LTE HO : Why two MME UE S1AP IDs present within the S1-AP HO Request message ?

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 ?

Comments

By the way nice question ...

Answer 1

Yes i also came under same question when i was going through Handover Request message .

This optional id importance is there when ENB (HeNB) and MME connected through Gateway instead of Directly .

Here GW creates two context one towards ENB and another towards MME.

See If Source ENB is connected to MME through Gateway (see figure) .

So during Handover the Target ENB should know two ids.
1>The MME UE S1-AP Gateway id (so that he can use Target ENB and Gateway connection ) as
2> The MME UE S1-AP id which is the generally we send.

So the 2nd ID is needed and it is optional as it is used in this case only.

For more description u can refer http://www.3gpp.org/ftp/tsg_ran/WG3_Iu/TSGR3_70bis/Docs/R3-110241.zip