The number dialed to reach a mobile subscriber (MSISDN) contains no information at all about the current location of the subscriber. In order to establish a complete connection to a mobile subscriber, however, one must determine the current location and the locally responsible switch (MSC). In order to be able to route the call to this switch, the routing address to this subscriber (MSRN) has to be obtained. This routing address is assigned temporarily to a subscriber by its currently associated VLR. At the arrival of a call at the GMSC, the HLR is the only entity in the GSM network which can supply this information, and therefore it must be interrogated for each connection setup to a mobile subscriber. An ISDN switch recognizes from the MSISDN that the called subscriber is a mobile subscriber, and therefore can forward the call to the GMSC of the subscriber's home PLMN based on the CC and NDC in the MSISDN. This GMSC can now request the current routing address (MSRN) for the mobile subscriber from the HLR using the MAP . By way of the MSRN the call is forwarded to the local MSC, which determines the TMSI of the subscriber and initiates the paging procedure in the relevant location area . After the MS has responded to the paging call, the connection can be switched through.Several variants for determining the route and interrogating the HLR exist, depending on how the MSRN was assigned and stored, whether the call is national or international and depending on the capabilities of the associated switching centers.
To locate a MS and to address the MS, several numbers are needed:
MOBILE STATION INTERNATIONAL ISDN NUMBER (MSISDN):
The only important number from the user of GSM is phone number. Remember that the phone number is not associated with certain device but with the SIM, which is personalized for user. The number consist of country code(CC) as eg.+49 179 1234567 with 49 for germany. National Destination Code (NDC) is used to locate the network provider and Subscriber number.
INTERNATIONAL MOBILE SUBSCRIBER IDENTITY(IMSI):
GSM uses the IMSI for internal unique identification of a subscriber. IMSI consist of a mobile country code (MCC) and mobile network code(MNC) and finally the mobile subscriber identification number(MSIN).
TEMPORARY MOBILE SUBSCRIBE IDENTITY(TMSI):
To hide the IMSI, which would give away the exact identity of user signaling over the air interface, GSM uses the 4 byte.TMSI is selected by current VLR and is only valid temporarily and within location area of VLR.
MOBILE STATION ROAMING NUMBER (MSRM):
Another temporary address which hides the identity and location of a subscriber is MSRN. The VLR generates this address on request from MSC, and the address is stored in the HLR . MSRN contains current visitory and visitor national destination code(VNDC).
MESSAGE TERMINATED CALL (MTC):
This figure shows the basic steps needed to connect the calling station with the mobile user.
Step 1: A user dials the phone number of GSM subscriber. The fixed network PSTN notices that the number belongs to the user in the GSM network and forwards the call setup to the Gateway MSC .
Step 2: The GMSC identifies the HLR for the subscriber and signals the call setup to the HLR.
Step 3: The HLR now checks whether the number exists and whether the user has subscribed to the requested services, and the requests an MSRN from the current VLR.
Step 4 : After receiving the request from MSRN
Step 5 : HLR can determine the MSC responsible for the MS and forwards this information to GMSC
Step 6 : GMSC can now forward the call setup request to MSC indicated.
Step 7: From this, MSC is responsible for all further steps. First it requests the current status of MS from VLR.
Step 8 : If MS is available, then MSC initiates paging in all cells it is responsible for as searching for the right cell would be too time consuming.
Step 9: This approach puts some load on signalling channels so optimization exist.
Step 10 : Location area (LA) can be determined.
Step 11 : The BTSs of all BSSs transmit this paging signal to MS.
Step 12 & 13 : If MS answers ( 12 and 13) the VLR has to perform security checks set up be encryption techniques.
Steps 14 to 17 : The VLR signals to MSC to setup a connection to MS.
MESSAGE ORIGINATED CALL (MOC):
It is simpler to perform message originated call(MOC)
compared to MTC.
The basic steps for MOC are,
Step 1: MS transmits a request for a new connection.
Step 2: BSS forwards the request to MSC.
Steps 3 & 4 : MSC then checks if the user is allowed to set up a call with the requested service(3 and 4) and checks the availability of resources through GSM network into PSTN.
Steps 5to 8 : If all resources are available, MSC sets up a connection between MS and fixed network.
Steps 9 & 10 : Its set up a call with the help of BSS and MS.
MESSAGE FLOW FOR MTC AND MOC :
In addition to the above steps mentioned above , the other messages are exxchanged between an MS and BTS during connection setup. These messages can be quite often heard in radios or badly sheileded speakers as crackling noise before the phone rings. Figure shows the message for an MTC and MOC. Paging is only necessary for an MTC, then similar message exchanges follow. The next step which are needed for a communication security comprises the authentication of MS and switching to encrytpted communication
The following steps which are mentioned in the figure denotes th euse of MSC and MOC. If someone is calling the MS, it answers now with ‗alerting‘ that MS is ringing and with ‗connect‘ that the user has pressed the connect button. The same actions happen the other way round if MS has initiated the call. Af.ter connection acknowledgement both parties are exchanged.