SRB in 5G-NR

SRB (Signaling Radio Bearer)

  • For carrying signaling message the type of bearer that required is know as SRB (Signaling Radio Bearer).
  • The SRBs are radio bearers that are used for the transmission of RRC and NAS messages.
  • There are four different type of SRB in NR as per TS38.331.
  • SRB0: This SRB is for RRC messages using the Common Control Channel (CCCH) logical channel.
  • SRB1: This SRB is for RRC messages (which may include a piggybacked NAS message) as well as for NAS messages prior to the establishment of SRB2, all using DCCH logical channel.
  • SRB2: This SRB is for NAS messages, all using DCCH logical channel. SRB2 has a lower-priority than SRB1 and is always configured by the network after security activation.
  • SRB3: This SRB is for specific RRC messages when UE is in EN-DC, all using DCCH logical channel.
  1. SRB3 can be setup at the request of the SG Secondary Node.
  2. SRB3 is used for signaling procedures which are time sensitive with respect to the gNode B, e.g. mobility procedures.
  3. SRB3 supports a limited number of signaling messages, i.e. RRC Reconfiguration,RRC Reconfiguration Complete and Measurement Report messages.

SRB 0 is only applicable to the Master Node

NAS and RRC Message Mapping with SRB

Reference : NR Bullets

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