What is UE Context Setup Request in simple terms?
UE Context Setup Request is the UE-associated F1AP message the gNB-CU sends to the gNB-DU to create a UE context, nominate the SpCell, transfer CU-to-DU RRC content, and request setup of SRBs, DRBs, and optional IAB, relay, sidelink, multicast, or mobility-related resources before the DU replies with UE Context Setup Response or UE Context Setup Failure.
Requests creation of the DU-side UE context and instructs the gNB-DU which radio, bearer, QoS, transport, and optional mobility or IAB-related resources to establish for that UE.
Why this message matters
UE Context Setup Request is the CU-to-DU command that tells the DU which serving cell, RRC payload, bearers, and transport endpoints to create for a UE on F1.
ASN.1 for 5G F1AP - UE Context Setup Request
UEContextSetupRequest ::= SEQUENCE {
protocolIEs ProtocolIE-Container { {UEContextSetupRequest-IEs} },
...
}
UEContextSetupRequest-IEs F1AP-PROTOCOL-IES ::= {
{ gNB-CU UE F1AP ID, mandatory } |
{ gNB-DU UE F1AP ID, optional } |
{ SpCell ID, mandatory } |
{ ServCellIndex, mandatory } |
{ CU to DU RRC Information, mandatory } |
{ DRX Cycle, optional } |
{ Candidate SpCell List, optional } |
{ SCell To Be Setup List, optional } |
{ SRB To Be Setup List, optional } |
{ DRB To Be Setup List, optional } |
{ RRC-Container, optional } |
{ Inactivity Monitoring Request, optional } |
{ Conditional Inter-DU Mobility Information, optional } |
{ Configured BAP Address, optional } |
{ F1-C Transfer Path, optional },
...
}
DRB-ToBeSetup-Item ::= SEQUENCE {
dRB-ID DRB-ID,
dRB-Information DRB-Information,
uL-UP-TNL-Information-toBeSetup-List UL-UP-TNL-Information-toBeSetup-List,
rLCMode RLC-Mode,
uL-Configuration UL-Configuration OPTIONAL,
duplicationActivation DuplicationActivation OPTIONAL,
dL-PDCP-SN-Length ENUMERATED { 12bits, 18bits, ... },
uL-PDCP-SN-Length ENUMERATED { 12bits, 18bits, ... } OPTIONAL,
...
}
UPTransportLayerInformation ::= CHOICE {
gTPTunnel SEQUENCE {
transportLayerAddress TransportLayerAddress,
gTP-TEID GTP-TEID
},
...
}
UEContextSetupFailure ::= SEQUENCE {
protocolIEs ProtocolIE-Container { {UEContextSetupFailure-IEs} },
...
}
How to read this ASN.1
The full ASN.1 for UE Context Setup Request is large because it covers normal NR context setup plus many optional mobility, relay, multicast, and IAB variants. For trace reading, the minimal mental model is: identifiers, target SpCell, CU-to-DU RRC payload, bearer setup lists, and any mobility or candidate-cell modifiers.
Detailed field explanation
gNB-CU UE F1AP ID
Mandatory CU-side UE identifier that anchors the UE-associated signaling context for the request.
Presence: Required
In practice: In practice, compare this field with the original request and with any later release-dependent optional fields so you can see whether the network accepted the same service model the UE asked for.
gNB-DU UE F1AP ID
Optional DU-side UE identifier. It becomes mandatory in the successful response, but may be absent in the initial request when the DU has not yet allocated it.
Presence: Optional
In practice: In practice, compare this field with the original request and with any later release-dependent optional fields so you can see whether the network accepted the same service model the UE asked for.
SpCell ID
Mandatory target special cell for the UE context. In handover-related use, this is the target cell the DU is asked to accept.
Presence: Required
In practice: In practice, compare this field with the original request and with any later release-dependent optional fields so you can see whether the network accepted the same service model the UE asked for.
ServCellIndex
Mandatory serving-cell index associated with the SpCell in the request.
Presence: Required
In practice: In practice, compare this field with the original request and with any later release-dependent optional fields so you can see whether the network accepted the same service model the UE asked for.
CU to DU RRC Information
Mandatory container carrying CU-originated RRC payloads and related control objects such as CellGroupConfig, measurement configuration, UE capability containers, assistance information, and gap-related requests.
Presence: Required
In practice: In practice, compare this field with the original request and with any later release-dependent optional fields so you can see whether the network accepted the same service model the UE asked for.
DRX Cycle
Optional desired DRX configuration including long cycle and optional short cycle or timer values.
Presence: Optional
In practice: In practice, compare this field with the original request and with any later release-dependent optional fields so you can see whether the network accepted the same service model the UE asked for.
Candidate SpCell List
Optional list of candidate SpCells that becomes important when the requested SpCell cannot be accepted and the DU may propose a Potential SpCell List in failure handling.
Presence: Optional
In practice: In practice, compare this field with the original request and with any later release-dependent optional fields so you can see whether the network accepted the same service model the UE asked for.
SCell To Be Setup List
Optional list of candidate SCells the DU may set up for the UE, with optional UL and servingCellMO attributes.
Presence: Optional
In practice: In practice, compare this field with the original request and with any later release-dependent optional fields so you can see whether the network accepted the same service model the UE asked for.
SRB To Be Setup List
Optional SRB configuration block. Each item names the SRB and can request duplication, SDT-specific bearer configuration, or relay mapping information.
Presence: Optional
In practice: In practice, compare this field with the original request and with any later release-dependent optional fields so you can see whether the network accepted the same service model the UE asked for.
DRB To Be Setup List
Optional but operationally central DRB block. Each item defines the DRB ID, QoS structure, mapped QoS flows, UL transport endpoints, RLC mode, PDCP SN lengths, duplication behavior, and related control options.
Presence: Optional
In practice: In practice, compare this field with the original request and with any later release-dependent optional fields so you can see whether the network accepted the same service model the UE asked for.
QoS Flow Level QoS Parameters
Mandatory inside DRB and several other setup items. It carries QoS characteristics, NG-RAN ARP, optional GBR information, QoS monitoring, PDU session AMBR context, and optional bitrate-adaptation or PDU-set signaling.
Presence: Required
In practice: QoS rules are the real service profile of the session. Inspect the QFI mapping, packet filters, and precedence because those values explain how user traffic will actually be classified and forwarded.
UL UP TNL Information to Be Setup List
Mandatory inside each DRB to be set up item. Each entry identifies the gNB-CU side F1-U tunnel endpoint by transport address and GTP TEID for uplink packets.
Presence: Required
In practice: In practice, compare this field with the original request and with any later release-dependent optional fields so you can see whether the network accepted the same service model the UE asked for.
RRC-Container
Optional DL-DCCH message encapsulated in a PDCP PDU. If present, the DU sends the corresponding RRC message to the UE via SRB1.
Presence: Optional
In practice: In practice, compare this field with the original request and with any later release-dependent optional fields so you can see whether the network accepted the same service model the UE asked for.
Inactivity Monitoring Request
Optional instruction asking the DU to perform UE inactivity monitoring if supported.
Presence: Optional
In practice: In practice, compare this field with the original request and with any later release-dependent optional fields so you can see whether the network accepted the same service model the UE asked for.
gNB-DU UE Aggregate Maximum Bit Rate Uplink
Conditional aggregate uplink bit rate to be enforced by the gNB-DU when DRBs are configured.
Presence: Optional
In practice: In practice, compare this field with the original request and with any later release-dependent optional fields so you can see whether the network accepted the same service model the UE asked for.
Conditional Inter-DU Mobility Information
Optional mobility-preparation block for CHO, CPAC, or related target-cell preparation. It can drive Requested Target Cell handling in the response or failure outcome.
Presence: Optional
In practice: In practice, compare this field with the original request and with any later release-dependent optional fields so you can see whether the network accepted the same service model the UE asked for.
Configured BAP Address
Optional BAP address used for the corresponding child IAB-node when the setup request also configures IAB-related transport behavior.
Presence: Optional
In practice: In practice, compare this field with the original request and with any later release-dependent optional fields so you can see whether the network accepted the same service model the UE asked for.
F1-C Transfer Path
Optional F1-C transfer-path control that the gNB-DU shall, if supported, take into account.
Presence: Optional
In practice: In practice, compare this field with the original request and with any later release-dependent optional fields so you can see whether the network accepted the same service model the UE asked for.
