscriber and signals the call setup to the HLR.
- Step 4: The HLR now checks whether the number exists and whether the user has subscribed to the requested services, and requests an MSRN from the current VLR.
- Step 5:MSRN is received
Step 6: the HLR determine the MSC responsible for the MS and forwards this information to the GMSC.
- Step 7: The GMSC now forward the call setup request to the MSC indicated.
- Step 8: MSC requests the current status of the MS from the VLR.
- Step 9&10: If MS is available MSC initiates paging in all cells.
- Step11: BTSs of all BSSs transmit this paging signal to MS If the MS answers (12 and 13), the VLR perform security checks (encryption). The VLR signals to the MSC to set up a connection to the MS (steps 15 to 17).