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4G/LTE - RRC |
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RRC Connection ReEstablishment
It is a kind of mechanism to make a recovery when radio link got broken for some reason. (See "When it happens" section for these reason)
< How it work >
it works in three steps as shown below (refer to 36.331 5.3.7 RRC connection re-establishment for the details) i) UE --> NW : RRCConnectionReestablishmentRequest ii) UE <-- NW : RRCConnectionReestablishment iii) UE --> NW : RRCConnectionReestablishmentComplete I think most of those who read this page may be pretty familiar with LTE protocol, so just by looking at the contents of these RRC messages, they would figure out a lot of details without much explanation.
RRCConnectionReestablishmentRequest
c1: rrcConnectionReestablishmentRequest (0) rrcConnectionReestablishmentRequest criticalExtensions: rrcConnectionReestablishmentRequest-r8 (0) rrcConnectionReestablishmentRequest-r8 ue-Identity c-RNTI: 0000 [bit length 16, 0000 0000 0000 0000 decimal value 0] physCellId: 0 shortMAC-I: 0000 [bit length 16, 0000 0000 0000 0000 decimal value 0] reestablishmentCause: reconfigurationFailure /handoverFailure/otherFailure/ spare1 spare: 00 [bit length 2, 6 LSB pad bits, 00.. .... decimal value 0]
RRCConnectionReestablishment (An example based on 36.508 - RRCConnectionReestablishment)
c1: rrcConnectionReestablishment (0) rrcConnectionReestablishment rrc-TransactionIdentifier: 0 criticalExtensions: c1 (0) c1: rrcConnectionReestablishment-r8 (0) rrcConnectionReestablishment-r8 srb-ToAddModList: 1 item Item 0 SRB-ToAddMod srb-Identity: 1 rlc-Config: defaultValue (1) defaultValue: NULL logicalChannelConfig: defaultValue (1) defaultValue: NULL mac-MainConfig: explicitValue (0) explicitValue ul-SCH-Config maxHARQ-Tx: n5 (4) periodicBSR-Timer: sf20 (3) retxBSR-Timer: sf320 (0) ..0. .... ttiBundling: False drx-Config: release (0) release: NULL timeAlignmentTimerDedicated: sf750 (1) phr-Config: setup (1) setup periodicPHR-Timer: sf500 (5) prohibitPHR-Timer: sf200 (5) dl-PathlossChange: dB3 (1) physicalConfigDedicated pdsch-ConfigDedicated p-a: dB-3 (2) pucch-ConfigDedicated ackNackRepetition: release (0) release: NULL pusch-ConfigDedicated betaOffset-ACK-Index: 9 betaOffset-RI-Index: 6 betaOffset-CQI-Index: 6 uplinkPowerControlDedicated p0-UE-PUSCH: 0dB deltaMCS-Enabled: en0 (0) ..1. .... accumulationEnabled: True p0-UE-PUCCH: 0dB pSRS-Offset: 3 filterCoefficient: fc4 (4) cqi-ReportConfig cqi-ReportModeAperiodic: rm30 (3) nomPDSCH-RS-EPRE-Offset: 0dB (0) soundingRS-UL-ConfigDedicated: setup (1) setup srs-Bandwidth: bw0 (0) srs-HoppingBandwidth: hbw0 (0) freqDomainPosition: 0 ..1. .... = duration: indefinite srs-ConfigIndex: 20 transmissionComb: 0 cyclicShift: cs0 (0) antennaInfo: explicitValue (0) explicitValue transmissionMode: tm3 (2) codebookSubsetRestriction: n2TxAntenna-tm3 (0) n2TxAntenna-tm3: c0 [bit length 2, 6 LSB pad bits, 11.. .... decimal value 3] ue-TransmitAntennaSelection: release (0) release: NULL schedulingRequestConfig: setup (1) setup sr-PUCCH-ResourceIndex: 60 sr-ConfigIndex: 30 dsr-TransMax: n4 (0) nextHopChainingCount: 0 HEX string : 00 12 9B 3E 86 03 B5 79 E8 96 6C 30 64 99 80 20 A0 28 68 3C 1E 00
There are several cases where this process get triggered. According to 36.331 5.3.7.2, there are several cases as described below.
Case 1 : When radio link failure happened Case 2 : when Handover failure happened Case 3 : when mobility from E-UTRA failure happened According to 36.331 5.4.3.5 Mobility from E-UTRA failure, revert back to the configuration used in the source PCell, excluding the configuration configured by the physicalConfigDedicated, mac-MainConfig and sps-Config; initiate the connection re-establishment procedure as described in How it works and Common UE side Process Case 4 : when integrity check failure indication was received from lower layers Case 5 : when RRC connection reconfiguration failure happened (UE cannot comply to the configuration set by RRC Connection Reconfiguration message)
< Causes of RRC Reconfiguration Re-establishment Request >
When anything happens where UE need to trigger RRC Connection Re-establishment process as described above, UE sends RRC Connection Re-establishment Request with the cause of one of the followings (36.331-5.3.7.4).
ReconfigurationFailure : if the re-establishment procedure was initiated due to reconfiguration failure (the UE is unable to comply with the reconfiguration) - Case 5 in previous section HandoverFailure : the re-establishment procedure was initiated due to handover failure as in intra-LTE handover failure or inter-RAT mobility from EUTRA failure - Case 2, 3 in previous section OtherFailure : Any failure caused by other than two mentioned above - Case 1, 4 and others.
Whatever the case is, overall process on UE side is similar as follows. (36.331 5.3.7.5) i) stop timer T301 ii) consider the current cell to be the PCell iii) re-establish PDCP for SRB1 iv) re-establish RLC for SRB1 v) perform the radio resource configuration procedure in accordance with the received radioResourceConfigDedicated vi) resume SRB1
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