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2014 YU-ANTL Lab Seminar

2014 YU-ANTL Lab Seminar. Performance Analysis of the IEEE 802.11 Distributed Coordination Function Giuseppe Bianchi. April 12 , 2014 Yashashree Jadhav Advanced Networking Technology Lab. (YU-ANTL) Dept. of Information & Comm. Eng, Graduate School, Yeungnam University, KOREA

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2014 YU-ANTL Lab Seminar

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  1. 2014 YU-ANTL Lab Seminar Performance Analysis of the IEEE 802.11 Distributed Coordination Function Giuseppe Bianchi April 12, 2014 Yashashree Jadhav Advanced Networking Technology Lab. (YU-ANTL) Dept. of Information & Comm. Eng, Graduate School, Yeungnam University, KOREA (Tel : +82-53-810-3940; Fax : +82-53-810-4742 http://antl.yu.ac.kr/; E-mail : yashashree@ynu.ac.kr)

  2. Outline (1) • Background • MAC • DCF • Basic Access Mechanism • RTS/CTS Mechanism • Main Idea • Contribution • Markov Model • Probabilities • Two Dimensional Markov chain • Packet Transmission Probability • Throughput

  3. Outline (2) • Basic Access Mechanism • RTS/CTS Access Mechanism • Model Validation & Simulation • Model Validation • Maximizing Saturation Throughput • Throughput vs Number of Stations • Throughput vs Initial Window Size • Throughput vs Max. Back‐off Stage • Throughput vs Packet Length • Conclusion

  4. MAC (1) • IEEE802.11 is a set of standards for wireless local area network (WLAN) • This paper’s interest is in MAC layer • The MAC layer is a set of protocols which is responsible for maintaining order in the use of a shared medium • The MAC layer defines two different access methods • The Distribution Coordination Function (DCF) • Random access scheme • Based on CSMA/CA Protocol • The Point Coordination Function (PCF) • Based on TDMA • Paper focus on DCF

  5. MAC (2) • WLAN MAC and PHY Layer

  6. DCF (1) • When a station wants to transmit a new packet Monitor the channel activity • If senses idle for DIFS (Distributed Inter Frame Space), the station transmits • CSMA/CA • If sensed busy (immediately or during the DIFS),the station persists to monitor until it is measured idle for DIFS • The station generates a random back‐off interval before transmitting to minimize the collision probability

  7. DCF (2) • It describes two techniques to employ for packet transmission • Basic access mechanism (two‐way handshaking) • Source transmits the packet • If destination receives successfully transmits a positive ACK • RTS/CTS mechanism (four‐way handshaking) • Source sends RTS • If destination receives RTS then sends CTS • So the channel reservation is done • Source then transmits the packet • If destination receives successfully transmits a positive ACK

  8. DCF (3) • IEEE 802.11 DCF • At each packet transmission, the back‐off time is uniformly chosen in the range(0,w‐1) where w=contention window • w depends on the number of transmissions failed for the packet • At first, w=CWmin (minimum contention window) • At each unsuccessful, w is doubled (binary back‐off) up to a maximum value CWmax=2mCWmin • The back‐off time counter is Decremented as long as channel is sensed idle • Frozen when a transmission is detected on the channel • Reactivated when the channel is sensed idle for more than a DIFS • The station transmits when the back‐off time reaches zero

  9. Basic Access Mechanism • Basic Access Mechanism • station has to wait for DIFS before sending data • receiver acknowledges at once (after waiting for SIFS) if the packet was received correctly (CRC) • automatic retransmission of data packets in case of transmission errors

  10. RTS/CTS Access Mechanism • RTS/CTS Access Mechanism • station can send RTS with reservation parameter after waiting for DIFS ( reservation determines amount of time the data packet needs the medium) • acknowledgement via CTS after SIFS by receiver (if ready to receive) • sender can now send data at once, acknowledgement via ACK • other stations store medium reservations distributed via RTS and CTS

  11. 802.11 – Slot Time in Bianchi’s Model • 802.11 – Slot Time in Bianchi’s Model

  12. Contribution • Analytical evaluation of the saturation throughput Ideal channel conditions (no hidden terminals and capture) • Fixed number of stations where each station having a packet available for transmission • Behavior of single station is studied with a Markov model • The packet transmission probability (τ) of a station in randomly chosen slot time is obtained which is independent of access mechanism • The throughput of the both access mechanism is expressed as a function of τ • In saturation, each station has immediately a packet available for transmission • Each packet needs to wait for a random back‐off time before transmitting • At each transmission attempt each packet collides with constant and independent probability (p)

  13. Markov Model (1) • s(t) : stochastic process of back‐off stage of a station at time t • b(t): stochastic process of back‐off time counter for a station • Defines W=CWmin • m=maximum back‐off stage such that CWmax=2mW • Wi= 2iW where i Є(0,m) is the back‐off stage • It is possible to model the bi‐dimensional process {s(t),b(t)} with the discrete‐timeMarkov chain

  14. Markov Model (2) • Probabilities • P{i, k |i, k+1}=1 kЄ(0,Wi ‐2) and iЄ(0,m) • At the beginning of each slot time the back‐off time is decremented • P{0, k |i, 0}=(1-p)/W0kЄ(0,W0 ‐1) and iЄ(0,m) • New packet following a successful transmission (probability=1‐p) and starts with back‐off stage 0.The back‐off is initially chosen between (0, W0‐1) • P{i, k |i-1, 0}=p/WikЄ(0,Wi ‐1) and iЄ(1,m) • Unsuccessful transmission (probability=p) occurs at back‐off stage i-1,The new back‐ off is uniformly chosen between (0, Wi) • P{m, k |m, 0}=p/WmkЄ(0,Wm ‐1) • Once the back‐off stage reaches the value m, it is not increased in subsequent packet transmission

  15. Markov Model (3) • Two Dimensional Markov chain

  16. Markov Model (4) • Packet Transmission Probability • bi, k= limt-> ∞ P{s (t)=i, b(t)=k} , kЄ (0,Wi ‐1) andiЄ(0,m) • Stationary distribution of the chain • Closed‐form solution is needed • All thebi, k values can be expressed as functions of the values b0,0and p • τ= probability that a station transmits in a randomly chosen slot time • transmission occurs when back‐off counter=0 regardless of the back‐off stage

  17. Markov Model (5) • Packet Transmission Probability • When m=0 (no exponential back‐off) • One station transmits, collision occurs when at least one of the other n‐1station transmits • Using the two equations it can be derived that • τ (p) Can be shown to be a monotone decreasing function that • Starts from ,reduces up to

  18. Throughput (1) • S=Normalized system throughput [fraction of time the channel is used to successfully transmit payload bits] • Ptr=probability that there is at least one transmission in the considered slot time=p=1‐(1‐τ)n • Ps=probability that a transmission in the channel is successful =

  19. Throughput (2) • E[P]=average packet payload size • PtrPs=probability of successful transmission in a slot time • 1-Ptr=probability of the empty slot time • Ptr (1-Ps)=probability of collision • Ts =average time the channel is busy due to successful transmission • Tc=average time the channel is busy during a collision • σ=duration of an empty slot time • S depends mainly on Ts and Tc

  20. Basic Access Mechanism • H=packet header=PHYhdr + MAChdr • δ=propagation delay • E[P* ]=Average length of the longest packet payload involved in a collision

  21. RTS/CTS Access Mechanism • H=packet header=PHYhdr + MAChdr • δ=propagation delay

  22. Model Validation & Simulation (1) • Used event‐driven custom simulation program in C++ • It closely follows all the 802.11 protocol details for each in dependent transmitting station • The analytic model is extremely accurate • The analytic results (lines) practically coincide with the simulation results (symbols) in both basic and RTS/CTS access

  23. Model Validation & Simulation (2) • Model Validation

  24. Model Validation & Simulation (3) • Maximizing Saturation Throughput • Max throughput achievable by Basic is very close to by RTS/CTS • Throughput of RTS/CTS is less sensitive on τ • RTS/CTS throughput has a much lower dependence on the system engineering parameters

  25. Model Validation & Simulation (4) • Throughput vs Number of Stations • The greater the network size, the lower is the throughput [Except W=32] • For Basic Access it varies with the values of n • For RTS/CTS it is almost independent of n

  26. Model Validation & Simulation (5) • Throughput vs Initial Window Size • For both Basic Access and RTS/CTS , a high value of W depends on the n

  27. Model Validation & Simulation (6) • Throughput vs Max. Back‐off Stage • For both Basic Access and RTS/CTS , with W=32 and n=10 –50 • Choice of m doesn’t practically affect the system throughput as long as is m is greater than 4 or 5

  28. Model Validation & Simulation (7) • Throughput vs Packet Length • RTS/CTS mechanism is effective when packet size increases

  29. Conclusion • Simple but extremely accurate analytical model to study 802.11 DCF • Covers both Basic Access and RTS/CTS mechanism as well as the hybrid one • Provides good simulation results with comparison • The best analytical model so far for DCF • Finite number of terminals • No hidden terminal • Fixed Data Rate

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