As per my knowledge, while doing handover procedure, target eNodeB receives "RRC connection reconfiguration complete" message as part of successful handover fromUE or it may receive "RRC connection re-establishment request" message from UE when handover fails.
Assume, UE is not able to latch on target cell. In this case, UE does cell-reselection and it may initiate re-establishment towards target cell. To entertain this UE at target cell, cell should prepare to handle re-establishment request from this UE.
The information contains within the AS-Context is used to authenticate the UE which sent re-establishment request at target cell.
Content of re-establishment request message as following:
RRCConnectionReestablishmentRequest ::= SEQUENCE {
criticalExtensions CHOICE {
rrcConnectionReestablishmentRequest-r8
RRCConnectionReestablishmentRequest-r8-IEs,
criticalExtensionsFuture SEQUENCE {}
}
}
RRCConnectionReestablishmentRequest-r8-IEs ::= SEQUENCE {
ue-Identity ReestabUE-Identity,
reestablishmentCause ReestablishmentCause,
spare BIT STRING (SIZE (2))
}
ReestabUE-Identity ::= SEQUENCE {
c-RNTI C-RNTI,
physCellId PhysCellId,
shortMAC-I ShortMAC-I
}
ReestablishmentCause ::= ENUMERATED {
reconfigurationFailure, handoverFailure,
otherFailure, spare1}
AS-context contains re-establishment info. which in turn contains so many other important information which is used by target eNodeB while handling re-establishment request message for an UE. If wee see re-establishment info of AS-context , it may contain "additionalReestabInfoList" which contains the information of list of all possible cells in which UE can send re-establishment request message.