Frame Aggregation: A-MSDU vs A-MPDU

Every Wi-Fi transmission pays a fixed price before any data moves: waiting for the channel, a preamble and an acknowledgement. Aggregation packs many packets into one transmission so that price is paid once. See how A-MSDU, A-MPDU and both together build a frame, what they save, and what happens when errors hit.

Pay the overhead once

Sending one packet costs a contention wait, a preamble (fixed time, however fast the data rate), MAC headers and an ACK. At high data rates the packet itself takes only a few microseconds, so most of the airtime is overhead. Aggregation sends many packets per transmission:

A-MSDU puts several packets inside one MAC frame: one header, one checksum. A-MPDU sends several complete MAC frames back to back in one transmission, each with its own checksum, and the receiver answers with one Block Ack that confirms each frame separately. Modern Wi-Fi usually does both: A-MSDUs inside an A-MPDU.

Aggregation

Wi-Fi generation

Packets to send Packet size

Typical packets

Channel errors

1 · What goes on the air

Waiting for the channel PHY preamble A-MPDU delimiter MAC header + encryption header A-MSDU subframe header + LLC/SNAP Your packet MIC + FCS (checksum) Padding ACK / Block Ack

2 · Airtime to send them all

Scroll

3 · When errors hit

A radio error corrupts a few bits somewhere in the transmission. The receiver checks each FCS (checksum): whatever that checksum covers is thrown away and has to be sent again. The difference is how much one checksum covers.

4 · Limits by generation

	Wi-Fi 4 (802.11n)	Wi-Fi 5 (802.11ac)	Wi-Fi 6 (802.11ax)	Wi-Fi 7 (802.11be)
Largest A-MSDU	7,935 bytes (3,839 on some devices)	about 11,400 bytes	about 11,400 bytes	about 11,400 bytes
Largest frame inside an A-MPDU	4,095 bytes, so an A-MSDU there holds only 2 full-size packets	11,454 bytes	11,454 bytes	11,454 bytes
Largest A-MPDU	65,535 bytes	1,048,575 bytes	6,500,631 bytes	15,523,200 bytes
Frames per Block Ack	64	64	256	1,024
Longest transmission	about 5.5 ms	about 5.5 ms	about 5.5 ms	about 5.5 ms

Whichever limit is reached first ends the transmission. At high data rates it's usually the Block Ack window; at low rates it's the 5.5 ms time limit.

5 · Pros and cons, and when each arrived

	A-MSDU	A-MPDU	Both (A-MSDU inside A-MPDU)
Arrived in	Wi-Fi 4 (802.11n, 2009)	Wi-Fi 4 (802.11n, 2009), with the compressed Block Ack	Wi-Fi 4 (802.11n), but small (inner frame ≤ 4,095 bytes); practical from Wi-Fi 5 on
What it packs	Several packets inside one MAC frame	Several complete MAC frames in one transmission	Several A-MSDUs, each a complete MAC frame, in one transmission
Checksums (FCS)	One for all the packets	One per frame	One per A-MSDU
Header overhead	Lowest: one MAC header, one security header	Highest: a MAC header, security header and delimiter per packet	Low
What an error costs	Everything in the frame is resent	Only the damaged frames are resent (the Block Ack says which)	Only the damaged A-MSDUs are resent
Needs	Both sides to support it and agree a maximum size; a normal ACK	A Block Ack agreement set up first (ADDBA)	A Block Ack agreement that allows A-MSDUs
Pros	Least header overhead; less work for the radio and CPU (one frame for many packets); great for bundling small packets like TCP ACKs	Robust on noisy links; very long transmissions possible; each frame can be encrypted, checked and retried on its own	Combines both: few headers and little processing, with errors limited to one small A-MSDU
Cons	One bad bit loses every packet; limited to about 11 KB; its "A-MSDU present" flag was abused in the 2021 FragAttacks, fixed by stricter checks and SPP A-MSDU	More header bytes per packet; the Block Ack agreement adds setup; the receiver must buffer and reorder	More complex to schedule; big A-MSDUs bring back the error cost, so they're kept to 2–3 packets
Used today	Not on its own: every Wi-Fi 5/6/7 data transmission is an A-MPDU	Always, in Wi-Fi 5 and later	Common on fast links, with A-MSDUs of 2–3 packets

Timeline

2005, 802.11e (WMM, QoS): introduces the Block Ack, confirming many frames at once. Optional and rarely used before 802.11n.
2009, Wi-Fi 4 (802.11n): brings both A-MSDU (up to 3,839 or 7,935 bytes) and A-MPDU (up to 64 KB and 64 frames), and the compressed Block Ack. Aggregation becomes the key to reaching 100+ Mbps.
2013, Wi-Fi 5 (802.11ac): every data transmission is sent as an A-MPDU, even a single frame. Frames up to 11,454 bytes and A-MPDUs up to about 1 MB.
2019/2021, Wi-Fi 6 (802.11ax): A-MPDUs up to about 6.5 MB, a 256-frame Block Ack window, A-MPDUs mixing traffic classes (multi-TID), and Block Acks for many users at once (multi-user).
2021, FragAttacks: researchers showed the unencrypted "A-MSDU present" flag could be used to inject packets. The fixes are stricter checks on each A-MSDU in patched drivers, now also written into the standard, and SPP A-MSDU, an older, optional feature of the standard that protects the flag (few devices used it).
2024, Wi-Fi 7 (802.11be): a 1,024-frame Block Ack window and A-MPDUs up to about 15.5 MB, to keep 320 MHz channels full.

Notes and simplifications

Bytes: each packet (an IP packet) carries an 8-byte LLC/SNAP header. A MAC frame adds a 26-byte QoS data header, an 8-byte CCMP encryption header, an 8-byte MIC and a 4-byte FCS. Each A-MSDU subframe adds a 14-byte header, and each A-MPDU subframe a 4-byte delimiter; subframes are padded to a multiple of 4 bytes.
Rates: Wi-Fi 4 at 144.4 Mbps (20 MHz, 2 streams, MCS 15, short guard interval, 40 µs preamble), Wi-Fi 5 at 866.7 Mbps (80 MHz, 2 streams, MCS 9, 44 µs preamble), Wi-Fi 6 at 1,201 Mbps (80 MHz, 2 streams, MCS 11, 0.8 µs guard interval, 50.4 µs preamble). Data is sent in whole OFDM symbols.
Each transmission: AIFS (43 µs) plus an average backoff of 7.5 slots (67.5 µs), the preamble and data, SIFS (16 µs), then an ACK (28 µs) or Block Ack (32 µs; 40 µs for Wi-Fi 6's 256-frame bitmap) at 24 Mbps. No other devices compete here; see CSMA/CA Basics for sharing the channel.
Errors: bits are corrupted independently at the chosen rate (Low 1 in a million, Medium 1 in 100,000, High 3 in 100,000). Throughput with errors is the expected value if every lost frame is resent until it arrives; real devices also lower their data rate when errors rise.
“Both” fills each A-MSDU up to the largest frame allowed inside an A-MPDU. Many devices use smaller A-MSDUs (often 2–3 packets) to limit the damage from errors.