A High-Throughput Path Metric for Multi-Hop Wireless Routing Douglas S. J. De Couto, Daniel Aguayo, John Bicket, Robert Morris MIT CSAIL Presented by Valentin Pistol, Duke University April 6 th, 2011 Credits: Includes slides borrowed and edited from authors

Test-bed map (indoor wireless network) 2

Motivation: Min hop-count not best 3 ‘Best’ for each pair is highest measured throughput of 10 promising static routes.

Throughput differs between paths 4

Outline 5  Testbed throughput challenges  Wireless routing challenges  ETX - New high-throughput metric  Evaluation

Wireless routing challenges 6  Longer routes have less throughput  Many lossy links  Many asymmetric links

Challenge: more hops, less throughput 7  Links in a route share radio spectrum  Extra hops reduce throughput Throughput = 1 Throughput = 1/2 Throughput = 1/3

Challenge: many links are lossy 8 ‘Good’ ‘Bad’

Challenge: many links are asymmetric 9 Broadcast delivery ratios in both link directions. Very asymmetric link.

Delivery Ratios 1 mW vs 30 mW 10

Metric: Maximize bottleneck throughput 11 Bottleneck throughput: 50%Delivery ratio = 100% 51% D A B C Actual throughput: A-B-C : ABBABBABB = 33% A-D-C : AADDAADD = 25% A-B-C = 50% A-D-C = 51%

Metric: Maximize end-to-end delivery ratio 12 A-B-C = 51% A-C = 50% 51%100% 50% A B C Actual throughput: A-B-C : ABBABBABB = 33% A-C : AAAAAAAA = 50% End-to-end delivery ratio:

New high-throughput metric (ETX) 13  ETX = expected transmission count  Finding the highest throughput path  Link throughput  1/ Link ETX Delivery Ratio 100% 50% 33% Throughput 100% 50% 33% Link ETX 1 2 3

Calculating link ETX 14  Actual TX  P(TX success) = P(Data success) x P(ACK success)  Link ETX = 1/ P(TX success) = 1/ [P(Data success) x P(ACK success)]  Estimating link ETX  Forward delivery ratio d fwd = P(Data success)  Reverse delivery ratio d rev = P(ACK success)  Link ETX  1 / ( d fwd x d rev )

Delivery ratios 15  Measured using dedicated link probe packets  All nodes broadcast link probes (134 bytes)  Each node remembers probes for last 10 seconds  Estimates  d fwd  # packets recv / # packets sent  d rev piggybacked on probes from neighbors

Route ETX 16  Route ETX = Sum of link ETXs Route ETX Throughput 100% 50% 33% 20%

ETX characteristics 17  Predicts throughput for short routes  Quantifies loss  Quantifies asymmetry  Quantifies throughput reduction of longer routes  Tends to minimize spectrum use which should maximize system capacity  May reduce energy/packet

ETX limitations 18  Does not specifically account for mobility  Link probes susceptible to MAC unfairness and hidden terminals  Route ETX measurements change under load  Estimates based on single small link probe packet (136 bytes)  Under-estimates data loss ratios  Over-estimates ACK loss ratios (38 bytes)  Assumes same bit-rate across all nodes  Assumes loss ratios cannot be controlled by the system

Evaluation Setup 19  Indoor network, b, ‘ ad hoc ’ mode  1 Mbps, 1 mW, small packets (134 bytes), RTS/CTS disabled  DSDV + modifications to metrics  DSR + modifications to metrics

ETX improves DSDV throughput 20 DSDV overhead ‘Best’ DSDV+ETX DSDV+hop-count

ETX slightly improves on DSR 21 DSR+ETX ‘Best’ DSR+hop-count

ETX + DSR (No TX feedback) 22 DSR+ETX DSR+hop-count ‘Best’

Summary 23  ETX is a new route metric for multi-hop wireless networks  ETX does find better routes  ETX considers  Throughput reduction of extra hops  Lossy and asymmetric links  Link-layer acknowledgements  DSR + ETX almost identical to DSR with link layer feedback  Feedback already helps avoid links with high loss ratios  No comparison between power levels for same runs

Thank you! Questions? 24 image © mrjanitor1.com

Backup slides 25

Large packets 26

Delivery ratio decreases for large packets 27

ETX vs. link handshaking 28

Hop-count penalty 29

ETX vs min hop-count at 30 mW 30