Design, Implementation and Evaluation of an Efficient Opportunistic

Retransmission ProtocolMei-Hsuan Lu Peter Steenkiste Tsuhan Chen

MobiCom 09

Outline

• Introduction• Estimating link quality• Protocol design• Collision and fairness• Multi-rate PRO• Evaluation

Introduction

• PRO - Protocol for Retransmitting Opportunistically

• IEEE 802.11 WLAN• S<->D distance

S

R

D

Estimating link quality

• Monitor success or failure of probe messages– Respond slowly to channel dynamics– Require extra bandwidth

• Monitor SNR of packets at receiver– – RSSI (received signal strength indicator)• Noisy• Thh

Estimating link quality

Protocol design

• Relay qualification• Relay selection• Relay prioritization• Retransmission

Protocol design

• Relay qualification– Relay->destination ≠ destination->relay– Thh, on-line calibration

• Relay selection (eligible relay)– Broadcast “I am qualified relay!”– Select the node has highest RSSI w.r.t destination– Add node nest highest …– Until the prob. of having a node hearing source >

threshold Thr

Protocol design

• Relay prioritization– Higher RSSI w.r.t destination -> higher priority ->

smaller CWmin(contention window size)

• Retransmission– Lack of ACK -> retransmit– Retransmission fail -> double CW, contend for channel

again– Terminate: an ACK heard or retry limit reached or a

new packet arrived– Re-ACK : to avoid collision, send “null” data packet

Collision and fairness

• Collision– Limiting number of eligible relays

• Fairness– More relays, more likely to gain access to channel– Mitigate unfairness: large initial CW, non uniform

selection of time slot in CW

Multi-rate PRO

• Rate adaption – reduce packet error rate by lowering bitrates (no relay)– SampleRate : probe-based– CHARM : SNR-based

• Combine PRO with CHARM– Transmission failed : eligible relay retransmit when

its rate ≥ source rate (having better link quality) – Aggressive rate selection

Evaluation

• Emulation– Static

• Overall• Per-relay

– Mobile– fairness

• Real world– Office building– Student lounge– 802.11g with multi-rate PRO

Emulation - static

• 3 environment scenarios– Freespace (outdoor)– Fading_k5 (small fading)– Fading_k0 (severe fading)

• 5 mechanisms– 802.11– 802.11 with CHARM– 802.11 with SampleRate– Mesh– Optimal PRO

Overall

Thr works well!

Per-relay

Emulation - mobile

S

D

Emulation - fairness

S1 S2 D2D1

100m 100m

S1 S2 D2D1

100m 50m

Real world

• Office building– Night

• Student lounge– Day– Severe fading

• Experiment– 10 laptops as nodes– Take turns as the source and send packet to other 9

nodes one by one– Nodes other than source and destination serve as relay

High contention High fading

Real world – 802.11g with multi-rate PRO

High contention High fading