Throughput in 802.11 Networks

The IEEE® 802.11™ working group is continually adding features to 802.11 specification [1] to improve the throughput and reliability in WLAN networks. Throughput is the amount of data transmitted over a period of time. Medium Access Control (MAC) layer throughput refers to the amount of data successfully transmitted by the MAC layer over a period of time. MAC protocol data unit (MPDU) is the unit of transmission at MAC layer. In 802.11n, MPDU aggregation was introduced to increase the throughput. When MPDU aggregation is supported, MAC layer aggregates multiple MPDUs into an aggregated MPDU (A-MPDU) for transmission. This reduces the overhead of channel contention for transmitting multiple frames, resulting in enhanced throughput. In 802.11ac [1] and 802.11ax [2], the maximum limits for an A-MPDU length were increased resulting in even better throughput in WLAN networks.

802.11网络

这个例子WLAN网络模型和五个节点as shown in this figure. These nodes implement carrier-sense multiple access with collision avoidance (CSMA/CA) with physical carrier sense and virtual carrier sense. The physical carrier sensing uses the clear channel assessment (CCA) mechanism to determine whether the medium is busy before transmitting. Whereas, the virtual carrier sensing uses the RTS/CTS handshake to prevent the hidden node problem.

The model in the example displays various statistics such as the number of transmitted, received, and dropped packets at PHY and MAC layers. Moreover, the runtime figures that help in analyzing/estimating the node-level and network-level performance are also displayed in this model. This model is validated against the published calibration results from the TGax Task Group [4] for Box 3 scenarios (Tests 1a, 1b, and 2a) specified in TGax evaluation methodology [3]。

WLAN Network

Components of a WLAN Node

The components of a WLAN node are shown in this figure. The information is retrieved by pressing the arrow button for each node in the above figure.

Application, EDCA MAC, PHY, and Channel Block Capabilities

应用：

该应用层具有生成具有不同优先级级别的数据，如本图所示。这些优先级级别是使用Access CategoryWLAN节点内的应用程序流量生成器块的掩模参数中的属性。您还可以为应用程序层配置数据包大小，包装间间隔和目标节点。

EDCA MAC：

The EDCA MAC block used in this example has the following capabilities:

PHY：

The PHY Transmitter and PHY Receiver blocks have the capability to generate and decode waveforms of Non-HT, HT-MF, VHT, HE-SU and HE-EXT-SU formats. You can configure the transmit gain and transmit power using theTx GainandTx Powerproperties in the mask parameters of the PHY Transmitter block inside a WLAN node.

Similarly, you can configure the receive gain and receive noise figure using theRx GainandRX噪声图properties in the mask parameters of the PHY Receiver block inside a WLAN node.

Channel:

通过自由空间路径模型和瑞利多径褪色确定的通道障碍被添加到传输的PHY波形中。您可以选择启用或禁用这些损伤模型。除了损伤模型外，信号接收范围还可以受到可选范围传播损耗模型的限制。为了建模这些损失中的任何一个，通道模型必须同时包含发件人和接收器位置以及传输信号强度。在将波形传递到PHY接收器块之前，该通道是在每个接收节点内部建模的。

Throughput Measurement

Throughput varies for different configuration parameters pertaining to the application, MAC & PHY layers. Any change in the configuration may either increase or decrease the throughput. You can vary the combination of these parameters to measure and analyze the throughput.

Along with above parameters, you can also vary the node positions, Tx & Rx gains, channel loss, number of nodes in the network, MAC contention parameters, number of transmit chains and rate adaptation algorithms to analyze MAC throughput. This example demonstrates the measurement and analysis of the MAC throughput by varying packet size in theApplication Traffic Generatorblock.

Application Packet Size

Throughput is directly proportional to the application packet size. Smaller packet size results in greater number of packets to be transmitted. At the MAC layer, there is an overhead of contention time for each transmitted packet. This is because the MAC layer makes sure that the channel is idle for a specific amount of time (Refer section 10.3.2.3 of [1]) before transmitting any packet. Therefore, as the packet size decreases, the contention overhead increases resulting in lower throughput.

Model Configuration

您可以使用以下步骤配置应用程序数据包大小：

运行模拟并观察吞吐量。测试1a的TGAX校准结果[4] are shown below:

The above plot compares the calibration results for WLAN Toolbox against the published results of other companies listed in [4]。The blue colored curve represents the results of WLAN Toolbox, while the grey colored curves represent the results of other companies.

Simulation Results

The simulation of the model generates:

This figure shows MAC state transitions with respect to simulation time.

You can also observe the live state of the MAC layer transmission buffers using the 'Observe MAC queue lengths' button in the above visualization.

This figure shows the network statistics at the end of simulation.

用TGAX校准结果验证应用层吞吐量

TGAX任务组[4] published application throughput results for different scenarios. You can observe the Layer 3 (above MAC layer) throughput of each node in the network in 'Throughput' column in 'statisticsTable' stored in 'statistics.mat'. The TGax calibration scenarios for MAC simulator published results of application throughput for a User Datagram Protocol (UDP) with Logical Link Control (LLC) layers overhead.

To calculate application throughput from simulation results use the code below:

% Load statistics.mat (Output of the simulation) filesimulationResults = load('statistics','statisticsTable');% Statisticsstats = simulationResults.statisticstable;% Successfully transmitted MAC layer bytes in the networktotalMACTxBytes = sum(stats.MACTxBytes);% UDP & LLC overheads (bytes)udpOverhead = 36; llcOverhead = 8;％UDP＆LLC开销（字节）网络中udpAndLLCOverhead = sum(stats.MACTxSuccess)*(udpOverhead + llcOverhead);% Successfully transmitted application bytestotalAppTxBytes = totalMACTxBytes - udpAndLLCOverhead;% Time at which last transmission is completed in the network (Microseconds)simulationTime = max(stats.MACRecentFrameStatusTimestamp);% Application throughput (Mbps)applicationThroughput = (totalAppTxBytes*8)/simulationTime; disp(['Application Throughput = 'num2str(applicationThroughput)' Mbps']);

Application Throughput = 4.7276 Mbps

不同的TGAX校准方案的应用程序吞吐量针对不同的Mac服务数据单元（MSDU）尺寸绘制了30秒的仿真时间：如下所示：

Further Exploration

Configuration options

您可以更改这些配置参数，以进一步探索此示例：

Related examples

为进一步探索参考这些示例:

此示例使您可以使用Simulink模型来创建和配置多节点802.11网络，以分析MAC和应用程序层吞吐量。金宝app在此模型中，通过仿真结果获得的MAC吞吐量用于计算应用程序层吞吐量。使用Box 3方案（测试1A，1B和2A）在TGAX评估方法中验证了该模型[3]确认它符合IEEE 802.11 [1]。该示例得出结论，计算出的应用层吞吐量在公开校准结果中指定的最小和最大吞吐量范围内[4]。

Appendix

The helper functions and objects used in this example are:

References