Why GUMMEI is included in RRC Connection setup complete message?

Question

During attach procedure, UE sends "Attach Request" along with "RRC Connection Setup Complete", eNodeB sends "Attach Accept" along with "RRC Connection Reconfiguration then why UE does not send "Attach Complete" along with "RRC Connection Reconfiguration Complete" message ? What could be the reason for defining the messages so ?

Answer 1

GUMMEI is used to identify a MME in MME pool. An eNodeB may have connection with multiple MME's. If UE already had connection or registered with a MME in MME pool, then it will send GUMMEI in subsequent RRC connection setup complete message while making RRC connection with eNodeB. When enodeb receives GUMMEI in RRC connection setup complete message, it prepares and forwards "Initial UE message" to relevant MME.

GUMMEI is an optional element in RRC connection setup complete message. It means it may come or not, based on the availability of GUMMEI or some other conditions.

Comments

During Initial attach GUMMEI will  come?if No How UE will identify MME?

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@Gnanendra Reddy,
If UE is doing storedInformation cell selection procedure, then it will have GUTI (GUMMEI + M-TMSI), which is included in rrcConnectionSetupComplete message.
If UE is doing initial cell selection, then it would not have GUTI and in rrcConnectionSetupComplete message IMSI will be sent.

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RRC Connection Setup Complete does not carry IMSI as a field rather it has  piggybacked NAS PDU which is decoded by MME-NAS layer.