THE PROCESS BEHIND MAKING A PHONE CALL (GSM)

THE PROCESS BEHIND MAKING A PHONE CALL (GSM)

 Ever wondered what happens when we dial a call to connect our recipients? For this process to be successful, it passes through series of protocols and validation in the service providers systems. This systems are base transceiver stations (BTS), base switching center (BSC), mobile switching center (MSC) and the database systems (ELR, VLR, EIR)

 

When I call is made, the callers device sends a signal wirelessly through a radio signal requesting to join a mobile network (referred to as random access channel) to the adjacent BTS, commonly known as the booster, the BTS then sends the call signal to the BSC. This BSC is responsible for handling signals and traffic to and from the BTS. The communication channel between the BTS and BSC is referred to as the ABIS interface. At this point, the call is kept on hold as the MSC uses the call setup message from the mobile device which contains the caller’s mobile number to validate the call using the stored database listed.

Home Location Register (HLR) - Main copy of the database

Visitor’s location Register (VLR - The copy of the HLR

Equipment Identification Register (EIR) – checks mobile devices ownership and information i.e. stolen or cloned.

The two databases ELR and VLR contains the following

IMSI (International mobile subscriber’s identity) which has

Ø  Country code

Ø  Callers ID

Ø  Mobiles phones unique ID (EMEI)

MSISDN (Mobile subscribers ISDN number) which has

Ø  Your mobile number

Ø  Subscriber’s mobile phones tariff. I.e. Texts, Minutes and data.

The channel between the BSC is referred to as the “A” interface .The MSC handles significant tasks and this makes it the Heart of the communication channel.

After a successful authentication, the MSC instructs the BSC to assign a voice channel and encryption to the HLR. The HLR then uses the recipient phone number to search the recipient IMSI and current location. Once this has been found, they are forwarded to the relevant MSC (as shown in the chart). The MSC in turn generates a mobile station roaming number (MSRN) code and forwards it back to the awaiting MSC via the HLR. The channel between to switched MSC is known as the Transit interface.

Once the call has been successfully rooted across the network using the generated MSRN code, the serving MSC examines its own VLR to find the recipient’s location area code then it broadcast recipients IMSI to the adjacent base station on the relevant geographical location.

 

The paging mechanism identifies the ring tone and sends back a status waiting tone to the caller. The process continues until a response is made.

BY: KEN RONOH.