How does Block ACK improve throughput?

Without Block ACK, each MPDU requires an individual ACK with SIFS spacing, consuming significant airtime on overhead. With A-MPDU aggregation and Block ACK, a burst of 64 MPDUs (up to 256 in 802.11ax) is acknowledged by a single Block ACK bitmap frame. This reduces ACK overhead from 64 frames to 1, improving MAC efficiency from approximately 50% to over 90%.

What is the Block ACK bitmap?

The Block ACK frame contains a 64-bit or 256-bit bitmap where each bit represents one MPDU in the burst. Bit=1 means successfully received; bit=0 means missing/corrupted. The sender retransmits only the failed MPDUs in the next burst. Starting sequence number identifies which MPDUs the bitmap corresponds to.

Immediate vs Delayed Block ACK?

Immediate Block ACK is sent within SIFS after receiving a Block ACK Request (BAR) or implicitly after an A-MPDU. Delayed Block ACK allows the receiver to process frames before responding but adds latency and is rarely used in practice. 802.11n introduced Immediate Block ACK as the standard mechanism.

Wireless Protocols

Block ACK

Block ACK (Block Acknowledgment) is a Wi-Fi MAC mechanism introduced in 802.11e and enhanced in 802.11n/ac/ax that acknowledges multiple MPDUs with a single acknowledgment frame containing a bitmap. When used with A-MPDU aggregation, a burst of 64 MPDUs (256 in 802.11ax) is acknowledged by one Block ACK frame, reducing protocol overhead from ~50% to under 10% and increasing effective throughput by up to 40%.

Category: Wireless Protocols

Bitmap: 64-256 bits

Understanding Block ACK

In legacy Wi-Fi, each data frame requires an individual ACK after a SIFS interval (10-16 μs). For high-rate transmissions, the ACK overhead becomes significant: at 1 Gbps PHY rate, a 1500-byte frame takes only 12 μs, but each ACK adds 44 μs of overhead (SIFS + ACK + DIFS + backoff). Block ACK eliminates this by batching acknowledgments.

A Block ACK session is established with an ADDBA (Add Block ACK) request/response exchange. Once established, the transmitter sends A-MPDUs (Aggregated MPDUs) and the receiver responds with a single Block ACK bitmap indicating which MPDUs were received correctly.

MAC Efficiency

Without Block ACK:
Efficiency = T_data/(T_data + T_ACK + T_overhead)
≈ 50-60% at high PHY rates

With Block ACK (64 MPDUs):
Efficiency = 64×T_data/(64×T_data + T_BA + T_overhead)
≈ 90-95%

ACK Mechanism Comparison

Mechanism	Standard	Frames/ACK	Efficiency
Normal ACK	802.11a/g	1	~50%
Block ACK	802.11n	64	~90%
Multi-STA BA	802.11ax	256 per STA	~95%

Common Questions

Frequently Asked Questions

How does it improve throughput?

64 MPDUs acknowledged by 1 frame vs 64 individual ACKs. MAC efficiency jumps from ~50% to 90%+. Up to 40% throughput gain.

Block ACK bitmap?

64 or 256-bit bitmap. Bit=1: received OK. Bit=0: missing. Sender retransmits only failed MPDUs next burst.

Immediate vs Delayed?

Immediate: sent within SIFS, standard in 802.11n+. Delayed: adds latency, rarely used in practice.

Related Terms

← Blind Via Block ACK Request →

Wi-Fi Testing

Request a Quote

Need Wi-Fi protocol analyzers, throughput test tools, or WLAN development kits? Contact our team.

Get in Touch