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The Masters Journey

An Empirical Performance Study of DCF in IEEE 802.11g Internet Access Networks Andrew Symington, 2008 . The Masters Journey. Started MSc in January 2007 Supervised by Prof. Kritzinger Interested in Wireless Networking Definitely wanted practical aspect to work

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The Masters Journey

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  1. An Empirical Performance Study of DCF in IEEE 802.11g Internet Access NetworksAndrew Symington, 2008

  2. The Masters Journey • Started MSc in January 2007 • Supervised by Prof. Kritzinger • Interested in Wireless Networking • Definitely wanted practical aspect to work • I took a while to find research my area • First, ad-hoc routing protocols • Then, WiMaX • Then, co-operation enforcement • Finally, channel access control • Decided on a DCF performance analysis

  3. Problem Statement • Infrastructure WLANs are everywhere • IEEE 802.11 is dominant technology • Uses DCF/HCF for channel access control • Analytic models do exist for DCF • Bianchi, Ziouva, Wu, Szczpiorski • All of the above are validated for • Saturation conditions • Against sample simulation • Research area needs performance studies for • A realistic workload model • On a prototype test bed

  4. Thesis Objective • An empirical performance study for DCF • Build a nine station 802.11g test bed • Implement popular analytic models • Perform saturation experiments • Analytically • On the prototype test bed • Compare results from [3.1] to [3.2] • Perform realistic workload experiments • On the prototype test bed • Compare results from [3.2] to [5.1]

  5. The Test Bed • Nine Mini-ITX stations • 1 GHz C3 Nehemia, 128 MB RAM • SMC wireless card • Atheros AR5212 802.11bg chipset • Antenna raised from chassis • Optional 30dB SMA attenuator • Ethernet backbone network • PXE-booting • Controls experiments • Operating Software • Custom Linux • R2500 per node

  6. Realistic Workload Models • Model will have parameters drawn from traces • Observations about WAN traffic traces • Session/flow/packet paradigm • Trimodal packet length distribution, due to TCP • Packet arrival is a self-similar process, which displays long-range dependency (LRD) • LRD Measured by Hurst exponent • Wavelet-based approximation • R/S statistic • Cluster-based model for packet-length • Two models implemented for packet arrival • Markov Modulated Bernoulli Process • Hierarchical Markov Modulated Poisson Process

  7. MMBP Model • k states, each representing a different arrival rate • DTMC modulates a clients movement between states • Parameters derived from traces • High level of flow parallelism must be removed • Flows merged under memoryless conditions • k-means clustering used to identify states • DTMC transition probability matrix calculated directly from traces and states

  8. Hierarchical MMPP Model • Infinite state CTMC model • Vertical → flows • Horizontal → sessions • Only 5 parameters • Arrival rate of sessions • Arrival rate of flows • Arrival rate of packets • Avg. number of flows per session • Avg. number of packet per flow • Two of the parameters are drawn directly from traces • Remaining three parameters are estimated via a convergence algorithm • Sample simulation run and Hurst value is compared between trace and synthetic trace • The three parameters are modified according to the comparison • Process stops when Hurst difference is less than a threshold

  9. Preliminary Results • DCF Saturation experiments for the 802.11g PHY - both basic and RTC/CTS access • Compares normalised throughput as a function of the number of contending stations • Each experiment lasts for 200 seconds and is repeated thirty times • Red bar shows 95% confidence interval, green bar is mean • Trends the same, but after 5 stations, real world performance decays faster

  10. Preliminary Results • Workload model for packet length based on k-means clustering • Model fit measured by R-squared statistic • Explains the proportion of natural variance explained by the clustering • Plotted # of clusters versus R-squared for both regular and log-transformed data • 3 clusters (log-transformed) explains 95% of variance and maintains simplicity

  11. Remaining Work • I plan to complete my experiments and hand-in draft thesis by September the 15th • I have just finished implementing both workload models and am busy comparing them • Trace Hurst is only ~0.63 for flows and ~0.6 for packets • Still need to perform statistical moment analysis and visual comparison • Experiment set two is scheduled for completion in a week in Franschhoek • Need clear channel conditions and UCT does not have an anechoic chamber ☹ • Completed Chapters: • Introduction • Background • Literature Review • Workload Models • Experiment Design • Remaining Chapters • Results • Conclusion & Future work

  12. Thank you Are there any questions?

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