Terminology » LoRaWAN Frame Counters : FCntUp, AFCntDwn, NFCntDwn

LoRaWAN Frame Counters : FCntUp, AFCntDwn, NFCntDwn

lorawan frame counter

Introduction : In LoRaWAN, secure and reliable communication depends not only on encryption keys but also on the correct sequencing of messages. To achieve this, the protocol employs frame counters such as FCntUp, AFCntDwn, and NFCntDwn, which act as running message numbers. These counters prevent replay attacks, maintain message order, and help the network server distinguish between new and duplicate packets. By tracking uplink and downlink communications, frame counters play a vital role in ensuring data integrity and synchronization between end devices and the network.

Key features

  • In LoRaWAN, every uplink and downlink frame carries a frame counter field.
  • They are incrementing counters (not reset during session).
  • Used for following :
    • Detect replay attacks (old packets being resent maliciously).
    • Maintain message ordering and integrity.
    • Ensure session synchronization between device ↔ network server.
  • Definition: Counts the number of uplink frames sent by the end device.
  • Usage:
    • Incremented by end device for every uplink.
    • The Network Server validates packets by checking if FCntUp > last known value.
    • Prevents attackers from replaying old uplink packets.
  • Definition: Counts application layer downlink frames (AppSKey-encrypted).
  • Usage:
    • Managed by the Network Server towards Device path.
    • Ensures the device ignores replayed or duplicated application downlinks.
    • Used when sending payloads intended for applications (not MAC commands).
  • Definition: Counts network/MAC layer downlink frames (NwkSKey-encrypted).
  • Usage:
    • Also incremented by the Network Server for each downlink.
    • Ensures devices accept only new network/MAC commands (e.g., ADR, LinkADRReq).
    • Prevents replay of network level control messages.

Conclusion: Frame counters in LoRaWAN provide an essential layer of security and reliability, ensuring that only fresh, valid messages are accepted by the network. By separating uplink and downlink counters and maintaining strict increment rules, LoRaWAN effectively protects against replay attacks while guaranteeing proper message flow.