Let’s imagine, how many simultaneous users can be connected to an eNodeB, of course it can be 100-150 users even more. But there is only one S1 interface between eNodeB and MME which is known as S1-MME interface. This interface is used to carry user control information. We mentioned, there is only one interface between eNodeB and MME, but how MME can distinguish control messages which are coming from eNodeB. Which message belongs to which UE?
There is an identifier to solve this issue, “eNodeB UE S1AP ID” is assigned for a UE by the eNodeB during first S1AP message. Consequently the MME can distinguish which control message belongs to which UE through “eNodeB UE S1AP ID”.
Second issue, there might be more than hundreds eNodeB under one MME which are connected via S1 interfaces. In this case, how the MME will distinguish which control message belongs to which UE under which eNodeB.
So, there is one more identifier for this case, “MME UE S1AP ID”. MME sends this identifier to eNodeB during first received S1AP message.
*Note: when a UE moves between eNodeBs, then eNodeB UE S1AP ID will be changed, if MME also will be changed during handover then MME UE S1AP ID will be renewed.