Investigating the DCF with Multiple Traffic Flows

The purpose of the 802.11 Distributed Coordination Function (DCF) is to allow multiple flows of traffic to contend for a shared wireless medium. In this application note, we investigate how the 802.11 Reference Design behaves when subjected to multiple traffic flows. This note provides a case study on how the WLAN Experiment Framework can be used to control and analyze the performance of the 802.11 Reference Design.

In this application note, we look at two things:

1. The effects of contention windows on medium access.
2. The effects of the hidden node problem on medium access.

This application note is not novel research. All of the insights provided here are known to the research community and have been for years. Instead, it is best to take this application note as an exercise of the WLAN Experiment Framework to re-discover known features of the 802.11 DCF.

Requirements

To run the code provided in this note, the following requirements must be met:

• 3 WARP v3 kits
• 802.11 Reference Design v.91 Beta

Experimental Setup

While all of the example code provided by this application note will operate over-the-air (OTA) with antennas, we chose to use RF cabling with attenuators to make the results repeatable and easier to discuss.

Block Diagram	Photo

The above figure shows the experimental setup. We use an ​Mini-Circuits Power Splitter/Combiner and rely on imperfect isolation between the two station-connected ports to allow the stations to carrier-sense one another. The three ports of the combiner are connected to three sets of attenuators, whose values we will define in the coming experimental description sections. In each of the below experiments, we refer to a common legend of traffic flows:

• Flow 1: Backlogged constant bit-rate (CBR) traffic from AP to STA_1
• Flow 2: Backlogged CBR traffic from AP to STA_2
• Flow 3: Backlogged CBR traffic from STA_1 to AP
• Flow 4: Backlogged CBR traffic from STA_2 to AP

The colors used to identify each flow in the above figure are common throughout this application note.

Experiment 1: Symmetric and Fully Connected

In this first experiment, we aim to see how the 802.11 Reference Design behaves when all nodes are "fully connected" with nearly-matched path losses between every combination of nodes. Topologically, this mimics the scenario where each of the three nodes sit on the vertex of an equilateral triangle.

Experiment Details

• Attenuation 1: 45dB
• Attenuation 2: 15dB
• Attenuation 3: 15dB
• Packet Length: 1400 byte payloads (1428 bytes OTA with MAC header and FCS)

Baseline: Performance of Each Flow in Isolation

As a first cut, we establish the best-case performance for each backlogged flow by running a series of experiments with only on flow enabled at a time.

Non-Simultaneous	Throughput (Mbps)
Flow 1	14.17
Flow 2	14.17
Flow 3	14.17
Flow 4	14.17

Experiment 2: Symmetric with Hidden Stations

Experiment Details

• Attenuation 1: 0dB
• Attenuation 2: 60dB
• Attenuation 3: 60dB
• Packet Length: 1400 byte payloads (1428 bytes OTA with MAC header and FCS)

Resources

Links to the data sets and experiment scripts used to perform this study will be posted shortly.