Mobility from EUTRA command

Query_1: Why MobilityFromEUTRACommand is needed?
When a UE is in RRC_Connected state and the UE needs to move to a different RAT (UTRA/GERAN/CDMA2000) then eNB triggers this procedure. This procedure is needed during:
  • Handover (the MobilityFromEUTRACommand message includes radio resources that have been allocated for the UE in the target cell)
  • Cell Change Order (the MobilityFromEUTRACommand message includes information for accessing Connection Establishment in the target cell. It includes system information. Cell change order is applicable only to GERAN)
  • CS fallback to CDMA2000 1xRTT (the MobilityFromEUTRACommand message includes radio resources that have been allocated for the UE in the target cell)

Query_2: When MobilityFromEUTRACommand is initiated?
eNB initiates this procedure when:
=  UE's in RRC Connected state, and in response to the following messages:
§  MeasurementReport message
§  reception of CS fallback indication for the UE from MME
=  AS Security is activated (if Security is not avtivated then eNB will not trigger this procedure)
=  SRB2 is established with atleast one DRB is setup and not suspended

Query_3: What happens when MobilityFromEUTRACommand message is received by UE?
The MobilityFromEUTRACommand message can carry the following information:
a. Cell Change Order (for GERAN cell)
b. CS fallback to CDMA system
c. Handover (to GERAN or UTRAN)

When this message is received by the UE, it shall:
=> stop timer T310 if it is running -T310 is started when UE detects physical layer problems-
=> if the MobilityFromEUTRACommand message includes the purpose set to 'handover' and the targetRAT-Type is set to 'utra' or 'geran' then:
    • UE will start accessing the respective RAT as mentioned in the targetRAT-Type IE
    • forward the nas-SecurityParamFromEUTRA to the upper layers
a. if the targetRAT-Type is set to 'geran' then:
  • use the contents of systemInformation IE to begin access to the target GERAN cell
  • UE will not indicate the release of the concerned DRBs to the upper layers if there are DRBs for which no radio bearers are established in the target RAT

b. if the targetRAT-Type is set to 'cdma2000-1XRTT' or 'cdma2000-HRPD' then:
  • forward the targetRAT-Type and the targetRAT-MessageContainer to CDMA2000 upper layers for the UE to access the cell as indicated in the inter-RAT message
=> if the MobilityFromEUTRACommand message includes the purpose set to 'cellChangeOrder':
    • start timer T304 with the value as received from MobilityFromEUTRACommand message
    • if the targetRAT-Type is set to 'geran' then acquire networkControlOrder and apply the value (in case this IE is not part of the MobilityFromEUTRACommand  message)
    • use the contents of systemInformation to access and establish the connection to the target cell indicated in the CellChangeOrder
=> if the MobilityFromEUTRACommand message includes the purpose set to 'e-CSFB' then forward the message to the CDMA2000 upper layers

Query_4: What happens when MobilityFromEUTRACommand message fails?
When MobilityFromEUTRACommand fails, UE shall initiate the MobilityFromEUTRAFailure procedure. (To know about the MobilityFromEUTRA failure procedure, click here)

Important Points to use:
  • When Mobility form UTRA/GERAN is received at UE and UE is attached to the LTE network and UE supports UTRA/GERAN then:
    • the message contents of the Mobility from UTRA is validated at UE RRC layer and then is passed to the other respective RAT to process the HO message
    • LTE PS shall deactivate the UE c-RNTI and will change it's master-ship to SLAVE mode, thereby giving the control to respective UTRA/GERAN
  • If the CDMA2000 upper layers in the UE receive both the messageContCDMA2000-1XRTT and messageContCDMA2000-HRPD then the UE performs concurrent access to both CDMA2000 1xRTT and CDMA2000 HRPD RAT

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