LTE - RRCConnectionSetup | LTE-RRC Message

Message Fact Sheet

Protocol	lte-rrc	Network	lte
Spec	3GPP TS 36.331	Spec Section	5.3.3, 6.2.2, Annex B
Direction	eNodeB -> UE	Message Type	Connection Control

Full message name	LTE - RRCConnectionSetup
Protocol	LTE-RRC
Technology	LTE
Direction	eNodeB -> UE
Interface	Uu
Signaling bearer / channel	SRB0 / DL-CCCH
Typical trigger	The eNodeB accepts RRCConnectionRequest and prepares the initial dedicated radio configuration for connected-mode continuation.
Main purpose	Accepts the connection request and delivers the initial dedicated radio configuration needed for LTE RRC connection establishment.
Main specification	3GPP TS 36.331, 5.3.3, 6.2.2, Annex B
Release added	Release 8
Procedures where used	RRC Connection Establishment, Initial Access, Paging Response from Idle
Related timers	T300
Related cause values	RRCConnectionReject is the alternative downlink outcome when setup is not granted

What is RRCConnectionSetup in simple terms?

Downlink LTE RRC message sent by the eNodeB to establish SRB1 and move the UE into connected-mode signaling.

Accepts the connection request and delivers the initial dedicated radio configuration needed for LTE RRC connection establishment.

Why this message matters

RRCConnectionSetup is the eNodeB saying yes to the LTE connection request and giving the UE its first dedicated connected-mode radio configuration.

Where this message appears in the call flow

LTE initial access

Call flow position: Network acceptance message sent after RRCConnectionRequest when the eNodeB proceeds with connection establishment.

Typical state: UE is still on SRB0 and has not yet entered stable connected-mode signaling.

Preconditions:

The eNodeB has accepted the connection request.
The eNodeB has prepared the initial dedicated radio configuration.

Next likely message: RRCConnectionSetupComplete

Paging response path

Call flow position: Fresh connection-establishment message used after paging when the UE must build a new connected context.

Typical state: UE is in RRC_IDLE and is moving back toward connected signaling after a paging-triggered access attempt.

Preconditions:

Paging or service trigger led to new access.
RRCConnectionRequest has already been accepted.

Next likely message: RRCConnectionSetupComplete

Call flow position

Previous message(s): RRCConnectionRequest

Next message(s): RRCConnectionSetupComplete, Security-related continuation

Message direction and transport

Sender and receiver: eNodeB -> UE

Interface: Uu

Domain: Access-side radio control

Signaling bearer: SRB0

Logical channel: DL-CCCH

Transport / encapsulation: RRC over CCCH on RLC TM before SRB1 is activated

Security context: Delivered before AS security activation. The message is not yet protected by the later dedicated AS security context.

Message Structure Overview

RRCConnectionSetup wraps the initial setup payload inside criticalExtensions and provides the first dedicated LTE RRC configuration.
The practical payload is radioResourceConfigDedicated, which creates SRB1 and prepares the UE for later dedicated signaling.
This is the message that turns a successful access attempt into a connected-mode control-plane path.

ASN.1 for LTE - RRCConnectionSetup

RRCConnectionSetup ::= SEQUENCE {
  rrc-TransactionIdentifier RRC-TransactionIdentifier,
  criticalExtensions CHOICE {
    c1 CHOICE {
      rrcConnectionSetup-r8 RRCConnectionSetup-r8-IEs,
      spare3 NULL,
      spare2 NULL,
      spare1 NULL
    },
    criticalExtensionsFuture SEQUENCE {}
  }
}

RRCConnectionSetup-r8-IEs ::= SEQUENCE {
  radioResourceConfigDedicated RadioResourceConfigDedicated,
  nonCriticalExtension         RRCConnectionSetup-v8a0-IEs OPTIONAL
}

How to read this ASN.1

The main trace question is whether radioResourceConfigDedicated is present, internally consistent, and appropriate for the UE and cell context.

Decoded Message Dump

DL-CCCH-Message
  message c1 : rrcConnectionSetup : {
    rrc-TransactionIdentifier 1,
    criticalExtensions c1 : rrcConnectionSetup-r8 : {
      radioResourceConfigDedicated {
        srb-ToAddModList {
          {
            srb-Identity 1
          }
        }
        mac-MainConfig present,
        physicalConfigDedicated present
      }
    }
  }

How to read this dump

SRB1 creation is one of the first things to confirm in a healthy LTE setup path.
radioResourceConfigDedicated is the main container to inspect when setup succeeds on paper but later dedicated signaling fails.
The following message should normally be RRCConnectionSetupComplete.

Important Information Elements

IE	Required	Description
`rrc-TransactionIdentifier`	Yes	Transaction identifier used to correlate the connection-establishment exchange.
`radioResourceConfigDedicated`	Yes	Initial dedicated radio configuration for SRB1 and other connected-mode parameters.
`nextHopChainingCount`	Optional	Appears in later security-related contexts rather than basic early setup and is not the focus of this message.
`nonCriticalExtension`	Optional	Release-extension branch used for forward compatibility.

Detailed field explanation

`rrc-TransactionIdentifier`

Transaction identifier used to correlate the connection-establishment exchange.

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.

`radioResourceConfigDedicated`

Initial dedicated radio configuration for SRB1 and other connected-mode parameters.

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.

`nextHopChainingCount`

Appears in later security-related contexts rather than basic early setup and is not the focus of this message.

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.

`nonCriticalExtension`

Release-extension branch used for forward compatibility.

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.

What to check in logs and traces

Confirm the message follows RRCConnectionRequest and not a rejected access attempt.
Check that SRB1 is present in the dedicated radio configuration.
Inspect radioResourceConfigDedicated for plausible MAC and PHY setup.
Verify the transaction identifier if correlating repeated setup attempts.
Confirm the next UE action is RRCConnectionSetupComplete.

Common Issues and Troubleshooting

RRCConnectionSetup arrives but the UE never completes setup.

Likely cause: The UE may fail to apply or accept part of radioResourceConfigDedicated.

What to inspect: Check SRB1 configuration, MAC main config, physical dedicated settings, and UE capability alignment.

Next step: Compare against a known-good setup for the same UE and cell profile.

The setup path repeats several times.

Likely cause: Earlier access may succeed, but the UE or network cannot hold the new dedicated context.

What to inspect: Correlate RRCConnectionSetup with the later setup-complete path and any immediate release or failure.

Next step: Walk forward from setup rather than stopping at the acceptance message.

Later security or reconfiguration fails.

Likely cause: The issue may start in the initial dedicated radio configuration delivered here.

What to inspect: Check the first connected-mode configuration before debugging only later DCCH messages.

Next step: Review RRCConnectionSetup together with the next dedicated control messages.

LTE / 5G / Variant Comparison

Compared with LTE RRCConnectionRequest

RRCConnectionRequest is the UE asking for access. RRCConnectionSetup is the network accepting that request and delivering the first dedicated LTE RRC configuration.

Compared with NR RRCSetup

LTE RRCConnectionSetup and NR RRCSetup play the same early acceptance role, but the field names and connected-mode setup containers differ between TS 36.331 and TS 38.331.

FAQ

What is RRCConnectionSetup in LTE?

It is the downlink LTE RRC message the eNodeB sends to accept the connection request and start dedicated connected-mode signaling.

What comes after RRCConnectionSetup?

The UE applies the received configuration and responds with RRCConnectionSetupComplete.

Does RRCConnectionSetup create SRB1?

Yes. The initial dedicated radio configuration in this message creates SRB1 for later dedicated LTE RRC signaling.

Why is RRCConnectionSetup important in troubleshooting?

It is the first dedicated LTE RRC configuration point, so later failures often trace back to what was delivered here.

Related message pages

Related Procedures

Related Tools

Use This Reference in Practice

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.