HLR (Home Location Register)
HLR or Home Location Register is the main database that stores varied data of subscribed customers. This data includes customer mobile numbers, billing info, and other indistinguishable information. Each SIM has a unique identifier called an IMSI which is the primary key to each HLR record. HLR directly receives and processes Mobile Application Part (MAP) SS7 protocol transactions and messages from the elements in the GSM network, for example, the location update messages received as mobile phones roam around.