Changes between Version 28 and Version 29 of 802.11/wlan_exp/app_notes/dcf_with_multiple_flows
- Timestamp:
- Apr 17, 2014, 12:15:34 PM (10 years ago)
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802.11/wlan_exp/app_notes/dcf_with_multiple_flows
v28 v29 8 8 1. The effects of the hidden node problem on medium access. 9 9 10 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.10 This application note does not present research results. Fairness in the 802.11 DCF is a [http://scholar.google.com/scholar?q=802.11+DCF+fairness well studied topic]. The goal of this app note is to demonstrate how the 802.11 Reference Design and its experiments framework can be used to study high- and low-level behaviors across many nodes in real time. 11 11 12 == = Requirements ===12 == Requirements == 13 13 14 14 To run the code provided in this note, the following requirements must be met: … … 19 19 === Experimental Setup === 20 20 21 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.21 In order to isolate inherent DCF behaviors from uncontrollable fading and interference, we run the first set of experiments using wired connections between the RF interfaces of the WARP v3 nodes. 22 22 23 23 || [[Image(wiki:802.11/wlan_exp/app_notes/dcf_with_multiple_flows/figs:experimental_setup.png, width=400)]] || [[Image(wiki:802.11/wlan_exp/app_notes/dcf_with_multiple_flows/figs:experimental_setup_photo.jpg, width=400)]] || 24 24 || '''Block Diagram''' || '''Photo''' || 25 25 26 The above figure shows the experimental setup. We use an [http://www.minicircuits.com/pdfs/ZAPD-2-272+.pdf 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:26 The above figure shows the experimental setup. We use an [http://www.minicircuits.com/pdfs/ZAPD-2-272+.pdf Mini-Circuits Power Splitter/Combiner] and discrete attenuators to establish a "shred" 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. 27 27 28 We use 4 traffic flows in our experiments: 28 29 * '''Flow 1:''' Backlogged constant bit-rate (CBR) traffic from AP to STA_1 29 30 * '''Flow 2:''' Backlogged CBR traffic from AP to STA_2 … … 31 32 * '''Flow 4:''' Backlogged CBR traffic from STA_2 to AP 32 33 33 The colors used to identify each flow in the above figure are common throughout this application note. 34 [[Image(wiki:802.11/wlan_exp/app_notes/dcf_with_multiple_flows/figs:experimental_flows.png, width=400)]] 35 36 The colors in the figure above correspond to the colors used in per-flow plots below. 34 37 35 38 == Experiment 1: Symmetric and Fully Connected == 36 39 37 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 vertexof an equilateral triangle.40 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. 38 41 39 === = Experiment Details ====42 === Experiment Details === 40 43 * Attenuation 1: 45dB 41 44 * Attenuation 2: 15dB … … 46 49 * Channel 1 47 50 48 === Baseline: Performance of Each Flow in Isolation ===51 === Baseline: Contention-Free Performance === #baseline_no_contention 49 52 50 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. To do this, we use the WLAN Experiment Framework to reset the nodes, set the PHY rate, and then start locally generated traffic from the nodes. The script used for this is provided below in the [#Resources Resources Section]. 53 We first establish the best-case performance for each backlogged flow in isolation. We use the [wiki:../../ Experiment Framework] to reset the nodes, set the PHY rate, and then start locally generated traffic from the nodes. The Tx/Rx statistics are then retrieved from every node twice spanning a 90 second interval. The per-flow throughput is calculated as the ratio of new Rx bytes received vs. the time span over which they were received. 54 55 The script used for this is provided below in the [#Resources Resources Section]. 51 56 52 57 ||= Non-Simultaneous =||= Throughput (Mbps) =|| … … 56 61 ||= Flow 4 =|| 14.07 || 57 62 58 Each flow , on its own, is capable of achieving ~14Mbps out of the 18Mbps PHY rate. This is near the theoretical maximum throughput, given the unavoidable MAC overhead of the 802.11 DCF. Each flow in the network sees a high-quality link through the RF cabling and attenuators -- there are virtually no PHY packet losses due to outage.63 Each flow is capable of achieving ~14Mbps out of the 18Mbps PHY rate. This is near the theoretical maximum throughput, given the normal overhead in the 802.11 MAC and OFDM PHY. Each flow sees a high-quality link through the RF cabling and attenuators -- there are virtually no PHY packet losses. 59 64 60 65 === Performance of Simultaneous Flows === … … 69 74 ||= Sum Throughput =|| 12.77 || 70 75 71 The first thing to notice is that the sum throughput achieved by all flows together is less than any one of the throughputs achieved by one of the flows on their own. The isolated throughputs in the [#Baseline:PerformanceofEachFlowinIsolation previous section] are upper bounds on perfomance because there are no losses dueto collisions. When all four flows contend for the same medium, collisions are inevitable, yielding an overall reduction in network sum throughput.76 The sum throughput achieved by all flows is less than the individual best-case per-flow throughput. The isolated throughputs in the [#baseline_no_contention previous section] are upper bounds on performance, reflecting the achievable performance when no packets are lost to collisions. When all four flows contend for the same medium, collisions are inevitable, yielding an overall reduction in network sum throughput. 72 77 73 78 || [[Image(wiki:802.11/wlan_exp/app_notes/dcf_with_multiple_flows/figs:connected_symmetric_simultaneous_xput.png, width=300)]] ||
