Mobile Originated Call

When a call is made by the user, the MS has to connect to the GSM network, for the MS first connects to a radio network, which is used for message handling between to set up connections between devices.

1. To start a call, the MS sends a request to the Base Station Subsystem(BSS), for creating a radio channel between them. The BSS reserves a channel for the MSS and assigns a frequency and time slot for that channel. this channel is used by the MS to communicate with the BSS.

2. A confirmation is sent by the MS implying that the radio channel has been established and the BSS can now create a connection to the MSC.

3. Next step after connection setup, the subscriber needs to be authenticated, which is done by verifying the number stored in the SIM card, and the subscriber’s authenticity is thus verified. After the verification, a connection is set up between the MS and Mobile Service Center(MSC).

4. The MSC encrypts the messages which are transmitted to the BTS so that data cannot be read or deleted by hackers. The BTS then forwards the message to the MS. The BTS also works with encrypted data. It both receives and sends encrypted data.

5. Before initiating the call setup, the MSC authenticates the service requested by the subscriber. This information about subscribers is checked in the Virtual Location Register, which stores temporary subscriber data like location, preferences, and allowed services. Once the VLR sends a green signal to the request from MS, the MSC can start the call setup.

6.  The MSC reserves a voice channel between the MSC and the BSS. The change to voice mode is notified by the BSS, and the MS returns a confirmation message. The MSC is in charge of setting up the call to the called number. When the call is received in the public switched telephone network(PSTN), the MSC is notified that the person whom we are calling is receiving the call in form of a ringtone as means of notification, at which point the originating MS receives a caller tune as a notification.

Pre-requisites: GSM in Wireless Communication
 

GSM stands for Global System for Mobile communication. This article explains two of the functionalities of GSM networks, which are mobile-originated and mobile-terminated voice calls. Whether or not a call is made from the Mobile Station, the Mobile Station(MS) always communicates with the Base Transceiver Station(BTS) by sending and receiving “HELLO” signals.