5G RRC Connection (Signalling Radio Bearer: SRB)

5G NR
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  • To exchange the layer 3 messages, i.e. the RRC (Radio Resource Control) and the NAS (Non-Access Stratum) messages between the UE and the 5G network, a signaling connection which is called “RRC Connection” should be established between the UE and the gNB .

  • To establish the RRC connection, a Signalling Radio Bearer (SRB) should be configured both in the UE and the gNB. The RRC or NSA messages are carried by the Signalling Radio Bearer.

  • For SRB configuration, PDCP, RLC parameters are delivered to the UE by the gNB.

  • Based upon which type of the layer 3 messages are going to be exchanged between the UE and the 5G network, a specific type of SRB is configured.

  • Here I am trying to depict the RRC connection as logical connection between the UE and the gNB.

5G RRC Connection (Signalling Radio Bearer: SRB)
5G RRC Connection (Signalling Radio Bearer: SRB)1232×918 143 KB

  • Dedicated Control Channel (DCCH) is used by SRB1, SRB2, and SRB3 for exchanging the dedicated control information between the UE and the 5G network , in both the downlink and uplink.

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Understanding 5G Signalling Radio Bearers (SRBs)

As we delve deeper into the world of 5G, it’s essential to understand the different Signalling Radio Bearers (SRBs) that play a critical role in the transmission of RRC and NAS messages. Here’s a quick overview of the various SRBs and their specific functions:

  • SRB0:

    • Used for RRC messages via the CCCH logical channel, excluding SRB0 of L2 U2N Remote UE.

  • SRB1:

    • Handles RRC messages (including piggybacked NAS messages) and NAS messages before SRB2 establishment, via DCCH logical channel, excluding SRB1 of L2 U2N Remote UE.

  • SRB2:

    • For NAS messages and RRC messages with logged measurement info, using DCCH logical channel; lower priority than SRB1 and configured post-AS security activation, excluding SRB2 of L2 U2N Remote UE.

  • SRB3:

    • Dedicated to specific RRC messages when UE is in (NG)EN-DC or NR-DC, using DCCH logical channel.

  • SRB4:

    • For RRC messages including application layer measurement reports, using DCCH logical channel; lower priority than SRB1 and configured by the network post-AS security activation.

  • SRB5:

    • For RRC messages with application layer measurement reports, using DCCH logical channel; lower priority than SRB1 and SRB3, configured by the SN serving the SCG when the UE is in NR-DC, post-AS security activation.

Understanding these SRBs is crucial for telecom professionals as they enhance network efficiency and performance, ensuring seamless communication in 5G networks.

Understanding 5G Signalling Radio Bearers (SRBs)

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