LTE: Why MME UE S1AP ID is of length 32 bits and eNodeB UE S1AP ID of length 24 bits only ?

Question

I found two MME UE S1-AP IDs i.e. MME UE S1AP ID (Mandatory) and MME UE S1AP ID 2 (optional).
I got why mandatory one is present but what's use of optional one.

Can someone please explain it ?

Answer 1

MME serves multiple eNodeB's. MME should allow to attach the UE's from all the eNodeB's under this MME.

For example, if eNodeB1 and eNodeB2 were registered under MME. eNodeB1 assigns x number of eNodeB UE S1AP ID's for x successful attached UE's. Also, eNodeB2 assigns y number of eNodeB UE S1AP ID's for y successful attached UE's. But MME should assign (x+y) MME UE S1AP ID's.