WLAN Performance Aspects Mohammad Hossein Manshaei Jean-Pierre - PowerPoint PPT Presentation

Mobile Networks Module C- Part 1 WLAN Performance Aspects Mohammad Hossein Manshaei Jean-Pierre Hubaux http://mobnet.epfl.ch 1

Performance Evaluation of IEEE 802.11(DCF) • Real Experimentations – HoE on IEEE 802.11b • Analytical Models – Bianchi’s Model • Simulations – HoE on ns-2 2

Bianchi’s Model: Topology and Parameters • N links with the same physical condition (single-collision domain): 3 2 4 1 2 3 N 1 AP N-2 N N-1 We want to calculate the throughput of this network.  = Probability of Transmission MAC Layer = Probability of Collision P PHY Layer = More than one transmission at the same time 3 = 1 – (1-  ) N-1

802.11 - CSMA/CA unicast (Review) • Sending unicast packets – 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 DIFS data sender SIFS ACK receiver DIFS data other stations t waiting time Contention window The ACK is sent right at the end of SIFS 4 (no contention)

802.11 – DCF with RTS/CTS (Review) • Sending unicast packets – 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 DIFS RTS data sender SIFS SIFS SIFS CTS ACK receiver DIFS NAV (RTS) data other NAV (CTS) stations t defer access Contention window RTS/CTS can be present for 5 NAV: Net Allocation Vector some packets and not for other

802.11 – Slot Time in Bianchi’s Model Idle Idle Idle Idle Idle Idle data Idle Idle Idle data Collision channel DIFS wait wait wait wait wait wait wait wait data Busy Busy sender1 wait wait DIFS Busy wait Busy data wait wait sender2 wait wait wait wait wait wait wait wait sender3 Busy Busy wait data DIFS wait wait wait wait wait wait wait wait DIFS wait sender4 Busy wait data Busy collision One slot time 6

Bianchi’s Model: Two Dimensional Markov chain ( s(t), b(t) ) ( Backoff Stage, Backoff Timer ) 1-p 1/CW 0 1 1 1 1 (0,0) (0,1) (0,2) (0,CW 0 -2) (0,CW 0 -1) p/CW 1 (i-1,0) p/CW i 1 1 1 1 (i,0) (i,1) (i,2) (i,CW i -2) (i,CW i -1) p/Cw i+1 (m-1,0) p/CW m 1 1 1 1 (m,0) (m,1) (m,2) (m,CW m -2) (m,CW m -1) p 1/CW m 7

802.11 – Slot Time in Bianchi’s Model Idle Idle Idle Idle Idle Idle data Idle Idle Idle data Collision channel DIFS (0, 9) (0, 8) (0, 8) (0, 7) (0, 6) (0, 6) (0, 5) (0, 4) data Busy Busy sender1 (0, 4) (0, 3) (0, 3) (0, 2) DIFS (1, 3) Busy (0, 6) (0, 5) Busy (0, 1) data sender2 sender3 (3, 6) Busy (2, 4) (2, 3) Busy (2, 3) data DIFS (2, 6) (2, 5) (2, 2) (2, 1) (7, 4) (7, 3) (7, 2) (7, 1) DIFS (0, 5) sender4 Busy data (0, 7) Busy (0, 7) (0, 6) collision One slot time 8

Bianchi’s Model: Two Dimensional Markov chain Stationary distribution:         b lim P s t ( ) i b t , ( ) k , i (0, m k ), (0, CW 1)  i k , t i Probability of transmission: 9

Bianchi’s Model: Two Dimensional Markov chain Successful Transmission 10

Bianchi’s Model: Two Dimensional Markov chain 1-p 1/CW 0 1 1 1 1 (0,0) (0,1) (0,2) (0,CW 0 -2) (0,CW 0 -1) p/CW 1 (i-1,0) p/CW i Collision 1 1 1 1 (i,0) (i,1) (i,2) (i,CW i -2) (i,CW i -1) p/Cw i+1 (m-1,0) p/CW m 1 1 1 1 (m,0) (m,1) (m,2) (m,CW m -2) (m,CW m -1) p 1/CW m 11

Bianchi’s Model: Stationary Distribution of Chain (i-1,0) p/CW i 1 1 1 (i,0) (i,1) (i,2) (i,CW i -2) (i,CW i -1) b i,0 = p b i-1,0 b m,0 = p b m-1,0 + p b m,0 12

Bianchi’s Model: Solution for p and  After some derivations  system of two nonlinear equations with two variables p and  :   Can be solved numerically to obtain p and  13

Bianchi’s model: Throughput Calculation • Throughput of node i: E Payload Transmitted by user i in a slot time [ ] P P L    s tr     i E Duration of slot time [ ] P P T P (1 P T ) (1 P T ) s tr s tr s c tr id – P tr : Probability of at least one transmission in slot time – P s : Probability of successful transmission during a random time slot – L: Average packet payload size – T s : Average time to transmit a packet of size L – T c : Average time of collision – T id : Duration of the idle period – t ACK : ACK transmission time – t H : Header transmission time – t L : Payload transmission time     N P 1 (1 ) tr     N 1 N (1 )  P    s N 1 (1 )          T t t SIFS t DIFS s H L ACK 14      T t t DIFS c H L

Numerical Results Basic Mode Basic Mode RTS/CTS RTS/CTS 15

Conclusion • Semi-analytical model to express the performance of IEEE 802.11 networks • More sophisticated models have been developed since then • Don’t forget checking the related write up: «Performance Analysis of the IEEE DCF: Bianchi Model» 16