ACK Procedure

Overview

IEEE 802.11 defines two acknowledgment mechanisms for confirming successful frame delivery over the wireless medium:

• Normal ACK (Immediate Acknowledgment)

• No-ACK Policy

Because Wi-Fi operates on a shared, half-duplex medium, collision detection is impossible. Hence, acknowledgments are essential to ensure reliability or, in some traffic types, deliberately disabled to reduce overhead.

Why Acknowledgments Are Needed

• The wireless channel is prone to collisions and bit errors.

• Transmitters cannot detect collisions directly (unlike Ethernet).

• The receiver explicitly confirms successful frame reception using an ACK frame.

• Lack of ACK indicates a collision or channel error, triggering retransmission.

Normal ACK Procedure

Normal ACK (Immediate Acknowledgment) is the standard and default mechanism for unicast data and management frames. Each frame sent by a transmitter must be acknowledged by the receiver within a strict timing window.

Sequence of Operations

1. Sender contends for the channel using the DCF procedure (DIFS + backoff).

2. Sender transmits the DATA frame.

3. Receiver waits for a SIFS interval.

4. Receiver transmits an ACK frame.

5. Sender, upon receiving ACK before timeout, declares success.

6. If ACK is not received, the sender retries using exponential backoff.

Transmission Sequence

|<-- DIFS -->|<-- Backoff -->|------ DATA ------|<-- SIFS -->|-- ACK --|

ACK Frame Format

The acknowledgment frame is a short control frame with the following fields:

Field	Description
Frame Control Duration / ID Receiver Address FCS	Type = Control, Subtype = ACK Usually 0 (not used for NAV) MAC address of the sender of DATA frame 32-bit CRC checksum

Properties:

• Length: 14 bytes

• Type/Subtype: Control (1) / ACK (13)

• Rate: Sent at the lowest basic rate supported by the BSS for reliability.

• Timing: Sent exactly after SIFS (Short InterFrame Space).

ACK Timeout

The transmitter starts a timer immediately after sending the DATA frame. If an ACK is not received before timeout, the transmission is considered failed.

ACK Timeout (simplified):

\[T_{ACK\_timeout} = SIFS + T_{ACK} + \text{propagation delay} + \text{margin}\]

Retransmission Procedure

If no ACK is received:

1. Increment retry counter.

2. Double Contention Window (CW) up to CWmax.

3. Retransmit the frame using the same Sequence Number.

4. If retry limit exceeded → frame dropped, upper layers notified.

Short Retry Limit (SRL): typically 7 Long Retry Limit (LRL): typically 4

Fragmented Frame Acknowledgments

When fragmentation is enabled:

• Each fragment is individually acknowledged.

• ACK is sent after SIFS following each fragment.

• The transmitter does not perform new backoff between fragments.

Example timing for fragmented frames:

Frag#0 → SIFS → ACK → SIFS → Frag#1 → SIFS → ACK → SIFS → Frag#2 → SIFS → ACK

Timing with RTS/CTS

When RTS/CTS protection is used:

|<-- DIFS -->| RTS | SIFS | CTS | SIFS | DATA | SIFS | ACK |

• RTS/CTS reserve the medium.

• ACK confirms completion of the final DATA transmission.

• The ACK still follows SIFS after DATA.

No-ACK Policy

No-ACK is an optional policy where the transmitter sends frames without expecting an acknowledgment. It is used when throughput is more critical than reliability.

Purpose and Use Cases

• Broadcast or multicast transmissions (always unacknowledged).

• Real-time or streaming data (tolerant to occasional loss).

• Certain management frames or test modes.

Sequence (No-ACK)

[Medium Idle for DIFS] → [Backoff Countdown] → DATA (no ACK expected)

Timeline:

|<-- DIFS -->|<-- Backoff -->|------ DATA (No ACK) ------|

Key Differences from Normal ACK:

• No SIFS + ACK exchange.

• No ACK timeout waiting.

• No retransmission on loss.

• Medium becomes available immediately after frame transmission ends.

NAV and Duration Behavior

For Normal ACK:

• The Duration field reserves the channel for the ACK exchange.

• Formula:

  \[Duration = SIFS + T_{ACK}\]

• Other stations set NAV to defer transmission until the ACK completes.

For No-ACK:

• Duration field = 0

• Other stations may attempt contention as soon as the frame ends.

Comparison — Normal ACK vs No-ACK

Feature	Normal ACK	No-ACK
Reliability Retransmission SIFS/ACK Timing NAV Reservation Efficiency Use Case	High (ACK confirms) Yes, per retry limit Required Includes SIFS + ACK Lower (overhead) Unicast data/ctrl	None (no feedback) No Not used Duration = 0 Higher Broadcast, streaming

ACK and No-ACK Timing Diagrams

Normal ACK Sequence

Sender:   |<-- DIFS -->|<-- Backoff -->|------ DATA ------|<-- SIFS -->|-- ACK --|
Receiver:                               Receives DATA     Wait SIFS    Sends ACK
Other STAs: NAV set for (SIFS + ACK) duration

No-ACK Sequence

Sender:   |<-- DIFS -->|<-- Backoff -->|------ DATA (No ACK) ------|
Receiver: (No ACK sent)
Other STAs: medium becomes idle immediately after DATA ends

ACK Rate and Robustness

• ACKs are sent using one of the basic rates supported by the BSS.

• Selected rate ≤ rate of received frame.

• Ensures sender can always decode ACK, even in poor channel conditions.

• Control frames (RTS/CTS/ACK) are never encrypted.

Implementation Notes

• ACKs must start exactly SIFS after the end of the received frame.

• ACK timeout must account for SIFS + ACK duration + PHY propagation delay.

• Retransmissions must follow exponential backoff with CWmin/CWmax limits.

• NAV updates depend on whether ACK is expected.

• Broadcast and multicast frames are never acknowledged.

• If receiver detects a CRC or PHY error, no ACK is sent (sender times out).

Edge Cases

1. Broadcast / Multicast Frames - Never acknowledged. - Duration = 0. - No NAV updates.

2. Hidden Node Collisions - Failure to receive ACK triggers retransmission.

3. PHY or CRC Errors - Receiver drops frame silently, no ACK sent.

4. Power Save Mode - Dozing stations do not send ACKs until awake.

Timing Summary

Advantages and Tradeoffs

References

• IEEE Std 802.11-2020, Clause 9.3.2.8 (Acknowledgment)

• IEEE Std 802.11-2016, Clause 9.2.8.3 (No-ACK transmission)

• 13. Gast, 802.11 Wireless Networks: The Definitive Guide

• Heusse et al., Performance Anomaly of 802.11b, IEEE INFOCOM 2003

Figures

Normal ACK exchange between sender and receiver.

No-ACK transmission with no acknowledgment or SIFS interval.