a high-throughput path metric for multi-hop wireless routing
DESCRIPTION
A High-Throughput Path Metric for Multi-Hop Wireless Routing. Douglas S. J. De Couto MIT CSAIL (LCS) Daniel Aguayo, John Bicket, and Robert Morris http://pdos.lcs.mit.edu/grid. Indoor wireless network. 29 PCs with 802.11b radios (fixed transmit power) in ‘ad hoc’ mode. 2 nd floor. - PowerPoint PPT PresentationTRANSCRIPT
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A High-Throughput Path Metric for Multi-Hop Wireless Routing
Douglas S. J. De CoutoMIT CSAIL (LCS)
Daniel Aguayo, John Bicket, and Robert Morris
http://pdos.lcs.mit.edu/grid
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5th floor
6th floor
29 PCs with 802.11b radios (fixed transmit power) in ‘ad hoc’ mode
Indoor wireless network
4th floor
3rd floor2nd floor
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Testbed UDP throughput
better
Better
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What throughput is possible?
‘Best’ for each pair is highest measured throughput of 10 promising static routes.
Routing protocol
‘Best’
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Talk outline
• Testbed throughput problems• Wireless routing challenges• A new high-throughput metric
(ETX)• Evaluation
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• Links in route share radio spectrum• Extra hops reduce throughput
Challenge: more hops, less throughput
Throughput = 1/2
Throughput = 1
Throughput = 1/3
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Challenge: many links are lossy
Smooth link distribution complicates link classification.
One-hop broadcast delivery ratios
‘Good’
‘Bad’
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Challenge: many links are asymmetric
Many links are good in one direction, but lossy in the other.
Broadcast delivery ratios in both link directions.
Very asymmetric link.
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Bottleneck throughput:
A straw-man route metric
Maximize bottleneck throughput
50%Delivery ratio = 100%
51% 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%
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A-B-C = 51% A-C = 50%
Another straw-man metric
Maximize end-to-end delivery ratio
51%100%
50%A
B
C
Actual throughput:A-B-C : ABBABBABB = 33% A-C : AAAAAAAA = 50%
End-to-end delivery ratio:
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Minimize total transmissions per packet(ETX, ‘Expected Transmission Count’)
New metric: ETX
Link throughput 1/ Link ETXDelivery Ratio
100%
50%
33%
Throughput
100%
50%
33%
Link ETX
1
2
3
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Calculating link ETXAssuming 802.11 link-layer acknowledgments (ACKs) and retransmissions:
P(TX success) = P(Data success) P(ACK success)
Link ETX = 1 / P(TX success) = 1 / [ P(Data success) P(ACK success) ]
Estimating link ETX: P(Data success) measured fwd delivery ratio rfwd
P(ACK success) measured rev delivery ratio rrev
Link ETX 1 / (rfwd rrev)
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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
rrev pkts received / pkts sent
• Forward delivery ratios obtained from neighbors (piggybacked on probes)
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Route ETX
Route ETX
1
2
2
3
Route ETX = Sum of link ETXs
5
Throughput
100%
50%
50%
33%
20%
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ETX Properties
• ETX predicts throughput for short routes(1, 2, and 3 hops)
• ETX quantifies loss• ETX quantifies asymmetry• ETX quantifies throughput reduction of
longer routes
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ETX caveats• ETX link probes are susceptible to MAC
unfairness and hidden terminals– Route ETX measurements change under
load
• ETX estimates are based on measurements of a single link probe size (134 bytes)– Under-estimates data loss ratios, over-
estimates ACK loss ratios
• ETX assumes all links run at one bit-rate
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Evaluation Setup
• Indoor network, 802.11b, ‘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
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ETX improves DSDV throughput
better
DSDV overhead
‘Best’
DSDV+ETX
DSDV+hop-count
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DSR with ETX
DSR+ETX
‘Best’
DSR+hop-count
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DSR with ETX (no TX feedback)
DSR+ETX
‘Best’
DSR+hop-count
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Some related work• Threshold-based techniques
– DARPA PRNet, 1970s–80s [Jubin87]: Minimum hop-count, ignore ‘bad’ links (delivery ratio 5/8 in either direction)
– Link handshaking [Lundgren02, Chin02]: Nodes exchange neighbor sets to filter out asymmetric links.
– SNR-based approaches [Hu02]: Mark low-SNR links as ‘bad’, and avoid them
• Mote sensors [Yarvis02]– Product of link delivery ratios
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What’s next: MIT Roofnet
QuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.
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Summary
• ETX is a new route metric for multi-hop wireless networks
• ETX accounts for – Throughput reduction of extra hops– Lossy and asymmetric links– Link-layer acknowledgements
• ETX finds better routes!
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http://pdos.lcs.mit.edu/gridDSDV & DSR implementations:
Roofnet info at poster session
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Extra slides follow
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Big packets
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Per-pair DSDV throughputs
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ETX vs. link handshaking
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Hop-count penalty
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Throughput differs between paths
Paths from 23 to 36
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Evaluation details• All experiments:
– 134-byte (including 802.11 overhead) UDP packets sent for 30 seconds
• DSDV:– 90 second warm-up (including ETX)– Route table snapshot taken at end of 90s
used to route UDP data for next 30s
• DSR:– Initiate route request by sending 1 pkt/s for
five seconds; followed by UDP data for 30s– ETX warms up for 15s before route request
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Effect of asymmetry on DSDV
100%
100%
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%100%