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.

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

In order to isolate inherent DCF behaviors (our measurement target) from fading and interference (variables we can't control), we run the first set of experiments using wired connections between the RF interfaces of the WARP v3 nodes.

Block Diagram	Photo

The above figure shows the experimental setup. We use an ​Mini-Circuits Power Splitter/Combiner and discrete attenuators to establish a "shared" medium among the three RF interfaces. The path loss between each pair of nodes is controlled by varying the attenuation values. The inherent path loss through the power splitter/combiner is ~3dB for paths S-1 and S-2 and ~28dB for path 1-2.

We use 4 traffic flows in our experiments:

• 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 in the figure above correspond to the colors used in per-flow plots below.

Experiment 1: Equilateral

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 node. This mimics topology of three nodes at the vertices of an equilateral triangle.

Experiment Details

• Attenuation 1: 35dB
• Attenuation 2: 15dB
• Attenuation 3: 15dB
• Packet Length: 1400 byte payloads (1428 bytes OTA with MAC header and FCS)
• PHY Rate: 18 Mbps
• Tx Power: 10 dBm
• Trial Duration: 300 seconds
• Channel 1

Experiment Results

Resources

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