GSM Call Flows
GSM Call Flow-Mobile Originated Call
Mobile originated (MO) call: – There are four distinct phase of a mobile originated call-
-Setup phase.
-Ringing phase.
-Conversation phase.
-Release phase.
Out of these phases the setup phase is the most important phase and includes authentication of the subscriber, Ciphering of data over radio interface, validation of mobile equipment, validation of subscriber data at VLR for requests service and assignment of a voice channel on A-interface by MSC. Whenever MS wants to initiate on outgoing call or want to send an SMS it requested for a channel to BSS over RACH. On receiving request from MS, BSS assigns a stand-alone dedicated control channel (SDCCH) to MS over access grant channel (AGCH). Once a SDCCH has been allocated to MS all the call set up information flow takes place over SDCCH.
A connection management (CM) entity initiates a CM Service Request message to the network. Network tries to establish an MM connections between the MS and the network and upon successful establishment of MM connection a CM Service Accept message is received by MS from the network. MS now sends a Call Set up Request to the network which contains the dialed digits (DD) of the called party. As the call setup message is received at the MSC/VLR certain check are performed at MSC/VLR like- whether the requested service is provisioned for the subscriber or not, whether the dialed digits are sufficient or not, any operator determined barring (ODB) does not allow call to proceed further etc. As these checks are performed at MSC/VLR a Call Proceeding Message is sent from the network towards the MS. After all the checks are successfully passed MSC sends Assignment command to the BSS which contains a free voice channel on A-interface On getting this message BSS allocates a free TCH to the MS and informs the MS to attach to it. MS on attaching to this TCH informs the BSS about it. On receiving a response from the BSS, MSC switches the speech path toward the calling MS. Thus at the end of Assignment the speech path is through from MS to MSC. It is important to note that at this stage mobile has not connected user connection as yet. MS at this stage does not listen anything.
After assignment MSC sends a network set-up message to the PSTN requesting that a call be set up. Included in the message are the MS dialed digits (DD) and details specifying which trunk should be used for the call. The PSTN may involve several switching exchanges before finally reaching the final local exchange responsible for applying the ringing tone to the destination phone. The local exchange will generate the ringing tone over the trunk, or series of trunk (if several intermediate switching exchange are involved), to the MSC. At this point in time MS will hear ringing tone. The PSTN notifies the MSC with a network-alerting message when this event occurs. MSC informs the MS that the destination number is being alerted. It is important to note that this is primarily a status message to the MS. The MS hears the ringing tone from the destination local exchange through the established voice path.
When the destination party goes off hook, PSTN informs the MSC of this event. At this point, MS is connected to the destination party and billing is started. MSC informs the MS that connection has been established and MS acknowledges the receipts of the connect message.
Under normal condition, the termination of a call is MS initiated or network initiated. In this scenario, we have assumed that MS initiates the release of the call by pressing “end” button and MS send a disconnect message to the MSC. The PSTN party is notified of the termination of the call by a release message from the MSC. The end- to- end connection is terminated. When MSC is left with no side task (e.g. charging indication etc.) to complete a release message is sent to the MS. MS acknowledges with a release complete message. All the resources between MSC and the MS are released completely.
