What is SRB3 in NR?

5G NR
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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.

What is SRB3 in NR
What is SRB3 in NR1245×574 132 KB

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During the execution phase of an intra-SgNB, intra-frequency handover initiated by the SgNB, is the RRC Reconfiguration and RRC Reconfiguration Complete message used?

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in 3GPP TS 37.340 version 15.7.0 Release 15,part 10.3 Secondary Node Modification (MN/SN initiated) — 10.3.1 EN-DC, it declared that it could be done.

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nitiated SN Modification without MN involvement, it

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Very well written. SRB3 in 5G-NR is an optional Signaling Radio Bearer used in EN-DC dual connectivity (LTE + NR). It provides a direct signaling path between the Secondary NR node and the UE for key RRC messages such as RRC Reconfiguration, RRC Reconfiguration Complete, and Measurement Reports, helping reduce signaling delay for the secondary connection.