What is SRB3 in NR?

And what is the expected gain of enabling SRB3 in NR?

  • First, Let`s quickly recap the meaning of Signaling Radio Bearer.

    • Signaling Radio Bearers (SRB) are used to transfer RRC messages between the UE and Base Station.
    • RRC messages can encapsulate NAS messages, so SRBs are also responsible for transferring NAS messages between the UE and Base Station.
    • The NG Application Protocol (NGAP) transfers NAS messages between the Base Station and AMF.
  • 3GPP has specified 4 types of SRB for New Radio (NR):

    • SRB 0 transfers RRC messages that use the Common Control Channel (CCCH) logical channel
    • SRB 1,2 and 3 transfer RRC messages that use the Dedicated Control Channel (DCCH) logical channel.
    • SRB 1 supports RRC signaling between the UE and Base Station but can also encapsulate NAS messages before the setup of SRB 2
    • SRB 2 is always set after security activation and encapsulates NAS messages.
    • SRB 2 messages are handled with lower priority than SRB 1 messages.

So, What is the SRB3?

  • SRB 3 is applicable when using the ‘E-UTRAN New Radio Dual Connectivity’ (EN-DC) configuration

  • SRB3 is optional and provides a direct SRB between the Secondary RAN Node and the device.

  • RRC Signaling through using NR SRB3 might reduce the latency.

  • The following RRC messages will be handled through SRB3 For EN-DC operation:

    1. RRC Reconfiguration.
    2. RRC Reconfiguration Complete.
    3. Measurement Report.

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