Uplink NR RRC message sent by the UE on SRB0 and CCCH to request establishment of an RRC connection.
Message Fact Sheet
Protocol
rrc
Network
5g
Spec
3GPP TS 38.331
Spec Section
5.3.3.1, 5.3.3.2, 5.3.3.3, 6.2.2
Direction
UE -> gNB
Message Type
Connection Control
Full message name
5G NR - RRC Setup Request
Protocol
RRC
Technology
5G
Direction
UE -> gNB
Interface
Uu
Signaling bearer / channel
SRB0 / UL-CCCH
Typical trigger
Upper layers request an RRC connection while the UE is in RRC_IDLE and has acquired the essential system information of the serving cell.
Main purpose
Starts the NR RRC connection establishment procedure and provides the initial UE identity and establishment cause needed by the gNB and lower layers.
Main specification
3GPP TS 38.331, 5.3.3.1, 5.3.3.2, 5.3.3.3, 6.2.2
Release added
Release 15
Procedures where used
RRC Connection Establishment, Initial Access, Mobile Originated Signalling, Mobile Originated Data, Mobile Terminated Access, Emergency Access
Related timers
T300
What is RRC Setup Request in simple terms?
Uplink NR RRC message sent by the UE on SRB0 and CCCH to request establishment of an RRC connection.
Starts the NR RRC connection establishment procedure and provides the initial UE identity and establishment cause needed by the gNB and lower layers.
Why this message matters
RRCSetupRequest is the UE asking the gNB for a new RRC connection. It carries just enough identity and cause information for the network to decide whether to continue with setup.
Where this message appears in the call flow
5G RRC Connection Setup
Call flow position: First dedicated uplink RRC message after essential access preparation.
Typical state: UE is in RRC_IDLE and requesting transition toward connected signaling.
Preconditions:
UE has selected a suitable cell.
Essential system information has been acquired.
Access control has not barred the attempt.
Next likely message: RRCSetup or RRCReject
5G Initial Registration
Call flow position: Opening access-side control message that enables the later NAS transfer.
Typical state: UE has not yet established SRB1 or transferred the NAS registration message.
Preconditions:
The UE intends to begin access signaling toward the network.
Next likely message: RRCSetup
Call flow position
Previous message(s): Random Access Response / Msg3 access completion
The practical reading order is simple: first determine which UE identity form is present, then inspect the establishment cause. Everything else in the message exists to support that early access decision.
randomValue is normal when the UE does not have a usable 5G-S-TMSI for this attempt.
establishmentCause should match the scenario under test. mo-Signalling is common for registration-driven access.
There is no NAS payload in this message. If you are looking for Registration Request, move to RRCSetupComplete.
Important Information Elements
IE
Required
Description
ue-Identity
Yes
Initial UE identity used to facilitate contention resolution by lower layers.
ng-5G-S-TMSI-Part1
Optional
Conditional form of ue-Identity carrying the rightmost 39 bits of the 5G-S-TMSI when upper layers provide a 5G-S-TMSI.
randomValue
Optional
Conditional form of ue-Identity carrying a 39-bit random value when no 5G-S-TMSI is provided.
establishmentCause
Yes
Indicates why the UE is requesting RRC connection establishment, such as mo-Signalling, mo-Data, mt-Access, emergency, or priority access.
spare
Yes
Reserved spare bit included by the ASN.1 message definition.
Detailed field explanation
ue-Identity
Initial UE identity used to facilitate contention resolution by lower layers.
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.
ng-5G-S-TMSI-Part1
Conditional form of ue-Identity carrying the rightmost 39 bits of the 5G-S-TMSI when upper layers provide a 5G-S-TMSI.
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.
randomValue
Conditional form of ue-Identity carrying a 39-bit random value when no 5G-S-TMSI is provided.
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.
establishmentCause
Indicates why the UE is requesting RRC connection establishment, such as mo-Signalling, mo-Data, mt-Access, emergency, or priority access.
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.
spare
Reserved spare bit included by the ASN.1 message definition.
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.
What to check in logs and traces
Check whether the message appears after essential system information acquisition and access preparation.
Verify whether the UE used ng-5G-S-TMSI-Part1 or randomValue.
Inspect establishmentCause and compare it with the expected scenario.
Confirm T300 starts before or with the procedure and stops only after the correct response.
Correlate the message with the following RRCSetup or RRCReject.
Common Issues and Troubleshooting
RRCSetupRequest is missing.
Likely cause: The UE may be barred by access control, may not have acquired essential SI, or may have failed before the dedicated RRC request stage.
What to inspect: Check cell selection, SIB1 availability, unified access control, and random access outcome.
Next step: Verify the access preparation stage before debugging later RRC messages.
The gNB rejects or deprioritizes the request.
Likely cause: The establishment cause may indicate an access class or request type that is barred or deprioritized.
What to inspect: Check establishmentCause and relevant barring / access control configuration.
Next step: Compare against a successful access scenario with the same cause.
Later signaling looks wrong even though RRCSetupRequest was sent.
Likely cause: The issue may be beyond this message, especially in RRCSetup or RRCSetupComplete.
What to inspect: Use RRCSetupRequest as the entry point, then walk forward through the connection establishment sequence.
Next step: Inspect RRCSetup, SRB1 creation, and the later NAS transfer.
LTE / 5G / Variant Comparison
Compared with LTE RRCConnectionRequest
The LTE analogue plays a similar role at the start of connection establishment, but the exact field names, causes, and procedure framing differ under TS 36.331.
FAQ
What is RRCSetupRequest in 5G NR?
It is the uplink RRC message the UE sends to request establishment of an RRC connection in NR.
What comes after RRCSetupRequest?
The gNB responds with either RRCSetup if the request is accepted or RRCReject if the procedure cannot be handled.
Does RRCSetupRequest carry a NAS message?
No. The initial NAS message is transferred later in RRCSetupComplete, not in RRCSetupRequest.
What identity is used in RRCSetupRequest?
The UE includes either ng-5G-S-TMSI-Part1 when a 5G-S-TMSI is available or a 39-bit randomValue when it is not.
Decode this message with the 3GPP Decoder, inspect the related message database, or open the matching call flow to see where this signaling step fits in the full procedure.