The LTE Attach procedure is the gateway for any UE to join the network and begin communicating. When you power on your phone, it first establishes an RRC connection with the nearest eNodeB by completing the random access procedure. The UE then sends an Attach Request containing its identity (IMSI or GUTI) through the eNodeB to the MME. This single Attach Request triggers a coordinated chain of signaling across every major element in the EPC, ultimately resulting in full network registration and an active data connection.