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5G-NR SRB (Signaling Radio Bearers)

5G-NR SRB (Signaling Radio Bearers)

SRB (Signaling Radio Bearer)

  • For carrying signaling message the type of bearer that required is known 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.

SRB 3 

  • SRB3 can be setup at the request of the SG Secondary Node.
  • SRB3 is used for signaling procedures which are time sensitive with respect to the gNode B, e.g. mobility procedures.

SRB3 supports a limited number of signaling messages, i.e. RRC Reconfiguration, rrc Reconfiguration Complete and Measurement Report messages.

NAS and RRC Message Mapping with SRB