Q21Mobile Computing

<h3>Location Management</h3><p>Location management ensures the GSM network knows the current location of a mobile station to deliver incoming calls. It heavily relies on the Home Location Register (HLR) and Visitor Location Register (VLR).</p><ul><li><strong>HLR:</strong> The primary database holding subscriber information and their current VLR location.</li><li><strong>VLR:</strong> A temporary database holding information about subscribers currently in the area served by its associated MSC.</li><li><strong>Location Update:</strong> When a Mobile Station (MS) moves into a new Location Area (LA), it detects a new Location Area Identity (LAI) broadcasted by the Base Station. The MS initiates a location update request to the new MSC/VLR. The new VLR registers the MS, updates the HLR with its new VLR address, and the HLR notifies the old VLR to delete the subscriber's record.</li></ul>

<h3>Mobile Terminated Call (MTC) Setup</h3><p>When an external party calls a GSM mobile subscriber, the following steps occur:</p><ol><li>The call is routed from the PSTN to the Gateway MSC (GMSC) of the subscriber's home network.</li><li>The GMSC queries the HLR using the subscriber's phone number (MSISDN) to find the current VLR.</li><li>The HLR requests a Mobile Station Roaming Number (MSRN) from the current VLR.</li><li>The VLR assigns an MSRN and sends it back to the HLR, which forwards it to the GMSC.</li><li>The GMSC routes the call to the current MSC using the MSRN.</li><li>The MSC asks the VLR for the exact Location Area of the MS and initiates a 'page' across all base stations in that Location Area.</li><li>The MS responds to the page, authentication is performed, ciphering is established, and the call is finally connected.</li></ol>