4G/LTE - SRB

  

 

 

 

SRB (Signaling Radio Bearer) mapping for LTE Signaling Message

 

One of the most common mistake we make and spend a lot of time for troubleshooting would be 'Network send some message but does not get any response from UE'. One of the most common reason in this situation (especially when you see the message has gone through L1) would be mismatchs between Signalling message and SRB.

 

3GPP 36.331 4.2.2 Signalling radio bearers says :

  • 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 RRC messages which include logged measurement information as well as for NAS messages, all using DCCH logical channel. SRB2 has a lower-priority than SRB1 and is always configured by E-UTRAN after security activation.

 

 

Default SRB Configuration

 

In many cases (especially with testing environment), default setting is used for SRBs.  In most cases, you wouldn't need to check on the detailed parameter of these default settings but you would need to check on the details in case that you need to do some troubleshooting.  The detailed parameter setting for the default SRB are specified in 36.331 - 9 Specified and default radio configurations as shown below.

 

 

< SRB1 >

 

< 36.331-9.1.2.1 SRB1 >

< 36.331-9.2.1.1 SRB1 >

 

 

< SRB1bis >

 

< 36.331-9.1.2.1a SRB1bis >

 

 

< SRB2 >

 

< 36.331-9.1.2.2 SRB2>

< 36.331-9.2.1.2 SRB2 >

 

 

SRB Mapping to Signalling Message

 

Also 36.331 defines the SRB mapping for each message as shown below. I recommend you to pay attention to these mapping examples. If SRB number does not match between the sender and the reciever, the message would not reach the destination even though the message is properly sent and reach the lower layer of the destination.

 

    MasterInformationBlock

    • Signalling radio bearer: N/A
    • RLC-SAP: TM
    • Logical channel: BCCH
    • Direction: E-UTRAN to UE

     

    SystemInformationBlockType1

    • Signalling radio bearer: N/A
    • RLC-SAP: TM
    • Logical channel: BCCH
    • Direction: E-UTRAN to UE

     

    RRCConnectionRequest

    • Signalling radio bearer: SRB0
    • RLC-SAP: TM
    • Logical channel: CCCH
    • Direction: UE to E-UTRAN

     

    RRCConnectionSetup

    • Signalling radio bearer: SRB0
    • RLC-SAP: TM
    • Logical channel: CCCH
    • Direction: E-UTRAN to UE

     

    RRCConnectionSetupComplete

    • Signalling radio bearer: SRB1
    • RLC-SAP: AM
    • Logical channel: DCCH
    • Direction: UE to E-UTRAN

     

    RRCConnectionReconfiguration

    • Signalling radio bearer: SRB1
    • RLC-SAP: AM
    • Logical channel: DCCH
    • Direction: E-UTRAN to UE

     

    MeasurementReport

    • Signalling radio bearer: SRB1
    • RLC-SAP: AM
    • Logical channel: DCCH
    • Direction: UE to E-UTRAN

     

    MobilityFromEUTRACommand

    • Signalling radio bearer: SRB1
    • RLC-SAP: AM
    • Logical channel: DCCH
    • Direction: E-UTRAN to UE

     

    UECapabilityEnquiry

    • Signalling radio bearer: SRB1
    • RLC-SAP: AM
    • Logical channel: DCCH
    • Direction: E-UTRAN to UE

     

    UEInformationRequest

    • Signalling radio bearer: SRB1
    • RLC-SAP: AM
    • Logical channel: DCCH
    • Direction: E-UTRAN to UE

     

    DLInformationTransfer

    • Signalling radio bearer: SRB2 or SRB1 (only if SRB2 not established yet. If SRB2 is suspended, E-UTRAN does not send this message until SRB2 is resumed.)
    • RLC-SAP: AM
    • Logical channel: DCCH
    • Direction: E-UTRAN to UE

     

    Paging

    • Signalling radio bearer: N/A
    • RLC-SAP: TM
    • Logical channel: PCCH
    • Direction: E-UTRAN to UE