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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