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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.
The SRB structure is well illustrated by Rahim Navael as
shown below. Rahim kindly approved me to share the illustration in my note.
Image Source : Rahim Navaei's linked in post
Default SRB Configuration
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RRC information elements
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SRB1/1S
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SRB2/2S
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SRB3
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PDCP-Config
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t-Reordering
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Infinity
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Infinity
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Infinity
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RLC-Config CHOICE
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AM
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AM
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AM
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ul-RLC-Config
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sn-FieldLength
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size12
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size12
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size12
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t-PollRetransmit
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ms45
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ms45
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ms45
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pollPDU
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Infinity
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Infinity
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Infinity
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pollByte
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Infinity
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Infinity
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Infinity
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maxRetxThreshold
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t8
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t8
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t8
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dl-RLC-Config
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sn-FieldLength
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size12
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size12
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size12
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t-Reassembly
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ms35
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ms35
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ms35
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t-StatusProhibit
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ms0
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ms0
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ms0
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logicalChannelIdentity
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1
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2
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3
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LogicalChannelConfig
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priority
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1
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3
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1
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prioritisedBitRate
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Infinity
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Infinity
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Infinity
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logicalChannelGroup
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0
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0
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0
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SRB Mapping to Signalling Message
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Message
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Direction
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Logical Channel
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RLC
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SRB
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MasterInformationBlock
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UE <-- NW
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BCCH
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TM
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N/A
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MeasurementReport
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UE --> NW
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DCCH
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AM
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SRB1,SRB3
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MobilityFromNRCommand
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UE <-- NW
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DCCH
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AM
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SRB1
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Paging
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UE <-- NW
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PCCH
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TM
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N/A
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RRCReestablishment
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UE <-- NW
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DCCH
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AM
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SRB1
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RRCReestablishmentComplete
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UE --> NW
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DCCH
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AM
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SRB1
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RRCReestablishmentRequest
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UE --> NW
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CCCH
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TM
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SRB0
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RRCReconfiguration
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UE <-- NW
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DCCH
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AM
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SRB1,SRB3
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RRCReconfigurationComplete
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UE --> NW
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DCCH
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AM
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SRB1,SRB3
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RRCReject
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UE <-- NW
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CCCH
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TM
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SRB0
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RRCRelease
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UE <-- NW
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DCCH
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AM
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SRB1
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RRCResume
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UE <-- NW
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DCCH
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AM
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SRB1
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RRCResumeComplete
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UE --> NW
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DCCH
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AM
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SRB1
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RRCResumeRequest
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UE --> NW
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CCCH
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TM
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SRB0
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RRCResumeRequest1
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UE --> NW
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CCCH1
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TM
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SRB0
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RRCSetup
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UE <-- NW
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CCCH
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TM
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SRB0
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RRCSetupComplete
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UE --> NW
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DCCH
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AM
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SRB1
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RRCSetupRequest
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UE --> NW
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CCCH
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TM
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SRB0
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RRCSystemInfoRequest
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UE --> NW
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CCCH
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TM
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SRB0
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SecurityModeCommand
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UE <-- NW
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DCCH
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AM
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SRB1
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SecurityModeComplete
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UE --> NW
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DCCH
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AM
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SRB1
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SecurityModeFailure
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UE --> NW
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DCCH
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AM
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SRB1
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SIB1
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UE <-- NW
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BCCH
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TM
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N/A
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SystemInformation
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UE <-- NW
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BCCH
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TM
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N/A
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UEAssistanceInformation
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UE --> NW
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DCCH
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AM
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SRB1
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UECapabilityEnquiry
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UE <-- NW
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DCCH
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AM
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SRB1
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UECapabilityInformation
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UE --> NW
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DCCH
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AM
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SRB1
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ULInformationTransfer
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UE --> NW
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DCCH
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AM
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SRB1,SRB2
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Reference
[1]
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