LTE - end-to-end TAU [Under LTE]
» E-UTRA and E-UTRAN overall description - 36.300, Rel 8 «
» Network architecture - 23.002, Rel 8 «
» E-UTRAN access - 23.401, Rel 8 «
» E-NAS - 24.301, Rel 8 «
» GTPv2-C - 29.274, Rel 8 «
» RRC - 36.331, Rel 8 «
As noted in quick bits on mobility, in RRC_IDLE state, mobility is managed at tracking area level. RRC continue to maintain DL synchronisation, do cell re-selection, and provide cell information to EMM. EMM, on finding TA change, triggers "tracking area update" procedure.
Below diagram shows an example of network architecture, wherein there are two MME areas, two SGW areas, and three tracking areas.
If there is MME change with no change in SGW, the UE context need to be exchanged between MMEs and SGW is to be informed about the MME change. No impact of SGW TEIDs.
If SGW changes too, TEIDs have to be created (the one towards eNB) at new SGW and also new sessions will have to be created between new SGW. Of course, old session/TEIDs would have be deleted at old SGW. As SGW has been changed, tunnel between new SGW and PGW need to created. Tunnel from PGW side remain same; new SGW gets this information from MME through "Create Session Request" and PGW gets information about new SGW TEIDs from new SGW through "Modify Bearer Request".
Note that in all above cases, eNB change, but there is no impact on eNB S1-U as S1-U bearers won't be present in RRC_IDLE state. S1-AP UE Context information will not be transferred from MME to eNB; it would be done as part service request procedure (when the need for service request arise).
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