How enodeb selects MME?

Question

Which parameters at eNodeB would be taken into consideration to trigger 'n' numbers of S1 connections with same MME or different MMEs ?
Note: My query is related to macro eNodeB.

Answer 1

During attach request, UE actually sends 'GUTI' if available (allocated during previous session by MME). GUTI includes MME ID (MMEC.MMEGroupID). So eNB in such case shall not lookup DNS or consider MME weight advertised. eNB simply drives MMEID from GUTI and checks if SCTP/S1-AP link is UP with that MME, if everything is OK then forwards attach request to MME. Otherwise it could lookup DNS using (current) TAI of UE, get IPV4 of all MME (nodes) matching TAI and selects based on TOPON/TOPOFF criterion.

Comments

Hello Asad, GUTI is a NAS Information element which eNodeB doesn't/can't decode.
I think you concern is related to RegisteredMME information which comes within the RRC connection setup complete message.

For reference:

RRCConnectionSetupComplete-r8-IEs ::= SEQUENCE {
    selectedPLMN-Identity                INTEGER (1..maxPLMN-r11),
    registeredMME                        RegisteredMME                        OPTIONAL,
    dedicatedInfoNAS                    DedicatedInfoNAS,
    nonCriticalExtension                RRCConnectionSetupComplete-v8a0-IEs    OPTIONAL
}

---

Thanks Vimal for your feedback and correcting me.

Answer 2

eNodeB selects an available MME to serve a particular UE based on typically network topology and location of the UE within the network, so that the best MME is selected e.g. to reduce the probability that the MME is changed.

As part of the UE Attach the eNB will query the DNS Server using the TAI for the address of the MME. Typically the DNS will have a pool of MMEs to select from in response to the query.

The format of the TAI FQDN is constructed as follows:
tac-lb.tac-hb.tac.epc.mnc.mcc.3gppnetwork.org

The DNS may also provide more than one MME address (candidate set) from which the eNB can select a MME.

Comments

If GUMMEI is not included then eNB free to select MME based on MME weight factor. eNB select it such MME who has less load (means more weight factor).