A High-Throughput Path Metric for Multi-Hop Wireless Routing Douglas S. J. De Couto, Daniel Aguayo, John Bicket, Robert Morris MIT Computer Science and Artificial Intelligence Laborotory Presented by Souvik Sen

Minimum Hop Count Links in route share radio spectrum Extra hops reduce throughput Throughput = 1 Throughput = 1/2 Throughput = 1/3

Min hop-count metric does not always choose highest-throughput path available!!!  Assumes links either work or don’t work  Maximize the loss ratio of each hop  Arbitrarily chooses among same length paths  Retransmission reduces throughput increases interference  Thresholds to discard lossy links may disconnect network Question – Is there a “better” metric ? Problems

Hop-count penalty Better

Challenge: many links are asymmetric Many links are good in one direction, but lossy in the other. Links with intermediate Loss Ratio Very asymmetric link.

Effect of asymmetry 100% 8% But, throughput of B-A = 0.08 B-C-A = 0.5 B successfully receives all of A’s route ads, and installs a one- hop route to A. A C B 100%

Throughput differs between paths Paths from 23 to 36

ETX metric: Design goals Find high-throughput paths Account for lossy links Account for asymmetric links Account for inter-link interference Independent of network load (don’t incorporate congestion)‏

A New Metric? SINR threshold?  Nodes Unreachable Multiply link ratios? 11 1 = 1.95 ~ 2 Throughput End to End Delay? -Depends on interface queue lengths

ETX: Minimize the Expected Transmission Count Link throughput  1/ Link ETX Delivery Ratio 100% 50% 33% Throughput 100% 50% 33% Link ETX 1 2 3

Calculating link ETX Assuming link-layer acknowledgements (ACKs) and retransmissions: P(TX success) = P(Data success)  P(ACK success)‏ Estimating link ETX: P(Data success)  measured fwd delivery ratio r fwd P(ACK success)  measured rev delivery ratio r rev Link ETX  1 / (r fwd  r rev )‏ ETX (path) = ∑ ETX(link)‏

Measuring Delivery Ratios Each node broadcasts small link probes (134 bytes), once per second Nodes remember probes received over past 10 seconds Reverse delivery ratios estimated as r rev  pkts received / pkts sent Forward delivery ratios obtained from neighbors (piggybacked on probes)‏

Test Specifics: Indoor network, b, ‘ad hoc’ mode 1 Mbps, 1 mW, small packets (134 bytes), RTS/CTS disabled DSDV + modifications to respect metrics  Packets are routed using route table snapshot to avoid route instability under load. DSR + modifications to respect metrics

DSDV overhead ‘Best’ DSDV+ETX DSDV+hop-count ETX and DSDV

Big packets

Comparison with min-hop (higher power)‏ DSDV hop-count DSDV ETX

DSR with ETX (no TX feedback)‏ DSR+ETX ‘Best’ DSR+ hop-count

DSR with ETX(With Tx Feedback)‏ DSR+ETX ‘Best’ DSR+ hop-count

ETX Performance “Broadcast” has lower priority Probe size ≠ Data/Ack size  Under-estimates data loss ratios, over-estimates ACK loss ratios For >= 4 hops, may choose a slower path with fewer hops ETX assumes all links run at one bit-rate, at the same Power Link-layer feedback already does a good job for DSR‏

Conclusion / Thoughts Proposed new metric to accommodate lossy/asymmetric links Detailed experiments on real testbed Favours shorter paths! Inter-hop interference accounting problematic  Probe Packets may suffer from interference due to a hidden terminal Is Traffic Independent:  Congestion / Link loss separation ?