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SRB (Signaling Radio Bearer)

 

 

As it says, SRB stands for Signaling Radio Bearer. In other words, SRB is a type of Radio Bearer that carries signaling message (i.e, RRC or/and NAS message).

 

According to 38.331 4.2.2   Signalling radio bearers, there are four different types of SRB in NR defined as follows.

  • SRB0 is for RRC messages using the CCCH logical channel;
  • SRB1 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 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 is for specific RRC messages when UE is in EN-DC, all using DCCH logical channel.

As you may notice, the type and functions of each SRB in NR is same as the SRBs in LTE except that NR has one more SRB called SRB3. Regarding SRB3, 36.331 4.2.2 has a short additional comments as below.

    In case of EN-DC, SRB3 may be configured for the transfer of some NR RRC messages between UE and SgNB via the NR radio interface.

 

 

 

Default SRB Configuration

 

RRC information elements

SRB1/1S

SRB2/2S

SRB3

PDCP-Config

     

     t-Reordering

Infinity Infinity Infinity

RLC-Config CHOICE

AM AM AM

ul-RLC-Config

     

     sn-FieldLength

size12 size12 size12

     t-PollRetransmit

ms45 ms45 ms45

     pollPDU

Infinity Infinity Infinity

     pollByte

Infinity Infinity Infinity

     maxRetxThreshold

t8 t8 t8

dl-RLC-Config

     

     sn-FieldLength

size12 size12 size12

     t-Reassembly

ms35 ms35 ms35

     t-StatusProhibit

ms0 ms0 ms0

logicalChannelIdentity

1 2 3

LogicalChannelConfig

     

     priority

1 3 1

     prioritisedBitRate

Infinity Infinity Infinity

     logicalChannelGroup

0 0 0

 

 

 

SRB Mapping to Signalling Message

 

 

Message

Direction

Logical Channel

RLC

SRB

MasterInformationBlock

UE <-- NW

BCCH

TM

N/A

MeasurementReport

UE --> NW

DCCH

AM

SRB1,SRB3

MobilityFromNRCommand

UE <-- NW

DCCH

AM

SRB1

Paging

UE <-- NW

PCCH

TM

N/A

RRCReestablishment

UE <-- NW

DCCH

AM

SRB1

RRCReestablishmentComplete

UE --> NW

DCCH

AM

SRB1

RRCReestablishmentRequest

UE --> NW

CCCH

TM

SRB0

RRCReconfiguration

UE <-- NW

DCCH

AM

SRB1,SRB3

RRCReconfigurationComplete

UE --> NW

DCCH

AM

SRB1,SRB3

RRCReject

UE <-- NW

CCCH

TM

SRB0

RRCRelease

UE <-- NW

DCCH

AM

SRB1

RRCResume

UE <-- NW

DCCH

AM

SRB1

RRCResumeComplete

UE --> NW

DCCH

AM

SRB1

RRCResumeRequest

UE --> NW

CCCH

TM

SRB0

RRCResumeRequest1

UE --> NW

CCCH1

TM

SRB0

RRCSetup

UE <-- NW

CCCH

TM

SRB0

RRCSetupComplete

UE --> NW

DCCH

AM

SRB1

RRCSetupRequest

UE --> NW

CCCH

TM

SRB0

RRCSystemInfoRequest

UE --> NW

CCCH

TM

SRB0

SecurityModeCommand

UE <-- NW

DCCH

AM

SRB1

SecurityModeComplete

UE --> NW

DCCH

AM

SRB1

SecurityModeFailure

UE --> NW

DCCH

AM

SRB1

SIB1

UE <-- NW

BCCH

TM

N/A

SystemInformation

UE <-- NW

BCCH

TM

N/A

UEAssistanceInformation

UE --> NW

DCCH

AM

SRB1

UECapabilityEnquiry

UE <-- NW

DCCH

AM

SRB1

UECapabilityInformation

UE --> NW

DCCH

AM

SRB1

ULInformationTransfer

UE --> NW

DCCH

AM

SRB1,SRB2

 

 

 

 

Reference

 

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