To initiate a call or to set up any call in GSM is to first called /dialed the number. When we dialed the no of the MSC (Main Switching Center), which is responsible for call connection and storing the information about the subscriber. MSC contains HLR(Home location register),which stores the user information and performing entire billing details and timing of calls. MSC also consist of AUC that stands for authentication. Then the call can goes to GMSC(Gateway Main Switching Center). When GMSC is find then the call gets transfer to the HLR, which checks whether the number is existing or not then HLR sends signal to VLR to find the MSRN. Then the MSRN(Mobile Station Switching Number) is find.Once the MSRN is find then HLR send the request to the HLR for finding the called MSC. If the MSC is found then the call is established between the two user. MSRN is used by the user which is using the roaming facility. If the mobile is in roaming mode then one number is given to the device called MSRN number. Once the user inter in foreign network then it has to first register themselfs with the use of HA(Home Agent)and FA(Foreign Agent). FA sents the request to Ha befor their registration. Once the registration is done then the subscriber can use interrupted services.
When a node that is mobile or any device moves from one cell to another cell that is one coverage area to another coverage area then their is chance that mobile is disconnected. This process is called handover mechanism. In this process the call is transfer to one base station to another base station. Generally handover is of two types:
1. Soft Handover : When a node move from one cell to another than their is a possibilities that call will continue,this is called soft handover.
2. Hard Handover: When a node moves from one cell to another than their is a chance that call will disconnect called hard handover.