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

Performance Evaluation of IEEE 802.11(DCF) Real Experimentations HoE on IEEE 802.11b Analytical Models Bianchi’s Model Simulations HoE on ns-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. PHY Layer MAC Layer P p = Probability of Transmission = Probability of Collision = More than one transmission at the same time = 1 – (1- p)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 (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 stations NAV (CTS) t defer access Contention window RTS/CTS can be present for some packets and not for other NAV: Net Allocation Vector

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

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

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

Bianchi’s Model: Two Dimensional Markov chain Stationary distribution: Probability of transmission:

Bianchi’s Model: Two Dimensional Markov chain Successful Transmission

Bianchi’s Model: Two Dimensional Markov chain Collision

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

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

Bianchi’s model: Throughput Calculation Throughput of node i: Ptr: Probability of at least one transmission in slot time Ps: Probability of successful transmission during a random time slot L: Average packet payload size Ts: Average time to transmit a packet of size L Tc: Average time of collision Tid: Duration of the idle period tACK: ACK transmission time tH: Header transmission time tL: Payload transmission time

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

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»