- SRB3 is supported in EN-DC, NGEN-DC and NR-DC, but not in NE-DC.
- The decision to establish SRB3 is taken by the SN (Secondary Node), which provides the SRB3 configuration using an SN RRC message. 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.
- Example: If SRB3 in EN-DC is established, then RRC Measurement Reports for NR quality can be sent directly to the SgNB. This enables the 5G node to make intra-SgNB handover decisions and start the handover execution (exchange of rrcReconfiguration and rrcReconfigurationComplete directly between UE & SgNB) without involving the master eNodeB of the connection. This will save lot of signaling delay (during handover) which would have happened if MN (Master Node) was involved.
- SRB3 will 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 Measurement Report messages are mapped to SRB3, regardless of whether the configuration is received directly from the SN or via the MN. No MN RRC messages are mapped to SRB3.
- SRB3 uses the NR-DCCH logical channel type. RRC PDUs on SRB3 are ciphered and integrity protected using NR PDCP, with security keys derived from secondary key 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.
- The integrity protection algorithm is common for SRB1, SRB2, SRB3 and DRBs configured with integrity protection, with the same keyToUse value. The ciphering algorithm is common for SRB1, SRB2, SRB3 and DRBs configured with the same keyToUse value.
- In EN-DC, NGEN-DC and NR-DC, only measConfig, radioBearerConfig, conditionalReconfiguration, bap-Config, iab-IP-AddressConfigurationList, otherConfig and/or secondaryCellGroup are included in RRCReconfiguration received via SRB3.
- 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.
