802.11 MAC Frame Decoding

Background

The general MAC frame format consists of a header, frame-body, and frame check sequence (FCS). The header holds information about the frame. The frame-body carries data that needs to be transmitted. The transmitter calculates the FCS over the header and frame-body. The receiver uses the FCS to confirm that the header and frame-body are properly received. The following diagram shows the structure of a general MAC frame.

For more information, see the WLAN MAC Frame Structure topic.

Introduction

This example shows how WLAN MAC frames specified in Section 9.3 of [ 1 ], [ 2 ], and [ 3 ] can be decoded. It also shows how aggregated MAC frames specified in Section 9.7 of [ 1 ], [ 2 ], or [ 3 ] can be deaggregated.

WLAN Toolbox™ supports MPDU decoding for the following MAC frames:

In addition to MPDU decoding, WLAN Toolbox also supports deaggregation of an A-MPDU.

MPDU Decoding

An MPDU can be a data, control or management frame type. wlanMPDUDecode can be used to decode an MPDU. This function processes the given MPDU and a physical layer configuration object to output the decoded MAC parameters.

To illustrate MPDU decoding, a valid MPDU is created using wlanMACFrame. The created MPDU is passed to the wlanMPDUDecode function and the outputs are observed.

Create an MPDU

A QoS Data frame is created for this example using wlanMACFrame. The following inputs are required to form a Non-HT format QoS Data frame containing a 40-octet payload:

% Create a MAC frame configuration object
txFrameCfg = wlanMACFrameConfig(FrameType='QoS Data', ...
                            FrameFormat='Non-HT');

% 40-octet payload for each 'QoS Data' frame
txMSDU = randi([0, 255], 40, 1);

% Physical layer configuration
phyCfg = wlanNonHTConfig;

% Create the MPDU
mpdu = wlanMACFrame(txMSDU, txFrameCfg);

Decode the MPDU

wlanMPDUDecode consumes an MPDU, a PHY configuration object of type wlanNonHTConfig, wlanHTConfig, wlanVHTConfig, or wlanHESUConfig and optionally a (Name, Value) pair for DataFormat specifying the input format of the MPDU. Since the MPDU generated using wlanMACFrame is in terms of octets, DataFormat is set to octets. wlanMPDUDecode decodes the MPDU and outputs the following information:

% Decode the MPDU.
[rxFrameCfg, rxMSDU, status] = wlanMPDUDecode(mpdu, phyCfg, ...
                                            DataFormat='octets');

% Check if the MPDU is decoded successfully
disp(['Status of the MPDU decoding: ' char(status)])

% Observe the outputs, if the MPDU is decoded successfully
if strcmp(status, 'Success')
    disp(['Type of the decoded MPDU: ' rxFrameCfg.FrameType])
    disp(['Number of MSDUs in the MPDU: ' num2str(numel(rxMSDU))])
    for i = 1:numel(rxMSDU)
        disp(['Size of MSDU-' num2str(i) ': ' num2str(size(rxMSDU{i}, 1)) ' octets'])
    end
end

Status of the MPDU decoding: Success
Type of the decoded MPDU: QoS Data
Number of MSDUs in the MPDU: 1
Size of MSDU-1: 40 octets

A-MPDU Deaggregation

An A-MPDU is an aggregation of multiple MPDUs. The type of MPDUs in an A-MPDU are restricted as specified in Section 9.7.3 of [ 1 ].

wlanAMPDUDeaggregate can be used to deaggregate an A-MPDU. This function processes the given A-MPDU and the corresponding physical layer configuration object to output the deaggregated list of MPDUs. wlanAMPDUDeaggregate is capable of decoding HT (High Throughput), VHT (Very High Throughput), HE-SU (High Efficiency Single User) and HE-EXT-SU (High Efficiency Extended Range Single User) format A-MPDUs as specified in [ 1 ] and [ 2 ].

To illustrate the A-MPDU deaggregation, a valid A-MPDU containing five MPDUs is created using wlanMACFrame. The created A-MPDU is passed to the wlanAMPDUDeaggregate function and the outputs are observed.

Create an A-MPDU

The following inputs are required to form an HE-SU format A-MPDU containing five MPDUs (QoS Data frames), each MPDU containing a 40-octet payload:

% Create a MAC frame configuration object
txFrameCfg = wlanMACFrameConfig(FrameType='QoS Data', ...
                            FrameFormat='HE-SU', ...
                            MPDUAggregation=true, ...
                            MSDUAggregation=false);

% 40-octet payload for each 'QoS Data' frame
txMSDUList = repmat({randi([0, 255], 40, 1)}, 1, 5);

% Physical layer configuration
phyCfg = wlanHESUConfig(MCS=3);

% Create the A-MPDU containing 5 MPDUs
ampdu = wlanMACFrame(txMSDUList, txFrameCfg, phyCfg);

Deaggregate the A-MPDU

wlanAMPDUDeaggregate consumes an A-MPDU, a PHY configuration object of type wlanHTConfig, wlanVHTConfig, or wlanHESUConfig and optionally a (Name, Value) pair for DataFormat specifying the input format of the A-MPDU. It finds and validates the MPDU delimiters, extracts the MPDUs and outputs the following information that can be used for further processing the MPDUs:

% Deaggregate the A-MPDU
[mpduList, delimCRCFails, ampduStatus] = wlanAMPDUDeaggregate(ampdu, phyCfg, ...
                                                    DataFormat='octets');

% Observe the outputs
disp(['Status of A-MPDU deaggregation: ' char(ampduStatus)])
disp(['Number of MPDUs extracted from the A-MPDU: ' num2str(numel(mpduList))])
disp(['Number of MPDUs with delimiter CRC fails: ' num2str(nnz(delimCRCFails))])

Status of A-MPDU deaggregation: Success
Number of MPDUs extracted from the A-MPDU: 5
Number of MPDUs with delimiter CRC fails: 0

Decode the list of MPDUs

The mpduList contains the list of MPDUs extracted from the A-MPDU. Each of the MPDUs present in the list can be decoded separately. However, if the delimCRCFails contains any true values, the MPDU present in mpduList at the corresponding index can be considered invalid as it may not be properly extracted because of the delimiter CRC failure.

% Decode the list of MPDUs
if strcmp(ampduStatus, 'Success')
    % Number of MPDUs in the list
    numMPDUs = numel(mpduList);

    for i = 1:numMPDUs
        % Decode the MPDU only if the corresponding delimiter CRC is valid
        if ~delimCRCFails(i)
            [rxFrameCfg, rxMSDU, mpduStatus] = wlanMPDUDecode(mpduList{i}, phyCfg, ...
                                                        DataFormat='octets');
            disp(['MPDU-' num2str(i) ' decoding status: ' char(mpduStatus)])
            disp(['MPDU-' num2str(i) ' type: ' rxFrameCfg.FrameType])
            disp(['MPDU-' num2str(i) ' payload size: ' num2str(size(rxMSDU{1}, 1)) ' octets'])
            disp(' ')
        end
    end
end

MPDU-1 decoding status: Success
MPDU-1 type: QoS Data
MPDU-1 payload size: 40 octets
 
MPDU-2 decoding status: Success
MPDU-2 type: QoS Data
MPDU-2 payload size: 40 octets
 
MPDU-3 decoding status: Success
MPDU-3 type: QoS Data
MPDU-3 payload size: 40 octets
 
MPDU-4 decoding status: Success
MPDU-4 type: QoS Data
MPDU-4 payload size: 40 octets
 
MPDU-5 decoding status: Success
MPDU-5 type: QoS Data
MPDU-5 payload size: 40 octets
 

Conclusion and Further Exploration

This example demonstrated how to deaggregate and decode IEEE 802.11 MAC frames. You can also explore WLAN Beacon Receiver Using Software-Defined Radio and Recovery Procedure for an 802.11ac Packet examples for decoding the MAC frames retrieved from the captured waveforms.

Selected Bibliography