Wireless Networking

Evaluation of IEEE 802.11e WLANs

M. Thottan and M.C. Weigle, Impact of 802.11e EDCA on Mixed TCP-based Applications, Proceedings of the International Wireless Internet Conference (WICON), Boston, MA, August 2006.

Abstract

There has been an explosive growth in the use of wireless LANs (WLANs) to support network applications ranging from web-browsing and file-sharing to voice calls. It is difficult to optimally configure WLAN components, such as access points (APs), to meet the quality-of-service requirements of the different applications, as well as ensuring flow-level fairness. Recent work has shown that the widely-deployed IEEE 802.11 MAC Distributed Coordination Function (DCF) is biased against downstream flows. The new IEEE 802.11e standard introduces QoS mechanisms, such as Enhanced Distributed Channel Access (EDCA), that allow this unfairness to be addressed. So far, only limited work has been done to evaluate the impact of these MAC protocols on TCP-based applications. In this paper, through ns-2 simulations, we evaluate the impact of EDCA on TCP application traffic consisting of both long and short-lived TCP flows. We find that the performance of TCP applications is very dependent upon the settings of the EDCA parameters and buffer lengths at the AP. We also show that the performance of the admission control strategy employed depends on the buffer lengths at the AP and the traffic intensity.

Experiment Scripts

• avg.pl - computes the average of a list of numbers
• compute-CI.pl - computes the 95% confidence interval for a set of runs
• edca.tcl - TCL script that sets-up the EDCA parameters
• exps.tcl - TCL script driving the model-based traffic
• format-time-mem.pl - script for recording the real-time and memory used in the simulation
• gogo - driver script for a set of experiments
• interval_sum.pl - computes the sum over a given time interval
• median.pl - computes the median
• p2p.tcl - TCL script driving the P2P traffic
• proc-args.tcl - base TCL script for processing arguments
• process-one - processing script for one run
• process-CI - processing script for multiple runs of the same experiment
• tcp-wlan.tcl - driver TCL script

Scripts for Running the Experiment

• gogo requires tcp-wlan.tcl, process-one, process-CI

• tcp-wlan.tcl requires proc-args.tcl, exps.tcl, edca.tcl, p2p.tcl
  - tcp-wlan.tcl contains this helper function that might be useful in other scripts:

  # helper function for accessing queue in the wireless node
  Node/MobileNode instproc get-ifq {} {
      $self instvar ifq_
      return $ifq_(0)
  }

• edca.tcl contains the following functions:
  • set_mobile_priority - set EDCA parameters on each of the 4 queues at a mobile node
  • set_AP_priority - set EDCA parameters on each of the 4 queues at an AP (cases follow scenarios in our WiCon paper)
  • set_qlim - set maximum queue sizes on the AP and mobile stations
  • dump_priority - print EDCA parameter settings on the mobile nodes and AP

• exps.tcl only contains the setup_flows function which sets up and starts the traffic used in the WiCon paper

• p2p.tcl contains functions for controlling the P2P traffic used in the WiCon paper (the functions are called from exps.tcl)

ns-2 Source

Our source is based on the 802.11 and 802.11e EDCA code developed at TKN by Sven Wietholter and Christian Hoene.

We made additions to priq.h, priq.cc, d-tail.h, and d-tail.cc to allow for monitoring of statistics. Our additions are labeled with "M. Weigle, Jul 2006" comments.

Note: If you want to use Full-TCP, you'll also want to make the changes described here.