Each MME can manage millions of UEs.
To handle this load the requirements of each subscriber for the MME must be as minimal and simple as possible so as to scale easily.
For each UE in the network a connection is setup between the UE and the MME.
This is done over the S1-AP’s Control Plane interface (sometimes calls S1-Control Plane or S1-CP) which carries control plane data to & from the UE via the eNB to the MME.
S1-CP is connection-oriented, meaning each UE has it’s own connection to the MME, so there are as many S1-CP connections to the MME as UE’s connected.
Each of these S1-CP connections is identified by a pair of unique connection IDs. The eNB keeps track of the connection IDs for each UE connected and hands this information off each time the UE moves to a different eNB.
The eNB keeps a lookup table between the RNTI of the UE and the LCID – the Logical Channel Identifier. This means that the eNB knows the sent and received ID of the S1-CP connection for each UE, and is able to translate that into the RNTI and LCID used to send the data over the air interface to the UE.
S1-CP Connect (Attach Procedure)
As we discussed in radio interfaces, when a UE connects to the network it is assigned an RNTI to identify it on the radio interface and allocate radio resources to it.
Once the RNTI is confirmed by both the eNB and the UE, a EMM Attach Request, which is put into an RRC Message called RRCConnectionSetupComplete.
The eNB must next choose a serving MME for this UE. It picks one based on it’s defined logic, and sends a S1-AP Intial UE Message (EMM Attach Request) to the MME along with the eNB’s connection identity assigned for this connection.
The MME stores the connection identity assigned by the eNB and chooses it’s own connection identity for it’s side, and sends back an S1AP Downlink NAS Transport response with both connection identities and the response for the attach request (This will be an EMM Authentication Request).
The eNB then stores the connection identity pair and the associated RNTI and LCID for the UE, and forwards the EMM Authentication Request to the RNTI of the UE via RRC.
The UE will pass the authentication challenge input parameters to the USIM which will generate a response. The UE will send the output of this response in a EMM Authentication Responseto the eNB, which will look at the RNTI and LCID received and consult the table to find the Connection Identifiers and IP of the serving MME for this UE.