In: Computer Science
In relation to the cellular telephone system , explain how a mobile call is established , maintained and terminated in a cellular system
The abreviations words that are t used in the process of call establishing and termination are
MS - Mobile station
GSM - Global System for Mobile Communication
MSC - Mobile Switching center
BSS - Base Station Subsystem
BSC - Base Station Controller
BTS -Base tranciever station
MO-mobile originated
PSTN-PUBLIC SWITCHED TELEPHONE NETWORK
VLR -Virtual Location Register
MT-Mobile termination
GMSC-Gateway Mobile Switching centre
How mobile call is established
A call originating from a mobile station (MS) in the GSM network is routed through the core network to the destination party. To connect to the GSM network, an MS first connects to a radio network, which is in charge of handling messaging between devices and the core network. An MS is constantly communicating with the Base Station Subsystem of the radio network to send and receive signals. The steps to establishing a GSM voice call are illustrated below.
To make sure the data sent over the radio network between the BSS and MS/MSC is secure, the MSC initiates a ciphering procedure which is transmitted to the BTS, which in turn forwards the message to the MS. The MS enables ciphering towards the BTS; as the BTS starts receiving ciphered data, it will start the ciphered transmission of information, finalizing the encryption procedure.
4. in order to initiate the call setup, the MSC verifies that the requested service is allowed for the subscriber. This information is available in the Virtual Location Register, which maintains temporary subscriber data (location, preferences, allowed services). Once the VLR confirms the service requested by the originating MS, the MSC starts the call setup.
5. for the call to take place, the MSC allocates a voice channel between the MSC and the BSS. The BSS notifies the MS about the change to voice mode, and the MS returns a confirmation message. The MSC routes the call to the dialled number. When the call is received in the PSTN, the MSC is notified that the called subscriber is being alerted, at which point the originating MS receives a ring notification.
To disconnect the call by either party, a disconnect message is sent to the MSC, which releases the communication channels created with the PSTN and the BSS.
Call termination
A terminated call in the GSM network is a call received on a mobile device. For an 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.
At the other end, the MSC notifies the GMSC, which notifies the PSTN that the destination number is being alerted