Bianchi’s Model Topology and Parameters比安奇的模型结构和参数.pptVIP

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Bianchi’s Model Topology and Parameters比安奇的模型结构和参数.ppt

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Bianchi’s Model Topology and Parameters比安奇的模型结构和参数

* Module C- Part 1WLAN Performance Aspects Mohammad Hossein Manshaei Jean-Pierre Hubaux Mobile Networks 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): 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 1 2 3 N AP 1 2 3 4 N N-1 N-2 We want to calculate the throughput of this network. * 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 t SIFS DIFS data ACK waiting time other stations receiver sender data DIFS Contentionwindow 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 t SIFS DIFS data ACK defer access other stations receiver sender data DIFS Contentionwindow RTS CTS SIFS SIFS NAV (RTS) NAV (CTS) NAV: Net Allocation Vector RTS/CTS can be present forsome packets and not for other * 802.11 – Slot Time in Bianchi’s Model channel sender1 sender2 sender3 One slot time sender4 Collision Idle data Busy data DIFS wait wait Idle DIFS Busy wait wait wait wait Idle wait wait wait wait Idle wait wait wait wait Idle data Idle Busy Busy wait wait wait wait data DIFS Busy wait wait Idle wait wait wait wait collision data DIFS