Signalling System 7 (SS7) is how the majority of the world’s mobile networks communicate with each other. It is a set of telephony protocols enabling the transmission of voice, data, control/session management information, and of course SMS. The SS7 protocol has layers based on the OSI (Open Systems Interchange) model. The MAP (Mobile Application Part) layer provides the functionality required for SMS transmission. The SMSC, HLR, VLR and MSC communicate together over the SS7 network utilizing MAP commands.