5G-NR: SRB-3

 

    In NSA, one more signaling radio bearer introduced, called SRB3. The decision to establish SRB3 is taken by the SN, which provides the SRB3 configuration using an SN RRC message. 

UE capability should support SRB-3. and also gNB should support SRB3.

 

UE-MRDC-Capability ::=  SEQUENCE {

    measParametersMRDC                  MeasParametersMRDC                  OPTIONAL,

    rf-ParametersMRDC                   RF-ParametersMRDC,

    generalParametersMRDC               GeneralParametersMRDC-XDD-Diff      OPTIONAL,

    fdd-Add-UE-MRDC-Capabilities        UE-MRDC-CapabilityAddXDD-Mode       OPTIONAL,

    tdd-Add-UE-MRDC-Capabilities        UE-MRDC-CapabilityAddXDD-Mode       OPTIONAL,

    fr1-Add-UE-MRDC-Capabilities        UE-MRDC-CapabilityAddFRX-Mode       OPTIONAL,

    fr2-Add-UE-MRDC-Capabilities        UE-MRDC-CapabilityAddFRX-Mode       OPTIONAL,

    featureSetCombinations              SEQUENCE (SIZE (1..maxFeatureSetCombinations))                    OPTIONAL,

    lateNonCriticalExtension            OCTET STRING                        OPTIONAL,

    nonCriticalExtension                SEQUENCE {}                         OPTIONAL

}

 

 

 

GeneralParametersMRDC-XDD-Diff ::= SEQUENCE {

    splitSRB-WithOneUL-Path             ENUMERATED {supported}      OPTIONAL,

    splitDRB-withUL-Both-MCG-SCG        ENUMERATED {supported}      OPTIONAL,

    srb3                                ENUMERATED {supported}      OPTIONAL,

    ...

}

     Above parameter can be seen in MeNB to SgNB contained under sgnb addition request from MeNB to SgNB over x2ap interface.

If UE capability and gNB supports SRB3, then CU include "SRB to be setup list for SRB3 " in uecontextsetup message from CU-to DU over f1ap interface during SgNB addition procedure. then DU configure SRB3 also for that UE context.

SRB3 establishment and release can be done at Secondary Node Addition and Secondary Node Change. 

SRB3 reconfiguration can be done at Secondary Node Modification procedure. 

SRB3 may be used to send SN RRC Reconfiguration, SN RRC Reconfiguration Complete and SN Measurement Report messages, only in procedures where the MN is not involved. 

SN RRC Reconfiguration Complete messages are mapped to the same SRB as the message initiating the procedure. SN Measurement Report messages are mapped to SRB3, if configured, regardless of whether the configuration is received directly from the SN or via the MN. 

No MN RRC messages are mapped to SRB3. SRB3 is modelled as one of the SRBs defined in TS 38.331 [4] and uses the NR-DCCH logical channel type. 

RRC PDUs on SRB3 are ciphered and integrity protected using NR PDCP, with security keys derived from S-KgNB. The SN selects ciphering and integrity protection algorithms for the SRB3 and provides them to the MN within the SCG Configuration for transmission to the UE. 

NOTE: A NR SCG RRC message sent via E-UTRA MCG SRB is protected by E-UTRA MCG SRB security (NR security is not used in this case). 

SRB3 is of higher scheduling priority than all DRBs. 

The default scheduling priorities of split SRB1 and SRB3 are the same. There is no requirement on the UE to perform any reordering of RRC messages between SRB1 and SRB3. 

When SCG is released, SRB3 is released.