Signaling Radio Bearer's:
ENDC (EUTRA-NR dual connectivity- 3gpp release-15 feature- deployment Option-3) is a feature that is introduced to support new radio NR (5G) data with existing LTE core and radio networks and without introducing 5G core network.
This ENDC (EUTRA New Radio dual connectivity) can be a useful feature for heterogeneous network where LTE provides reliable coverage and NR can be used for improving data rates.
In ENDC, the schedulers in the eNB and gNB are independent from each other and make their own decisions how much and when the data is to be send or receive from UE.
This is also applicable to RRC, and the device now has one RRC context to the eNB and another separate and independent RRC context to the gNB.
Now the question is how RRC messages from th eNB and en-gNB are sent to and from UE?
There are two types of bearers SRB and DRB (Data radio bearer). SRB (Signaling Radio Bearer) is a type of radio bearer that carries signaling message i.e. RRC or NAS messages or both. DRB is used to carry the data associated with EPS bearer. There are four types of SRB in NR defined as follows:
SRB0:
SRB0 is for RRC messages using the CCCH logical channel (before UE attached with network, common signaling not assigned to UE) it will used RLC-TM mode. SRB0 is used During RRC Connection setup and re-establishment process.
SRB1:
SRB1 is for RRC (which may include a piggybacked NAS message) as well as for NAS (encapsulate) messages prior to the establishment of SRB2. All SRB1 msg are transmitting using DCCH logical channel and it will used RLC-AM mode. SRB1 priority is always higher than SRB2. It used for High priority DCCH messages.
SRB2:
SRB2 is for NAS messages only. All SRB2 msg are transmitting using DCCH logical channel and it will used RLC-AM mode. It has a lower-priority than SRB1 and is always configured by the network after security activation. It is used for Lower priority DCCH messages and established after security has been configured.
SRB3:
SRB3 is for specific RRC messages when UE is in EN-DC, All SRB3 msg are transmitting using DCCH logical channel and it will used RLC-AM mode. It is used for DCCH messages for secondary node in dual connectivity. It is established and changed by the secondary node. Also carries SN RRC reconfiguration and SN measurement report messages.
What is NAS?
In LTE it is a signaling protocol that operates between UE and MME whereas in 5G it operates between UE and AMF/SMF.
Which AP (Application Protocol) is used to transfer NAS messages between UE and AMF?
NGAP is used to transfer NAS msg between UE and AMF.
Note: in NR NAS message associated with session management terminate at SMF rather than at AMF. The AMF acts a relay between the BS and SMF for session management NAS messages.
Can we configure all SRB simultaneously?
Yes, in ENDC case SRB 0,1,2 is established between UE and Master node (eNB) and SRB 3 is established between UE and Secondary Node(en-gNB).
Is it mandatory to configure SRB3?
No, it is not mandatory. If SRB3 is not configured by network then LTE RRC (vai X2 interface) are able to pass the NR RRC messages between the UE and en-gNB and it’s utilizing SRB1
SRBs (Signaling Radio Bearers) are nothing but the Radio Bearers that are used only for the transmission of NAS and RRC messages. In more convenient way SRBs can be defined as follow:
In downlink piggybacking of NAS messages is used only for bearer establishment or modification or release. Whereas, in uplink piggybacking of NAS message is used only for transferring the initial NAS message during connection resume and connection setup.FFS Piggybacking of NAS messages in other procedures than bearer establishment or modification or release.
NOTE: The NAS messages transferred through SRB2 are also contained in RRC messages, which however do not include any RRC protocol control information.
As soon as the security is activated, all RRC messages on SRB1, SRB2 and SRB3, including those NAS messages, are integrity protected as well as ciphered by PDCP. Integrity protection and ciphering to NAS messages are applied independently by NAS itself.
The integrity protection and ciphering algorithm is common for SRB1 and SRB2. When not configured with any kind of DC, the integrity protection and ciphering algorithm is common for all SRBs that is SRB1, SRB2 and DRBs. The security configuration for all DRBs related to same PDU is same.
Does integrity Protection and ciphering are applied to SRB0?
The integrity protection and ciphering are not applied to SRB0.
What is the motivation for SRB3?
· Below are the motivations for SRB3:
· Lower signaling delay over NR interface
· Less processing at MCG.
· Can only be used for message that does not need coordination between MCG and SCG.
Do SCG SRB has higher priority than DRBs?
SCG SRB has higher scheduling priority than all DRBs.
What is the Range of LCID ?
Logical channel ID range is 1 to 10.
What is a Control channel?
It is carrying control plane information (SRB) between UE and eNodeB.
LCID | SRB-ID |
1 | SRB1 |
2 | SRB2 |
Message Mapping:
What is a traffic channel?
It is carrying user plane information (DRB) between UE and eNodeB/gNB
What is maximum range of DRB in LTE?
Maximum DRB as per spec 36.331 is 11 for LTE.
What is maximum range of DRB in NR?
its governed by the IE drb-ToAddModList which is .
You can find it from below as>
//Maximum number of DRBs (that can be added in DRB-ToAddModLIst).
maxDRB INTEGER ::= 29
RRC Constrains in ASN (38.331)
//Max number of serving cells (SpCell + SCells) per cell group minus 1
maxNrofServingCells-1 INTEGER ::= 31
maxNrofAggregatedCellsPerCellGroup INTEGER ::= 16
//Max number of secondary serving cells per cell group
maxNrofSCells INTEGER ::= 31
//Maximum number of entries in each of the cell lists in a measurement object
maxNrofCellMeas INTEGER ::= 32
//Max number for the (max) number of SS blocks to average to determine cell measurement
maxNrofSS-BlocksToAverage INTEGER ::= 16
//Max number for the (max) number of CSI-RS to average to determine cell measurement
maxNrofCSI-RS-ResourcesToAverage INTEGER ::= 16
//Maximum number of PDSCH time domain resource allocations
maxNrofDL-Allocations INTEGER ::= 16
//Maximum number of SR configurations per cell group
maxNrofSR-ConfigPerCellGroup INTEGER ::= 8
//Maximum value of LCG ID
maxLCG-ID INTEGER ::= 7
//Maximum value of Logical Channel ID
maxLC-ID INTEGER ::= 32
//Maximum number of Timing Advance Groups
maxNrofTAGs INTEGER ::= 4
maxNrofTAGs-1 INTEGER ::= 3
//Maximum number of BWPs per serving cell
maxNrofBWPs INTEGER ::= 4
//Maximum number of reported MR-DC combinations for IDC
maxNrofCombIDC INTEGER ::= 128
//Maximum index identifying a symbol within a slot (14 symbols, indexed from 0..13)
maxNrofSymbols-1 INTEGER ::= 13
//Maximum number of slots in a 10 ms period
maxNrofSlots INTEGER ::= 320
maxNrofSlots-1 INTEGER ::= 319
//Maximum number of PRBs
maxNrofPhysicalResourceBlocks INTEGER ::= 275
maxNrofPhysicalResourceBlocks-1 INTEGER ::= 274
maxNrofPhysicalResourceBlocksPlus1 INTEGER ::= 276
//Max number of CoReSets configurable on a serving cell minus 1
maxNrofControlResourceSets-1 INTEGER ::= 11
//Max number of OFDM symbols in a control resource set
maxCoReSetDuration INTEGER ::= 3
// Max number of Search Spaces minus 1
maxNrofSearchSpaces-1 INTEGER ::= 39
//Max number payload of a DCI scrambled with SFI-RNTI
maxSFI-DCI-PayloadSize INTEGER ::= 128
maxSFI-DCI-PayloadSize-1 INTEGER ::= 127
//Max number payload of a DCI scrambled with INT-RNTI
maxINT-DCI-PayloadSize INTEGER ::= 126
maxINT-DCI-PayloadSize-1 INTEGER ::= 125
//Max number of rate matching patterns that may be configured
maxNrofRateMatchPatterns INTEGER ::= 4
maxNrofRateMatchPatterns-1 INTEGER ::= 3
//Max number of rate matching patterns that may be configured in one group
maxNrofRateMatchPatternsPerGroup INTEGER ::= 8
//Maximum number of report configurations
maxNrofCSI-ReportConfigurations INTEGER ::= 48
maxNrofCSI-ReportConfigurations-1 INTEGER ::= 47
//Maximum number of resource configurations
maxNrofCSI-ResourceConfigurations INTEGER ::= 112
maxNrofCSI-ResourceConfigurations-1 INTEGER ::= 111
//Maximum number of DL band combinations
maxBandComb INTEGER ::= 65536
//Maximum number of NR blacklisted cell ranges in SIB3, SIB4
maxCellBlack INTEGER ::= 16
//Maximum number of inter-Freq cells listed in SIB4
maxCellInter INTEGER ::= 16
//Maximum number of intra-Freq cells listed in SIB3
maxCellIntra INTEGER ::= 16
//Maximum number of cells in EUTRAN
maxCellMeasEUTRA INTEGER ::= 32
//Maximum value of EUTRA carrier frequency
maxEARFCN INTEGER ::= 262143
//Maximum number of EUTRA-blacklisted physical cell identity ranges in SIB5
maxEUTRA-CellBlack INTEGER ::= 16
//Maximum number of NS and P-Max values per band
maxEUTRA-NS-Pmax INTEGER ::= 8
//Maximum number of additional frequency bands that a cell belongs to
maxMultiBands INTEGER ::= 8
//Maximum value of NR carrier frequency
maxNARFCN INTEGER ::= 3279165
//Maximum number of NS and P-Max values per band
maxNR-NS-Pmax INTEGER ::= 8
//Max number of serving cells (SpCell + SCells) per cell group
maxNrofServingCells INTEGER ::= 32
maxNrofAP-CSI-RS-ResourcesPerSet INTEGER ::= 16
//Maximum number of triggers for aperiodic CSI reporting
maxNrOfCSI-AperiodicTriggers INTEGER ::= 128
//Maximum number of report configurations per trigger state for aperiodic reporting
maxNrofReportConfigPerAperiodicTrigger INTEGER ::= 16
//Maximum number of Non-Zero-Power (NZP) CSI-RS resources
maxNrofNZP-CSI-RS-Resources INTEGER ::= 192
maxNrofNZP-CSI-RS-Resources-1 INTEGER ::= 191
//Maximum number of NZP CSI-RS resources per resource set
maxNrofNZP-CSI-RS-ResourcesPerSet INTEGER ::= 64
//Maximum number of NZP CSI-RS resources per cell
maxNrofNZP-CSI-RS-ResourceSets INTEGER ::= 64
maxNrofNZP-CSI-RS-ResourceSets-1 INTEGER ::= 63
//Maximum number of resource sets per resource configuration
maxNrofNZP-CSI-RS-ResourceSetsPerConfig INTEGER ::= 16
//Maximum number of resources per resource configuration
maxNrofNZP-CSI-RS-ResourcesPerConfig INTEGER ::= 128
//Maximum number of Zero-Power (NZP) CSI-RS resources
maxNrofZP-CSI-RS-Resources INTEGER ::= 32
maxNrofZP-CSI-RS-Resources-1 INTEGER ::= 31
maxNrofZP-CSI-RS-ResourceSets-1 INTEGER ::= 15
maxNrofZP-CSI-RS-ResourcesPerSet INTEGER ::= 16
maxNrofZP-CSI-RS-ResourceSets INTEGER ::= 16
//Maximum number of CSI-IM resources. See CSI-IM-ResourceMax in 38.214.
maxNrofCSI-IM-Resources INTEGER ::= 32
maxNrofCSI-IM-Resources-1 INTEGER ::= 31
//Maximum number of CSI-IM resources per set. See CSI-IM-ResourcePerSetMax in 38.214
maxNrofCSI-IM-ResourcesPerSet INTEGER ::= 8
//Maximum number of NZP CSI-IM resources per cell
maxNrofCSI-IM-ResourceSets INTEGER ::= 64
maxNrofCSI-IM-ResourceSets-1 INTEGER ::= 63
//Maximum number of CSI IM resource sets per resource configuration
maxNrofCSI-IM-ResourceSetsPerConfig INTEGER ::= 16
//Maximum number of SSB resources in a resource set
maxNrofCSI-SSB-ResourcePerSet INTEGER ::= 64
//Maximum number of CSI SSB resource sets per cell
maxNrofCSI-SSB-ResourceSets INTEGER ::= 64
maxNrofCSI-SSB-ResourceSets-1 INTEGER ::= 63
//Maximum number of CSI SSB resource sets per resource configuration
maxNrofCSI-SSB-ResourceSetsPerConfig INTEGER ::= 1
//Maximum number of failure detection resources
maxNrofFailureDetectionResources INTEGER ::= 10
maxNrofFailureDetectionResources-1 INTEGER ::= 9
//Maximum number of measurement objects
maxNrofObjectId INTEGER ::= 64
//Maximum number of page records
maxNrofPageRec INTEGER ::= 32
//Maximum number of PCI ranges
maxNrofPCI-Ranges INTEGER ::= 8
//Maximum number of PLMNs broadcast and reported by UE at establisghment
maxPLMN INTEGER ::= 12
//Maximum number of CSI-RS resources for an RRM measurement object
maxNrofCSI-RS-ResourcesRRM INTEGER ::= 96
maxNrofCSI-RS-ResourcesRRM-1 INTEGER ::= 95
//Maximum number of configured measurements
maxNrofMeasId INTEGER ::= 64
//Maximum number of quantity configurations
maxNrofQuantityConfig INTEGER ::= 2
//Maximum number of FFS
maxNrofCSI-RS-CellsRRM INTEGER ::= 96
//Maximum number of SRS resource sets in a BWP.
maxNrofSRS-ResourceSets INTEGER ::= 16
maxNrofSRS-ResourceSets-1 INTEGER ::= 15
//Maximum number of SRS resources.
maxNrofSRS-Resources INTEGER ::= 64
maxNrofSRS-Resources-1 INTEGER ::= 63
//Maximum number of SRS resources in an SRS resource set
maxNrofSRS-ResourcesPerSet INTEGER ::= 16
//Maximum number of SRS trigger states minus 1, i.e., the largest code point.
maxNrofSRS-TriggerStates-1 INTEGER ::= 3
maxNrofSRS-TriggerStates-2 INTEGER ::= 2
//Maximum number of interworking RAT containers (incl NR and MRDC)
maxRAT-CapabilityContainers INTEGER ::= 8
//Maximum number of simultaneously aggregated bands
maxSimultaneousBands INTEGER ::= 32
//Maximum number of Slot Format Combinations in a SF-Set.
maxNrofSlotFormatCombinationsPerSet INTEGER ::= 512
maxNrofSlotFormatCombinationsPerSet-1 INTEGER ::= 511
//Maximum number of PUCCH Resources
maxNrofPUCCH-Resources INTEGER ::= 128
maxNrofPUCCH-Resources-1 INTEGER ::= 127
//Maximum number of PUCCH Resource Sets
maxNrofPUCCH-ResourceSets INTEGER ::= 4
maxNrofPUCCH-ResourceSets-1 INTEGER ::= 3
//Maximum number of PUCCH Resources per PUCCH-Resour©ceSet
maxNrofPUCCH-ResourcesPerSet INTEGER ::= 32
//Maximum number of P0-pucch present in a p0-pucch set
maxNrofPUCCH-P0-PerSet INTEGER ::= 8
//Maximum number of RSs used as pathloss reference for PUCCH power control.
maxNrofPUCCH-PathlossReferenceRSs INTEGER ::= 4
maxNrofPUCCH-PathlossReferenceRSs-1 INTEGER ::= 3
//Maximum number of P0-pusch-alpha-sets (see 38,213, section 7.1)
maxNrofP0-PUSCH-AlphaSets INTEGER ::= 30
maxNrofP0-PUSCH-AlphaSets-1 INTEGER ::= 29
//Maximum number of RSs used as pathloss reference for PUSCH power control.
maxNrofPUSCH-PathlossReferenceRSs INTEGER ::= 4
maxNrofPUSCH-PathlossReferenceRSs-1 INTEGER ::= 3
//Maximum number of supported NAICS capability set
maxNrofNAICS-Entries INTEGER ::= 8
//Maximum number of supported bands in UE capability.
maxBands INTEGER ::= 1024
maxBandsMRDC INTEGER ::= 1280
maxBandsEUTRA INTEGER ::= 256
//
maxCellReport INTEGER ::= 8
//Maximum number of DRBs (that can be added in DRB-ToAddModLIst).
maxDRB INTEGER ::= 29
//Max number of frequencies.
maxFreq INTEGER ::= 8
//Maximum number of candidate NR frequencies for MR-DC IDC indication
maxFreqIDC-MRDC INTEGER ::= 32
//
maxNrofCSI-RS INTEGER ::= 64
//Max number of PRACH-ResourceDedicatedBFR that in BFR config.
maxNrofCandidateBeams INTEGER ::= 16
//Maximun number of PCIs per SMTC.
maxNrofPCIsPerSMTC INTEGER ::= 64
//
maxNrofQFIs INTEGER ::= 64
//Maximum number of triggers for semi persistent reporting on PUSCH
maxNrOfSemiPersistentPUSCH-Triggers INTEGER ::= 64
//Maximum number of SR resources per BWP in a cell.
maxNrofSR-Resources INTEGER ::= 8
//
maxNrofSlotFormatsPerCombination INTEGER ::= 256
//
maxNrofSpatialRelationInfos INTEGER ::= 8
//
maxNrofIndexesToReport INTEGER ::= 32
//
maxNrofIndexesToReport2 INTEGER ::= 64
//Maximum number of SSB resources in a resource set.
maxNrofSSBs INTEGER ::= 64
maxNrofSSBs-1 INTEGER ::= 63
//Maximum number of S-NSSAI.
maxNrofS-NSSAI INTEGER ::= 8
//
maxNrofTCI-StatesPDCCH INTEGER ::= 64
//Maximum number of TCI states.
maxNrofTCI-States INTEGER ::= 128
maxNrofTCI-States-1 INTEGER ::= 127
//Maximum number of PUSCH time domain resource allocations
maxNrofUL-Allocations INTEGER ::= 16
maxQFI INTEGER ::= 63
maxRA-CSIRS-Resources INTEGER ::= 96
//Maximum number of RA occasions for one CSI-RS
maxRA-OccasionsPerCSIRS INTEGER ::= 64
//Maximum number of RA occasions in the system
maxRA-Occasions-1 INTEGER ::= 511
maxRA-SSB-Resources INTEGER ::= 64
maxSCSs INTEGER ::= 5
maxSecondaryCellGroups INTEGER ::= 3
maxNrofServingCellsEUTRA INTEGER ::= 32
maxMBSFN-Allocations INTEGER ::= 8
maxNrofMultiBands INTEGER ::= 8
//Maximum number of cells for SFTD reporting
maxCellSFTD INTEGER ::= 3
maxReportConfigId INTEGER ::= 64
//Maximum number of codebooks suppoted by the UE
maxNrofCodebooks INTEGER ::= 16
maxNrofSRI-PUSCH-Mappings INTEGER ::= 16
maxNrofSRI-PUSCH-Mappings-1 INTEGER ::= 15
// Maximum number of SIBs
maxSIB INTEGER::= 32
maxSIB-1 INTEGER::= 31
//Maximum number of SI messages
maxSI-Message INTEGER::= 32
//Maximum number of Acccess Categories minus 1
maxAccessCat-1 INTEGER ::= 63
//Maximum number of Acccess Categories
maxBarringInfoSet INTEGER ::= 8
//Maximum number of EUTRA cells in SIB list
maxCellEUTRA INTEGER ::= 8
// Maximum number of EUTRA carriers in SIB list
maxEUTRA-Carrier INTEGER ::= 8
// Maximum number of PLMN identites in RAN area configurations
maxPLMNIdentities INTEGER ::= 8
// (for NR DL) Total number of FeatureSets (size of the pool)
maxDownlinkFeatureSets INTEGER ::= 1024
//(for NR UL) Total number of FeatureSets (size of the pool)
maxUplinkFeatureSets INTEGER ::= 1024
//(for EUTRA) Total number of FeatureSets (size of the pool)
maxEUTRA-DL-FeatureSets INTEGER ::= 256
//(for EUTRA) Total number of FeatureSets (size of the pool)
maxEUTRA-UL-FeatureSets INTEGER ::= 256
//(for NR) The number of feature sets associated with one band.
maxFeatureSetsPerBand INTEGER ::= 128
//(for NR) Total number of CC-specific FeatureSets (size of the pool)
maxPerCC-FeatureSets I NTEGER ::= 1024
//(for MR-DC/NR)Total number of Feature set combinations (size of the pool)
maxFeatureSetCombinations INTEGER ::= 1024
maxInterRAT-RSTD-Freq INTEGER ::= 3
What is EPS-Bearer ID and its range?
NAS identifies the bearer base on the EPS-Bearer ID, which has a range from 0 to 15. But in that 0 to 4 are reserved, so 5 to 15 are used.
LCID (logical channel ID) & DRB ID & EPS-Bearer ID mapping:
Note: Most of the operator use same LCID and DRB-ID for better understand. It is not necessary to start DRB ID from 3, as its maximum range is 11 so we can give any DRB ID number between 1 to 11.
What is maximum possible DRB ID, UE can establish in the LTE case?
Since LCID is having range from 3 to 10, hence as per above table maximum 8 DRB’s can be established per UE at a time in LTE.
In EN-DC, SRB3 can be used for measurement configuration and reporting, to configure and reconfigure MAC, RLC, physical layer and RLF timers and constants of the SCG configuration, and to reconfigure PDCP for DRBs associated with the S-KgNB or SRB3, given that the (re-)configuration does not require any MeNB involvement.
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