How eNodeB selects MME while executing attach procedure for an UE?

Question

I have seen many wireshark logs and found mainly two types of attach request, one is known as "EPS attach" and another one is known as "Combined attached". I want to know, how does UE nas layer decide on "attach type" in attach request ?

Answer 1

Every MME might have different capacity in then network. So assuming that point enodeB consider a weight factor which is communicated by MME to eNodeb at the time S1 SETUP procedure.
So if eNB doesn't received S-TMSI from UE in RRC Connection request message then eNodeB executes algorithm based on weight factor of each connected MME's and get a MME to which It will forward InitialUeMessage.

If eNodeB receives S-TMSI then it fetch MME code from S-TMSI and forward to respective MME.

Answer 2

eNodeB selects MME on the basis of parameters sent in Attach Request by UE. If GUTI is there, then eNB uses it to identify the MME. After identifying the MME, if eNB has got connectivity to that MME; then eNB forwards the attach request ( Inital UE message + PDN Connectivity Request NAS message) to the that particular MME. If eNB doesn't have connectivity to that MME, then eNB uses the MME selection function and on the basis of weight factor ie loading factor, chooses MME from the MME pool area.

Answer 3

When an eNodeB selects an MME, the eNodeB may use a selection function which distinguishes if the GUMMEI is mapped from P-TMSI/RAI or is a native GUMMEI. The indication of mapped or native GUMMEI shall be signalled by the UE to the eNodeB as an explicit indication. The eNodeB may differentiate between a GUMMEI mapped from P TMSI/RAI and a native GUMMEI based on the indication signalled by the UE. Alternatively, the differentiation between a GUMMEI mapped from P-TMSI/RAI and a native GUMMEI may be performed based on the value of most significant bit of the MME Group ID, for PLMNs that deploy such mechanism. In this case, if the MSB is set to "0" then the GUMMEI is mapped from P-TMSI/RAI and if MSB is set to "1", the GUMMEI is a native one. Alternatively the eNodeB makes the selection of MME only based on the GUMMEI without distinguishing on mapped or native