Fragmentation

Overview

IEEE 802.11 fragmentation allows a large MAC Service Data Unit (MSDU) to be split into smaller MAC Protocol Data Units (MPDUs), known as fragments, before transmission.

Fragmentation improves reliability over noisy channels by ensuring that only the lost fragment must be retransmitted, rather than the entire frame.

Purpose

Increase reliability under high Bit Error Rate (BER) conditions.
Limit retransmission overhead.
Maintain better efficiency on lossy or interference-prone wireless links.

Fragmentation Threshold

Parameter: dot11FragmentationThreshold
Range: 256–2346 bytes
Default: 2346 bytes (effectively disables fragmentation)

Rule:

if MSDU_size > fragmentation_threshold:
    fragment MSDU
else:
    send as single frame

Fragmentation at the MAC Layer

The MAC layer divides an MSDU into smaller MPDUs (fragments). Each fragment:

Is transmitted independently.
Shares the same Sequence Number.
Has a unique Fragment Number (0, 1, 2, …).
Uses the More Fragments (MF) bit in the Frame Control field to indicate whether more fragments follow.

Fragment fields summary

Field	Description
Sequence Number Fragment Number More Fragments Duration/ID Frame Control	Same for all fragments Increments from 0 1 for all but last fragment, 0 for last one Updated per fragment for NAV reservation Contains MF bit

Frame Structure

Each fragment is transmitted as a complete 802.11 frame:

+----------------------------+
| Frame Control (MF bit)     |
| Duration / ID              |
| Address 1 (Receiver)       |
| Address 2 (Transmitter)    |
| Address 3 (BSSID)          |
| Sequence Control (Seq+Frag)|
| Payload (MSDU fragment)    |
| FCS                        |
+----------------------------+

MF Bit: 1 for all but the last fragment.
Sequence Control: Upper bits = Sequence Number, lower 4 bits = Fragment Number.

Timing Relationships

Fragmentation uses separate ACKs for each fragment. Once a station wins medium access, it transmits all fragments sequentially separated by SIFS intervals — no further backoff.

Timing example:

[DIFS + Backoff] → Frag#0 → SIFS → ACK → SIFS → Frag#1 → SIFS → ACK → SIFS → Frag#2 → SIFS → ACK

Key rules:

SIFS separates each fragment and ACK.
No DIFS/backoff between fragments of the same MSDU.
Each fragment is ACKed individually.

NAV and Duration Field Handling

Each fragment updates the Duration field to indicate how long the medium will be busy for the remaining fragments and their ACKs. Other stations set their NAV (Network Allocation Vector) accordingly.

Example NAV update sequence:

Stations that overhear these frames set their NAVs to the corresponding durations.

Acknowledgment and Retransmission

Each fragment must be individually acknowledged.
ACK sent after SIFS.
If no ACK is received, only that fragment and subsequent ones are retransmitted.

Retransmission logic

for (i = 0; i < num_fragments; i++) {
    send(fragment[i]);
    if (wait_for_ack()) {
        continue; // next fragment
    } else {
        if (retry_count++ > SHORT_RETRY_LIMIT) {
            drop_msdus();
            break;
        }
        retransmit(fragment[i]);
        i--; // retry current fragment
    }
}

Reassembly at Receiver

Receiver reassembles fragments based on: - Transmitter Address - BSSID - Sequence Number - Fragment Number

Fragments must arrive in order. If any fragment is missing or corrupt, the receiver discards the entire MSDU.

MF=1, Frag#0 → MF=1, Frag#1 → MF=0, Frag#2  → reassembly complete

Reassembly Steps

Collect all fragments with same Sequence Number and source.
Sort by Fragment Number.
Verify integrity (CRC/FCS).
Deliver MSDU to LLC layer when final fragment (MF=0) arrives.
Discard incomplete MSDUs after timeout or CRC failure.

Interaction with RTS/CTS

If RTS/CTS is enabled: - The RTS/CTS exchange occurs only before Fragment #0. - CTS Duration field covers the time for all fragments + ACKs. - All subsequent fragments are protected by NAV values set by the CTS.

This minimizes collision risk across the entire fragment burst.

Security Considerations

Encryption is applied after fragmentation.

Each fragment is individually encrypted.
Each fragment carries its own IV (for WEP/TKIP) or PN (for CCMP).
Receiver decrypts and verifies each fragment separately.

Fragmentation → Encryption → Transmission

This ensures message integrity and replay protection for every fragment.

Performance Implications

Advantages: - Less retransmission overhead on noisy channels. - Better performance with high BER links.

Disadvantages: - Higher MAC overhead (headers, ACKs, SIFS). - Slightly reduced throughput in clean channels.

Optimal fragmentation threshold depends on: - PHY rate - Channel quality (BER) - Traffic type (bulk vs. latency-sensitive)

Fragmentation Example

Given a 4000-byte MSDU and a fragmentation threshold of 1500 bytes:

Fragment	Size (B)	MF Bit	Frag #	Seq #
Frag #0 Frag #1 Frag #2	1500 1500 1000	1 1 0	0 1 2	110 110 110

Transmission timeline:

DIFS + Backoff
    ↓
Frag#0 → SIFS → ACK
SIFS → Frag#1 → SIFS → ACK
SIFS → Frag#2 → SIFS → ACK

ASCII Timing Diagram

Sender:  [DIFS] [Backoff] ──Frag#0── SIFS ──ACK── SIFS ──Frag#1── SIFS ──ACK── SIFS ──Frag#2── SIFS ──ACK──
Receiver:             Receive0  SIFS SendACK0  SIFS Receive1  SIFS SendACK1  SIFS Receive2  SIFS SendACK2
Others:                     NAV set for entire burst duration (from Duration fields)

Implementation Notes

Always recompute FCS for each fragment.
Maintain same Sequence Number across fragments.
Increment Fragment Number sequentially.
Apply same priority and QoS parameters for all fragments.
Respect SIFS and ACK timing precisely (µs granularity).

References

IEEE Std 802.11-2020, Clause 9.3.2.10 — Fragmentation and Defragmentation
IEEE Std 802.11-2016, Clause 9.2.5
1. Gast, 802.11 Wireless Networks: The Definitive Guide
Heusse et al., Performance Anomaly of 802.11b, IEEE INFOCOM 2003

Figures

Fragmentation timing diagram — Example of Fragmentation and ACK exchange timing.

Fragment reassembly process — Receiver defragmentation process showing MSDU reconstruction.