Tools / 3gpp-decoder

3GPP Decoder - Decode RRC, NAS, NGAP, GTP-U, PFCP, and SIP / IMS Messages

The 3GPP Decoder is an online tool that helps engineers decode telecom signaling messages used in 3G, 4G LTE, and 5G NR networks.

Telecom protocols defined by the 3rd Generation Partnership Project are typically encoded using ASN.1 encoding rules such as PER or BER. These encoded messages appear as binary or hexadecimal data, which is difficult to interpret without decoding.

Using this tool, you can quickly decode telecom messages and view their structure in a human-readable format.

The current decoder supports telecom access and control-plane protocols such as NR RRC, NAS-5GS, NGAP, LTE RRC, NAS-EPS, S1AP, and packet/core protocols such as GTP-U, PFCP, and SIP / IMS.

Raw Decode Output

Decoded Summary

Payload Hex: N/A

Payload ASCII: N/A

Supported Protocol Families

The decoder supports telecom signaling across radio access, access-core control plane, and packet/core procedures. Use the grouped protocol families below to pick the right decode path more quickly.

Use the tool above to decode message dumps captured from network traces or device logs.

What is a 3GPP Message Decoder?

A 3GPP decoder converts encoded telecom signaling messages into readable protocol structures.

Most cellular protocols are defined using ASN.1 (Abstract Syntax Notation One). When these messages are transmitted over the network, they are encoded using rules such as:

• PER (Packed Encoding Rules)
• UPER (Unaligned PER)
• BER (Basic Encoding Rules)

Without decoding, the message appears as raw hexadecimal data.

Example encoded message:

7E 00 41 02 00

After decoding, the structure becomes readable:

Registration Request
  Security Header Type: Plain NAS
  Registration Type: Initial Registration
  UE Identity: SUCI

This allows engineers to understand the message contents and debug network behavior.

Why Message Decoding is Important in Telecom Networks

Decoding signaling messages is essential for troubleshooting and protocol analysis.

Engineers commonly decode messages for:

Network Troubleshooting

Analyzing signaling failures during call setup or registration.

UE Log Analysis

Interpreting device logs captured from smartphones or modems.

Protocol Debugging

Validating telecom protocol implementations during development.

Interoperability Testing

Ensuring devices and networks follow 3GPP specifications correctly.

Example Decoded RRC Message

Below is a simplified example of a 5G NR RRCReconfiguration message decoded from an ASN.1 structure.

DL-DCCH-Message ::= {
  message c1 : rrcReconfiguration : {
    rrc-TransactionIdentifier 1,
    criticalExtensions rrcReconfiguration : {
      measConfig {
        measObjectToAddModList {
          {
            measObjectId 1,
            measObjectNR {
              ssbFrequency 631968,
              ssbSubcarrierSpacing kHz30
            }
          }
        }
      }
    }
  }
}

Key Parameters

RRC messages control the radio connection between the UE and base station.

Example NAS Message Decode

NAS messages operate between the UE and the core network.

Encoded NAS message:

7E 00 41 02 00

Decoded message:

Registration Request
  Extended Protocol Discriminator: 5G Mobility Management
  Security Header Type: Plain NAS
  Message Type: Registration Request
  5GS Registration Type: Initial Registration
  UE Identity: SUCI

Important Fields

NAS messages handle mobility management, authentication, and session management.

Example NGAP Message Decode

NGAP messages are exchanged between the gNB and AMF in a 5G network.

Example decoded NGAP message:

NGAP-PDU ::= {
  initiatingMessage : {
    procedureCode id-InitialUEMessage,
    criticality ignore,
    value InitialUEMessage {
      protocolIEs {
        AMF-UE-NGAP-ID 1024,
        RAN-UE-NGAP-ID 2001,
        NAS-PDU RegistrationRequest
      }
    }
  }
}

Key Fields

NGAP coordinates signaling between the radio access network and the core network.

Example GTP-U Header Decode

GTP-U is the user-plane tunneling protocol used to carry subscriber traffic between access and core user-plane nodes. Engineers commonly decode it to check TEID, header flags, and whether the packet really belongs to the expected tunnel.

Flags: 0x30
Message Type: 0xff (G-PDU)
Length: 4
TEID: 0x00000001

Why this matters

Example PFCP Message Decode

PFCP is used between control-plane and user-plane functions to establish, modify, and delete forwarding state. It is especially useful when debugging PDU session setup, UPF behavior, or QoS flow activation.

Version: 1
Message Type: 50 (Session Establishment Request)
SEID: 0x0000000000000001
Sequence Number: 258

Why this matters

Example SIP / IMS Decode

SIP / IMS decoding is useful for VoLTE and VoNR workflows such as registration, session setup, and release. It helps engineers correlate radio and core events with the actual voice-session signaling.

INVITE sip:user@example.com SIP/2.0
Via: SIP/2.0/UDP host
Call-ID: 123456@example.com
From: <sip:caller@example.com>
To: <sip:user@example.com>

Why this matters

Example SIB1 Message Decode

System Information Blocks broadcast essential cell information.

Example decoded SIB1:

SystemInformationBlockType1 ::= {
  cellAccessRelatedInfo {
    plmn-IdentityList {
      mcc 260
      mnc 01
    }
    trackingAreaCode '0001'H
  }
  cellSelectionInfo {
    q-RxLevMin -65
  }
}

Key Parameters

UE devices read SIB1 during cell selection procedures.

Supported Telecom Protocols

The decoder supports multiple telecom signaling protocols.

These protocols are defined in specifications published by the 3rd Generation Partnership Project.

How the 3GPP Decoder Works

The decoder performs several steps to convert encoded telecom messages into readable structures.

1. Parse binary or hexadecimal input
2. Identify the protocol format
3. Apply ASN.1 schema definitions
4. Generate a readable protocol tree

The result shows each Information Element (IE) contained in the message.

Typical Use Cases

Engineers use a 3GPP decoder in many scenarios.

UE Log Analysis

Smartphones and modems generate logs containing encoded protocol messages.

Network Debugging

Engineers analyze signaling exchanges between network nodes.

Protocol Development

Developers verify that messages follow specification formats.

Telecom Education

Students use decoded messages to understand cellular signaling procedures.

Related Telecom Tools

You may also find these tools useful:

• NR ARFCN Calculator
• 5G Link Budget Calculator
• Free Space Path Loss Calculator
• 5G Throughput Calculator

These tools help engineers analyze and design wireless communication systems.

References

The protocols decoded by this tool are defined in specifications published by the 3rd Generation Partnership Project.

Important standards include:

• TS 38.331 - NR Radio Resource Control
• TS 24.501 - NAS Protocol
• TS 38.413 - NGAP Protocol
• TS 36.331 - LTE RRC
• TS 29.281 - GTP-U
• TS 29.244 - PFCP
• RFC 3261 - SIP

Summary

The 3GPP Decoder is a powerful tool for analyzing telecom signaling messages used in 3G, 4G LTE, and 5G NR networks.

By decoding encoded ASN.1 messages into readable structures, the tool helps engineers:

• troubleshoot network signaling
• analyze UE logs
• debug telecom protocols
• study 3GPP message structures

Use the decoder above to quickly interpret telecom message dumps and understand how cellular networks communicate internally.