Signaling Radio Bearer basic Part-3

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In this section we will learn about SRB and its basics as well as interview questions. Please read our previous SRB article basic Part-2.

Signaling Radio Bearer's:

In ENDC, for SRB1, the random access is triggered by RRC layer itself because there is not necessarily other UL transmission. Also for SRB3 the random access is triggered by MAC layer because of arrival of RRC reconfiguration complete.

There are 4 SRB's – SRB 0, 1 ,2 & 3


SRB0 is mapped to CCCH in Downlink and Uplink. The RRC messages, RRC Connection Setup & RRC Connection request ( in NR Case RRCResumeRequest ) will be transmitted in SRB0.

Signaling Radio Bearer 0 is for RRC messages using the CCCH logical channel.

Direction: UL/DL
RLC - Mode: TM (Transparent Mode)
Logical Channel: CCCH

Messages Transmitted in SRB0:
(Interview Qus: What messages are included by SRB0?)

RRC Connection request
RRC Connection setup
RRC Connection Reestablishment
RRC Connection Reestablishment Reject
RRC System info Request


SRB1 is mapped to DCCH in downlink and  uplink. RRC messages & RRC with NAS piggybacked messages prior to the establishment of SRB2 will be transmitted in the SRB1

Direction: UL/DL
RLC - Mode: AM (Acknowledged Mode)
Logical Channel: DCCH

Messages Transmitted in SRB1:
(Interview Qus: What messages are included by SRB1?)

RRC Connection setup Complete
RRC Resume
RRC Resume Complete
RRC Connection Reconfiguration
RRC Connection reconfiguration Complete
RRC Connection Re establishment Complete
Security Mode Command
Security Mode Complete
UE Capability Enquiry
UE Capability Information
Measurement reports
RRC Release
DLlnformation Transfer (ifSRB2 has not been setup)
Counter Check
Mobility from NR Command
RRC Reestablishment
Security Mode Failure
ULlnformation Transfer (if SRB2 has not been setup)
Measurement Report
UE Capability information
Location Measurement indication
UE Assistance information
Counter Check Response


SRB2 is mapped to the DCCH in Uplink & Downlink, but after the Ciphering & integrity protected. All the NAS piggybacked messages for the UE, will be transferred in SRB2 if its present hereafter.

For the Signaling Radio Bearer 2 & Data Radio Bearer’s to establish, the AS has to be always Integrity Protected. Signaling Radio Bearer 2 establishment will be taken place with the help of RRC connection reconfiguration coming from Base station to UE.

SRB2 is always configured by the network after security activation.

Direction: UL/DL
RLC - Mode: AM (Acknowledged Mode)
Logical Channel: DCCH

Messages Transmitted in SRB2:
(Interview Qus: What messages are included by SRB2?)

1. UL Information Transfer
2. DL Information Transfer

Note: Signaling Radio Bearer 1 will always have the high priority in transferring the messages over SRB2. LCID is only for DRB's.


SRB3 which is available in various MRDC options.SRB3 is for specific RRC messages when UE is in EN-DC, all using DCCH logical channel.

Direction: UL/DL
RLC - Mode: AM (Acknowledged Mode)
Logical Channel: DCCH

SRB3 contain only below three messages.
(Interview Qus: What messages are included by SRB3?)

1. RRC Reconfiguration
2. RRC Reconfiguration complete
3. Measurement report

In the next section we will learn more about SRB. So keep reading……. And Stay tune……………

Pinal Dobariya………..

3GPP TS 38.331
3GPP TS 37.340


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