Performance Analysis of IEEE 802.11 Distributed Coordination Function

1. Performance Analysis of IEEE 802.11 Distributed Coordination Function Instructor: Prof. Liu Presented By: Abhishek Girotra and Yicai Jiang EECS: 557 Project Presentation 05’

2. OVERVIEW OF 802.11 DCF SATURATION THROUGHPUT ANALYSIS OUTLINE PERFORMANCE MEASURES ANALYTICAL MODEL FOR DCF DYNAMICS CONCLUSION EECS557 course project

3. DCF • In the 802.11 protocol, the fundamental mechanism to access the medium is called distributed coordination function (DCF). DCF is based on Carrier Sense Multiple Access with Collision Avoidance( CSMA/CA) with Binary Exponential Back Off Algorithm • Basic Access Mechanism – 2 Way Handshaking (ARQ Type- DATA, ACK) • RTS/CTS Mechanism – 4 Way Handshaking (RTS, CTS, DATA, ACK) • RTS/CTS more useful • It reduces the duration of collision when a long packet is transmitted • It solves the hidden node problem as done in class • Hidden Node just need to know RTS or CTS EECS557 course project

4. BACKOFF / CONTENTION WINDOW • For the transmission of each packet, a random Backoff time (in slot) is selected uniformly between 0 and CW-1. The value of CW is called contention window. • Station also waits for random Backoff Time after successfully transmitting 1 packet to avoid Channel Capture • Initial Value of CW = CWmin • On Collision: CW is Doubled (Binary Backoff), upto CWmax • Successful transmission: CW comes back to CWmin • Example: 802.11b uses CWmin = 32, CWmax = 1024 • Backoff COUNTER: • Initial: Selected randomly between 0 and CW-1 • Decrements at each idle slot and “freeze” during a busy slot • When Counter hits 0, Station transmits • Reset after successful transmission EECS557 course project

5. RTS/CTS Mechanism Example DIFS: distributed interframe space ( decide if it is idle) SIFS: short interframe space ( shorter than DIFS) NAV: Network Allocation Vector ( contains info about packet length being Tx) SIFS BO = 3 BO = 5 BO = 7 A DIFS RTS DATA DIFS RTS DIFS collision CTS ACK B BO = 4 BO = 8 BO = 5 BUSY DIFS NAV (RTS) DIFS RTS DIFS C NAV(CTS) EECS557 course project

6. CRITICAL ASSUMPTIONS • Ideal Channel conditions and finite number of terminals • Ideal Channel conditions include (No Hidden Terminals, No Channel Capture) • Constant & independent collision probability P for each transmitted packet • System is in Overload Condition (Every station is always ready to Transmit a Packet) EECS557 course project

7. Mathematical Model for Dynamics of DCF • s(t) – stochastic process representing back off stage (0, …. , m) of a given station • b(t) – stochastic process representing back off time counter (k, k-1,……,1,0) of a station • Bi-dimensional Process {s(t), b(t)} with state space (i, k) • and W = CWmin (minimum contention window length) • p – Conditional Collision Probability seen by a packet being transmitted (const and indep) • -1 • 2 • 3 • 4 Transition Probabilites P for process {s(t),b(t)} • Eq 1 – Once Back off Counting Starts, Counting has to decrement with Probability 1 • Eq 2 – Counter hits zero at t, Tx is a success, s(t+1) = 0, b(t+1) = k (uniform distribution in 0) • Eq 3 – Counter hits zero at t, Tx is a collision, s(t+1) = i, b(t+1) = k (uniform distribution in i) • Eq 4 – Counter is zero at t, Tx is a collision but s(t) = m, s(t+1) = m, b(t+1) = k, no new CW • t – Probability that station transmits a packet (remember SLOTTED ALOHA) • n – number of stations What can we do now ? • STATE TRANSITION DIAGRAM OF THE CHAIN BASED ON ABOVE TRANSITION MATRIX • STEADY STATE ANALYSIS OF THE CHAIN TO FIND SOLUTION TO • THROUGHPUT ANALYSIS OF RTS/CTS and Basic Access SCHEMES EECS557 course project

8. USE Global Balance Equations and Sum of Probability Distributions of all states=1 to solve for State Transition Diagram for the Chain S C EECS557 course project

9. Remember Slotted Aloha Stabilization ? • tdepends on m and W and can be changed adaptively • But m and W fixed because of Physical Layer Standard • Result – S can be significantly lower than maximum Throughput Analysis Based on Model • S := Fraction of Time channel is used to successfully transmit payload bits • As an outside observer, see a random slot and observe what is happening • Probability, Exactly 1 TX Occurring on the channel is successful given someone transmits • Hybrid Scheme also possible. • Packet Length may vary and throughput may relate itself to packet size distribution mean • Ts, Tc, s,P are constant for model verification constant, and determined by standard • Maximizing throughput over probabilities which are in terms of t, we get S is max when EECS557 course project

10. IMPORTANT RESULTS • For Sufficiently Large n, Smax is practically independent of no. of stations in wireless network • Maximum throughput achievable by BAS is very close to RTS/CTS mechanism • RTS/CTS scheme throughput is less insensitive to transmission probability t • RTS/CTS scheme is network size independent for W <= 64 values. Basic Mechanism throughput increases but significantly decreases with network size • Key to these results – RTS/CTS mechanism reduces the time spent during a collision, and it becomes more effective than Basic Access when W and n increases the collision probability • RTS/CTS even more effective when packet length are longer • SEE PERFORMANCE EVALUATION NEXT EECS557 course project

11. PERFORMANCE EVALUATION • Performance is based on following Parameters • Network size n • Transmission probability t • Initial contention window size CWmin • Maximum Backoff stage m • Packet size EECS557 course project

12. Basic access strongly depends on it n Throughput (except W = 128) RTS/CTS not depends on it much Performance Evaluation: Network Size EECS557 course project

13. Performance evaluation: transmission probability Both decrease dramatically when n is large, but the basic access is more sententive EECS557 course project Basic access RTS/CTS

14. Performance evaluation: CWmin • Basic Access: increases when station CWmin gets closer to 64, decreases as n increases • RTS/CTS is almost independent on CWmin and n when CWmin<64 EECS557 course project RTS/CTS Basic access

15. Performance Evaluation: Maximum Backoff stage Almost no effect when m > 5 EECS557 course project

16. Performance Evaluation: Packet Size RTS/CTS is effective when packet size increases EECS557 course project

17. CONCLUSION Giuseppe Bianchi, “ Performance Analysis of the IEEE 802.11 Distributed Coordination Function”, IEEE Journal on selected areas in Communications, Vol. 18, No. 3, March 2000 • Contributions of the referenced Paper • Proposed analytical model • Accurate: verified by comparison with simulations • Simple • Account for all exponential backoff details • Evaluate basic and RTS/CTS access schemes • Performance evaluation on saturation throughput • Other Remarks • Model lacks in considering non-ideal channel conditions (like hidden terminals, interfering stations, or multiple access points) • It can be extended towards study of throughput for different classes of customer with different access priorities • Only considers saturation throughput (overload conditions) EECS557 course project

18. THANK You! Questions? EECS557 course project