NR NTN Call Flow

This page walks through a successful 5G NR Standalone registration and PDU session establishment over a simulated non-terrestrial network. The decoded messages come from an Amarisoft Callbox export captured on 17 June 2026. The cell operates in FDD band n256 with PCI 500 and a Release 17 NTN system-information configuration.

NOTE: The extracts below retain only fields needed to explain the procedure. Subscriber identifiers and security material are omitted. Amarisoft also provides an interactive sample-log tutorial.

Test Profile

The test profile establishes the radio, cell, UE, and data-session context for interpreting the call flow. These values describe the supplied sample trace and are not general configuration requirements for every NR NTN deployment.

Item	Value observed in the sample
Radio access	NR Standalone, FDD, band n256, subcarrier spacing 15 kHz
Cell	PCI 500, 79 PRB downlink and uplink, PLMN 001/01
NTN scenario	LEO satellite information signaled by NG-RAN; NTN assistance delivered in SIB19
UE capability	Release 18 UE with Release 17 NTN capability, uplink pre-compensation, uplink TA reporting, and NTN HARQ extensions
Data session	DNN `ntn-internet`, PDU session ID 1, IPv4 address assigned

Call Flow Overview

The overview follows the complete procedure from NTN system-information acquisition and random access through 5GS registration, security activation, UE capability exchange, and establishment of an operational PDU session.

Step	Phase	Direction	Message
1	System information	gNB to UE	SIB1, SIB2, and SIB19NTN: SIB19 supplies the common TA and drift, orbital ephemeris, cell-specific K offset, service time, and TA-report enablement.
2	Random access	UE to gNB	PRACH preamble (Msg1)NTN: The UE applies uplink timing and frequency pre-compensation derived from SIB19 assistance before transmitting PRACH.
3	Random access	gNB to UE	Random Access Response (Msg2)NTN: The RAR completes residual timing alignment while the scheduler allows for the extended satellite propagation delay.
4	RRC connection	UE to gNB	RRCSetupRequest on Msg3NTN: The RRC payload has no NTN-specific IE; Msg3 follows the NTN-adjusted RAR grant and UE uplink pre-compensation.
5	RRC connection	gNB to UE	RRCSetup and contention resolution (Msg4)NTN: The RRCSetup PDSCH uses `k1=40`, extending HARQ-ACK timing for the satellite round-trip delay.
6	Registration	UE to gNB	RRCSetupComplete carrying NAS Registration RequestNTN: The RRC/NAS payload is unchanged; its uplink PUSCH grant uses `k2=40` during initial setup.
7	NAS security	UE and AMF	Authentication Request/Response and NAS Security Mode Command/Complete
8	AS security	gNB and UE	RRC SecurityModeCommand/Complete
9	Capability	gNB and UE	UECapabilityEnquiry/UECapabilityInformationNTN: The UE reports NTN operation, uplink pre-compensation, TA reporting, NTN HARQ modes, and NTN mobility support.
10	Registration	AMF to UE	Registration Accept in RRCReconfigurationNTN: The RRC configuration sets `tar-Config-r17`, a 0.5 ms TA-offset threshold, and TA-triggered scheduling requests.
11	RRC configuration	UE to gNB	RRCReconfigurationComplete
12	Registration	UE to AMF	Registration Complete
13	PDU session	UE to SMF	PDU Session Establishment Request
14	PDU session	SMF to UE	PDU Session Establishment Accept and DRB configurationNTN-related: DRB1 uses `t-Reordering=ms3000` to tolerate the extended delay; this timer is not itself an NTN-specific IE.
15	PDU session	UE to gNB	RRCReconfigurationComplete; PDU Session Resource Setup Response

Detailed Message Sequence

The following sections expand the overview with selected decoded fields from each signaling stage. They distinguish standard NR and 5GS procedures from the NTN assistance, timing extensions, and capability signaling observed in the trace.

1. NTN system information: SIB1, SIB2, and SIB19

The UE first acquires SIB1. In addition to the normal cell-selection and initial BWP parameters, SIB1 schedules SIB19 using si-SchedulingInfo-v1700. SIB2 supplies common reselection parameters. SIB19 is the NTN-specific block and provides the timing and orbital assistance needed before access.

{
  "SIB1": {
    "frequencyBandList": [
      { "bandNR": 256 }
    ],
    "carrierBandwidth": 79,
    "si-SchedulingInfo-v1700": ["sibType19"]
  },
  "SIB19-v1700": {
    "ntn-Config-r17": {
      "ntn-UlSyncValidityDuration-r17": "s5",
      "cellSpecificKoffset-r17": 36,
      "ta-Info-r17": {
        "ta-Common-r17": 3564382,
        "ta-CommonDrift-r17": -222329,
        "ta-CommonDriftVariant-r17": 695
      },
      "ephemerisInfo-r17": {
        "type": "orbital-r17"
      },
      "ta-Report-r17": "enabled"
    },
    "t-Service-r17": 399071060523
  }
}

cellSpecificKoffset-r17 extends scheduling timing to account for the long NTN propagation delay.
The common TA, drift, and drift-variation values give the UE the cell-level timing model.
The orbital ephemeris lets a location-aware UE estimate satellite motion and perform uplink timing/frequency pre-compensation.
ta-Report-r17 indicates that UE timing-advance reporting is enabled for this cell.

2. Random access and RRC setup

After applying the NTN assistance, the UE transmits a PRACH preamble. The gNB responds with a Random Access Response, including the uplink grant and timing adjustment. The UE then sends RRCSetupRequest in Msg3 and receives RRCSetup in Msg4.

{
  "randomAccessAndRrcSetup": [
    {
      "timestamp": "14:31:15.662",
      "direction": "UL",
      "channel": "PRACH",
      "sequenceIndex": 6,
      "timingAdvance": 28
    },
    {
      "timestamp": "14:31:15.663",
      "direction": "DL",
      "message": "RandomAccessResponse",
      "raRnti": "000f"
    },
    {
      "timestamp": "14:31:15.709",
      "direction": "UL",
      "channel": "CCCH-NR",
      "message": "RRCSetupRequest",
      "establishmentCause": "mo-Signalling"
    },
    {
      "timestamp": "14:31:15.709",
      "direction": "DL",
      "channel": "CCCH-NR",
      "message": "RRCSetup",
      "radioBearerConfig": {
        "srbToAdd": [1]
      }
    }
  ]
}

The initial-setup physical-layer records use k1=40 and k2=40. Later dedicated scheduling uses k1=35 and k2=35. These large offsets are trace-level evidence that the scheduler has allowed for a substantially larger round-trip delay than a terrestrial cell.

3. RRCSetupComplete and Registration Request

The UE confirms the RRC connection with RRCSetupComplete. Its dedicatedNAS-Message contains a 5GMM Registration Request for initial registration. The UE also signals support for uplink RRC segmentation.

{
  "timestamp": "14:31:15.865",
  "direction": "UL",
  "channel": "DCCH-NR",
  "message": "RRCSetupComplete",
  "rrcSetupComplete": {
    "selectedPLMN-Identity": 1,
    "ul-RRC-Segmentation-r16": true,
    "dedicatedNAS-Message": {
      "protocol": "5GMM",
      "message": "RegistrationRequest",
      "registrationType": "initial-registration",
      "followOnRequest": true,
      "securityCapability": {
        "encryptionAlgorithms": ["EA0", "EA1", "EA2"],
        "integrityAlgorithms": ["IA0", "IA1", "IA2"]
      }
    }
  }
}

4. 5GMM authentication and NAS security

The AMF authenticates the UE through NAS messages transported in RRC DLInformationTransfer and ULInformationTransfer. After authentication succeeds, the AMF activates NAS integrity protection and ciphering.

Direction	NAS message
AMF to UE	Authentication Request
UE to AMF	Authentication Response
AMF to UE	Security Mode Command
UE to AMF	Security Mode Complete

5. Access-stratum security

Once the initial context is available, the gNB separately activates RRC and user-plane security. This AS procedure must not be confused with the NAS Security Mode exchange in the previous step.

{
  "accessStratumSecurity": [
    {
      "timestamp": "14:31:16.101",
      "protocol": "NGAP",
      "direction": "core-to-gNB",
      "message": "InitialContextSetupRequest"
    },
    {
      "timestamp": "14:31:16.101",
      "protocol": "RRC",
      "direction": "DL",
      "channel": "DCCH-NR",
      "message": "SecurityModeCommand"
    },
    {
      "timestamp": "14:31:16.220",
      "protocol": "RRC",
      "direction": "UL",
      "channel": "DCCH-NR",
      "message": "SecurityModeComplete"
    }
  ]
}

6. UE capability exchange

The gNB requests the UE radio-access capability. The returned UE-NR-Capability identifies a Release 18 UE and explicitly lists NTN support required by this test.

{
  "UE-NR-Capability": {
    "accessStratumRelease": "rel18",
    "supportedBandListNR": [
      { "bandNR": 256 }
    ],
    "nonTerrestrialNetwork-r17": "supported",
    "uplinkPreCompensation-r17": "supported",
    "uplink-TA-Reporting-r17": "supported",
    "ntn-Parameters-r17": {
      "inactiveStateNTN-r17": "supported",
      "ra-SDT-NTN-r17": "supported",
      "mac-ParametersNTN-r17": {
        "harq-FeedbackDisabled-r17": "supported",
        "uplink-Harq-ModeB-r17": "supported"
      }
    },
    "ntn-CHO-OnlyLocationTimeTrigger-r18": "supported"
  }
}

The gNB forwards the resulting radio-capability information toward the core network using NGAP UE Radio Capability Info Indication.

7. Registration Accept and initial dedicated configuration

The AMF returns Registration Accept. In this trace it is delivered as a dedicated NAS message inside an RRC reconfiguration that also establishes the initial dedicated radio configuration.

{
  "downlinkMessage": {
    "direction": "DL",
    "channel": "DCCH-NR",
    "message": "RRCReconfiguration",
    "dedicatedNAS-Message": "RegistrationAccept",
    "tar-Config-r17": {
      "action": "setup",
      "offsetThresholdTA-r17": "ms0dot5",
      "timingAdvanceSR-r17": "enabled"
    },
    "nrofHARQ-ProcessesForPDSCH": "n16",
    "maxMIMO-Layers": 1
  },
  "uplinkResponse": {
    "direction": "UL",
    "channel": "DCCH-NR",
    "message": "RRCReconfigurationComplete"
  }
}

tar-Config-r17 configures when the UE should report a significant timing-offset change and permits a scheduling request for timing-advance reporting. The excerpt contains the configuration, but no separately decoded TA Report MAC CE was observed during this short procedure.

8. Registration Complete

The UE acknowledges the NAS registration result with Registration Complete, transported in ULInformationTransfer. At this point the UE is registered and can request a data session.

9. PDU Session Establishment Request

The UE sends an uplink NAS transport carrying a PDU Session Establishment Request. The request selects session ID 1, asks for IPv4v6 service, and specifies the NTN data network name.

{
  "ULNASTransport": {
    "payload": {
      "message": "PDUSessionEstablishmentRequest",
      "pduSessionIdentity": 1,
      "requestedPDUSessionType": "IPv4v6",
      "dnn": "ntn-internet"
    }
  }
}

10. PDU Session Accept and DRB setup

The core accepts the session with IPv4 service and QoS flow identifier 1. The gNB then uses another RRCReconfiguration to add SRB2 and DRB1 and map the QoS flow to the data radio bearer.

{
  "pduSessionEstablishmentAccept": {
    "selectedPDUSessionType": "IPv4",
    "pduAddress": "192.168.3.2",
    "qosFlow": {
      "qfi": 1,
      "5qi": 10
    },
    "dnn": "ntn-internet"
  },
  "rrcReconfiguration": {
    "direction": "DL",
    "channel": "DCCH-NR",
    "srbToAdd": [2],
    "drbToAdd": [1],
    "mappedQoSFlows": [1],
    "pdcpSnSizeBits": 18,
    "t-Reordering": "ms3000"
  },
  "completion": [
    {
      "protocol": "RRC",
      "direction": "UL",
      "message": "RRCReconfigurationComplete"
    },
    {
      "protocol": "NGAP",
      "direction": "gNB-to-core",
      "message": "PDUSessionResourceSetupResponse"
    }
  ]
}

The completed NGAP response confirms that the radio and core-network resources for the PDU session are active.

What Is NTN-Specific in This Flow?

Most registration and session-management messages are unchanged from terrestrial NR. The table below isolates the NTN-specific information and delay-related behavior that adapts the normal procedure to a moving satellite link.

Feature	Where it appears	Purpose
SIB19	Before random access	Broadcasts NTN timing, ephemeris, service time, and TA-report configuration.
Cell-specific K offset	SIB19	Extends scheduling timelines for the satellite propagation delay.
Common TA and drift	SIB19	Provides a moving timing reference that the UE can extrapolate.
Uplink pre-compensation	UE capability	Declares that the UE can compensate its uplink using NTN assistance.
TA reporting	SIB19, UE capability, RRC reconfiguration	Allows the network to configure and receive UE timing-change reports.
NTN HARQ behavior	UE capability	Advertises NTN-specific HARQ feedback suppression and uplink mode support.
Long scheduling offsets	PHY trace	Allows grants and feedback to remain valid across the long round-trip time.

References

The following ShareTechnote material and 3GPP specifications provide the normative protocol definitions and supporting background for the decoded RRC, MAC, NAS, and NGAP messages used in this analysis.

NR NTN RRC Parameters - SIB19 and dedicated NTN RRC configuration.
3GPP TS 38.331 - NR Radio Resource Control protocol.
3GPP TS 38.321 - NR Medium Access Control protocol.
3GPP TS 24.501 - 5GS NAS protocol.
3GPP TS 38.413 - NG-RAN NGAP protocol.
ShareTechnote NB-IoT NTN call-flow format reference.

Trace timestamps and selected decoded fields are taken from the supplied Amarisoft export. Values may vary with satellite orbit, service link, UE implementation, and network configuration.