Expected Data Rate (EDR): An Accurate High-Throughput Path Metric For Multi-Hop Wireless Routing
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Expected Data Rate (EDR): An
Accurate High-Throughput Path Metric
For Multi-Hop Wireless Routing
Jun Cheol Park ([email protected])
Sneha Kumar Kasera ([email protected])
School of Computing
University of Utah
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labtop
PDA
labtop
PDA
PDA
The Internet
Multi-hop wireless networks
Flexible solution regardless of existence of fixed wired infrastructure
Efficient ad hoc routing necessary to achieve high throughput
Path metric crucial in selecting ad hoc paths
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Related Work
ETX (Expected Transmission Count) [MobiCom’03] considers packet loss, but does not accurately model
transmission interference
Existing transmission interference models do not consider packet loss
None of existing work has comprehensively addressed packet loss, transmission interference
together
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ETX
Average # transmissions (including retransmissions) needed for successful packet delivery on wireless link with loss rate p
ETX sum of ad hoc path sum of ETX of individual links used as path metric for selecting best ad hoc path
Achievable Data Rate of a link:
Maximal data rate / ETX
Maximal data rate delivery ratio
ETX =1 - p
1
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Limitations of ETX sum
PathA:ETX=1.5 ETX=1.5
ETX=1.0 ETX=2.0PathB:
ETX=1.7 ETX=1.7PathC:
UDP packet size: 1500 bytes
Source node always backlogged (11 Mbps)
ETX sum cannot accurately differentiate ad hoc paths
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Goal
Develop an accurate high-throughput path metric for multi-hop wireless
networks
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Outline
Problem Setting
EDR (Expected Data Rate)Transmission Contention DegreeBack-off procedure
Performance Evaluation
Summary
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Problem Setting
IEEE 802.11 networks Distributed Coordination Function (DCF) all links use single data rate
Load-insensitive path metric, routing does not consider “dynamic interference” due to other
flows considers “unavoidable” transmission interference within
single flow1 2 3 4
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Basic Ideas of EDR
Every link relies on supplying rate from previous link
EDR : achievable data rate of whole ad hoc path = achievable data rate of bottleneck link
B: Bottleneck linkD: Maximal Data rate on
link BETX(B)
DEDR =
ETX(B) for wired links
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Basic Ideas of EDR
Every link relies on supplying rate from previous link
EDR : achievable data rate of whole ad hoc path = achievable data rate of bottleneck link
B: Bottleneck linkD: Maximal Data rate on
link B
I: Total transmission interference factorETX(B)
D
ETX(B) I EDR = for wireless
links
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Total Transmission Interference Factor
Depends upon TCD: Transmission Contention Degree RTCD: Relatively Increased TCD
I = Sum of all TCD and RTCD on links that interfere with bottleneck link B
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Transmission Contention Degree for Link k
Represents how busy link k transmitting, retransmitting packets range [0.0, 1.0], normalized value compared
maximal data rate of link k when node always backlogged, TCD = 1.0
Considers load due to original transmission, retransmissions
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How to calculate TCD?
TCD(k)
ETX(k)Supplying rate at link k+1 =
TCD(k+1) ?TCD(k)
ETX(k) ETX(k+1)
ETX(k)
TCD(k)TCD(k+1) = Min { 1, ETX(k+1) }
Assume ETX values of links are given TCD(k+1) in terms of TCD(k)?
TCD(1) = 1.0
Original load
Increased load due to lost packets
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Effect of 802.11 Back-off
No mechanism to differentiate packet loss due to collisions, channel noise
Upon packet loss exponential back-off used for occupying shared medium
Different loss rates between adjacent links different average contention window sizes different medium occupancy probabilities relatively increased TCD (RTCD) on higher
loss rate link
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How to calculate RTCD?
Assume W(1) = 5, W(2) = 10
Node 1 twice more likely to occupy shared medium than Node 2
Thus, higher loss rate node (Node 2) experiences relative increase in TCD due to different window sizes
RTCD(k+1) = W(k+1)/W(k) -1
1 2 3
105Window size W(k)
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EDR
ETX(B) IEDR =
D
D: Maximum data rate on bottleneck link B
ETX(B): ETX of link B
I: Sum of (TCD+ RTCD) over all links that interfere with link B
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Performance Evaluation NS-2 simulations
Independent, temporally correlated loss models
Randomly generate 270 ad hoc paths hop lengths: 2 - 5 link loss rates: 0.0 - 0.5 (ETX: 1.0 - 2.0)
Construct groups of 4 ad hoc paths between source, destination for given group as input set, find how well each metric
selects best ad hoc path
Use 1500-byte UDP packets, send rate at source node = 11 Mbps
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Independent loss
EDR performs much better than ETX sum
EDR: for 90% of input cases, throughput more than 90 % of best
00.10.20.30.40.50.60.70.80.9
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1 27 53 79 105 131 157 183 209 235 261
Input sets, each having four different ad hoc paths
Thp
ut r
atio
(ch
osen
/ be
st)
ETX sum EDR
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Temporally correlated loss
Packet burst loss modeled using two-state continuous time Markov chain
Burst length borrowed from experimental results [Divert, MobiSys ’04]
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1 27 53 79 105 131 157 183 209 235 261
Input sets, each having four different ad hoc paths
Thpu
t rat
io (c
hose
n/be
st)ETX sum EDR
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Summary
Proposed a new metric, EDR
Showed that EDR can accurately determine achievable data rates of ad hoc paths
Future work investigate TCP over EDR routing apply EDR in multi-radio wireless networks
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Backup
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EDR for TCP on multi-rate paths
IEDR =
R
Bottleneck link B such that R = Min { D(k) / ETX(k) }
I = TCD(k)/TCDmax, k over interference range of link B, Normalized total transmission contention degree in terms of B
For TCP flows, EDR does not include RTCD in I because TCP window mechanism is able to avoid unnecessary overhead of RTCD by adjusting send rate at source node
ETX(k+1)ETX(k)
TCD(k+1) = TCD(k) D(k)D(k+1)
, TCD(1) = 1.0