The User Equipment (UE) needs to register with the network to receive Evolved Packet System (EPS) services like Internet connectivity. This registration is referred to as network attachment, in short Attach. “Always-on” Internet Protocol (IP) connectivity is enabled in EPS by establishing a so-called default EPS bearer during Attach (see Figure A.1).
- The UE initiates the Attach procedure by sending an Attach Request that is encapsulated in the Radio Resource Control (RRC) connection setup complete message. The RRC message is sent to the Evolved NodeB (eNodeB). The UE provides its Globally Unique Temporary Identity (GUTI) in the Attach, if this is available. If GUTI is not available, it provides its International Mobile Subscriber Identity (IMSI). If an UE without Universal Subscriber Identity Module (USIM) initiates the Attach procedure for emergency services, it may provide its International Mobile Equipment Identity (IMEI). The Attach Type can either be an Initial Attach, Handover, or Emergency. If the UE has valid security parameters, then the Attach Request is integrity protected.
- The eNodeB forwards the Attach Request to the selected Mobility Management Entity (MME) (either derived from the old Globally Unique Mobile Management Entity Identifier (GUMMEI) or selected based on MME selection function) in a S1-MME control message.
- If the UE context is not present anywhere in the network (in old or new MME) and the Attach Request was ...