The SAE has a flat, all-IP architecture with separation of control plane and user plane traffic. The MME is the key control-node for the LTE access-network. MME and it is epc and 4g packet networks pdf responsible for generation and allocation of temporary identities to UEs. NAS signaling and handles the security key management.
Lawful interception of signaling is also supported by the MME. 3G access networks with the S3 interface terminating at the MME from the SGSN. The MME also terminates the S6a interface towards the home HSS for roaming UEs. For idle state UEs, the SGW terminates the downlink data path and triggers paging when downlink data arrives for the UE. It manages and stores UE contexts, e. IP bearer service, network internal routing information.
It also performs replication of the user traffic in case of lawful interception. The PDN Gateway provides connectivity from the UE to external packet data networks by being the point of exit and entry of traffic for the UE. A UE may have simultaneous connectivity with more than one PGW for accessing multiple PDNs. The functions of the HSS include functionalities such as mobility management, call and session establishment support, user authentication and access authorization. The main function of the ePDG is to secure the data transmission with a UE connected to the EPC over an untrusted non-3GPP access.
NAS protocols support the mobility of the UE and the session management procedures to establish and maintain IP connectivity between the UE and a PDN GW. They define the rules for a mapping between parameters during inter-system mobility with 3G networks or non-3GPP access networks. They also provide the NAS security by integrity protection and ciphering of NAS signaling messages. EPS provides the subscriber with a “ready-to-use” IP connectivity and an “always-on” experience by linking between mobility management and session management procedures during the UE attach procedure.
It also provides control of security for the NAS protocols. The procedures belonging to this type are initiated by the network. They include GUTI reallocation, authentication, security mode control, identification and EMM information. At any time only one UE-initiated EMM specific procedure can run. Service request: Initiated by the UE and used to establish a secure connection to the network or to request the resource reservation for sending data, or both.
Paging procedure: Initiated by the network and used to request the establishment of a NAS signalling connection or to prompt the UE to re-attach if necessary as a result of a network failure. Generic transport of NAS messages: Initiated by the UE or the network and used to transport protocol messages from other applications. The UE and the network execute the attach procedure, the default EPS bearer context activation procedure in parallel. During the EPS attach procedure the network activates a default EPS bearer context.
The EPS session management messages for the default EPS bearer context activation are transmitted in an information element in the EPS mobility management messages. The UE and network complete the combined default EPS bearer context activation procedure and the attach procedure before the dedicated EPS bearer context activation procedure is completed. The success of the attach procedure is dependent on the success of the default EPS bearer context activation procedure. If the attach procedure fails, then the ESM session management procedures also fails. The transmission of ESM messages is suspended during EMM procedures except for the attach procedure.