How Incoming Calls To a Mobile Phone are Handled in GSM?

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.

Introduction to GSM Network

GSM (Global System for Mobile Communications) is the most widely used mobile communication system worldwide. It is a second-generation (2G) mobile communication system that uses digital technology to transmit voice and data. In a GSM network, calls are routed and transmitted through a combination of various components such as Mobile Station (MS), Base Station System (BSS), and Mobile Switching Center (MSC).

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.

Mobile-Terminated Call

A terminated call in the GSM network is a call received on a mobile device. For a Mobile Terminal(MT) to be placed, it is necessary to locate the network to which the called subscriber is currently connected, and, once this is done, to route the call towards the destination MS.

1. When a call is made, the country code of the phone number is used to find the location of the receiver and also to find the MSC which the subscriber is linked to. 

2. It is the job of the HLR to keep track of the MSC/VLR the MS visits so that the authentication process of the subscriber becomes easier.

3. A temporary roaming number called the Mobile Station Roaming Number(MSRN) is generated by the MSC for the call to be transferred to the MSC

4.  After that all the BSCs are paged by the MSC and also the BTSs are paged by the BSC.

5. After that a radio channel is established between MS and BTS. 

6. the last step is to send the reply to MSC so that it initiates encoding of calls and sends a call confirmation to the BS and the mobile starts to ring.

Procedure for Call Establishment

When a call is made to a mobile phone in a GSM network, it is first received by the nearest Base Station System (BSS). The BSS then establishes a connection with the Mobile Switching Center (MSC) which acts as a control center for the entire network. The MSC then sends a paging request to the Mobile Station (MS), which is the mobile phone, to indicate an incoming call. The MS sends an acknowledgment to the MSC, which then sends a setup request to the BSS to establish a connection between the calling and called parties.
Once the connection is established, the audio signals are transmitted between the calling and called parties through the BSS. The MSC continuously monitors the call and ensures that it remains connected until it is terminated either by the calling or called party.