The MME is responsible for the NAS connection with the UE. All NAS signaling messages are exchanged between the UE and MME to trigger further procedures in the core network if necessary.
A new function of the E-UTRAN is NAS signaling security. The purpose of this feature is to protect the signaling messages that could reveal the true subscriber's identity and location from unauthorized eavesdropping.
The MME is also responsible for paging subscribers in the EPS Connection Management (ECM) IDLE state (including control and execution of paging retransmission) and is concerned with tracking area list management. The list of tracking areas is the list of locations where the UE will be paged.
To route the user plane data streams the MME will select the best fitting PDN-GW and S-GW. It will also connect the E-UTRAN with the 3G UTRAN using the S3 interface (MME to SGSN). When necessary, a relocation of gateways will be triggered and controlled by the MME.
As its name suggests, the MME will perform management of handovers by selecting a new (target) MME or SGSN for handovers to 2G or 3G 3GPP access networks. Also, it is the MME that hosts the connection to the HSS across the S6a interface and, hence, it is responsible for roaming management and authentication of subscribers.
Last but not least, the MME sets up, modifies, and releases default and dedicated bearers. This function is commonly known as the bearer management function.
According to standard documents, the MME will allow lawful interception of signaling traffic and transfer of warning messages (including selection of an appropriate eNB). The purpose of warning message transfer is to inform people living in a larger area about upcoming natural disasters like storms, bush fires, or tsunamis.
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