department of computer science university of virginia
Post on 22-Jan-2016
27 Views
Preview:
DESCRIPTION
TRANSCRIPT
Addressing Burstiness for Reliable Communication and Latency Bound
Generation in Wireless Sensor Networks
Department of Computer ScienceUniversity of Virginia
Sirajum Munir, Shan Lin, Enamul Hoque, S. M. Shahriar Nirjon, John A. Stankovic, and Kamin Whitehouse
Problem Definition
• Reliable delivery– Retry on each link until the
packet is receivedN1 N2 N3
N4 N5N6 N7
N8 N9N10
Source
Destination
23 3
5
Problem Definition
• Reliable delivery– Retry on each link until the
packet is received
• Problem:– Unbounded E2E latency– Not acceptable for real-time
applications
N1 N2 N3
N4 N5N6 N7
N8 N9N10
Source
Destination
Overview
• Basic Approach– Estimate maximum “burst”
length for each link• #consecutive failures
N1 N2 N3
N4 N5N6 N7
N8 N9N10
Source
23
3 200
335
3
5
100
5
4 562
2
Destination
Overview
• Basic Approach– Estimate maximum “burst”
length for each link
– Key insight:• Burstiness caused by physical
world dynamics• Some links are relatively
insulated from these dynamics
N1 N2 N3
N4 N5N6 N7
N8 N9N10
Source
23
3 200
335
3
5
100
5
4 562
2
Destination
Overview
• Basic Approach– Estimate maximum “burst”
length for each link– Choose routes that only use
non-bursty links
N1 N2 N3
N4 N5N6 N7
N8 N9N10
Source
23
3 200
335
3
5
100
5
4 562
2
Destination
Overview
• Basic Approach– Estimate maximum “burst”
length for each link– Choose routes that only use
non-bursty links– Schedule packet transmission
to avoid interference between links
N1 N2 N3
N4 N5N6 N7
N8 N9N10
Source1 Destination2
Source2 Destination1
Outline
• Modeling link burstiness• E2E latency bounds• Evaluation
Modeling Burstiness
• Modeling link Bursts– Bmax, B’min per link
• Bmax = Maximum No. of time slots where transmission can fail
• B’min = Minimum No. of time slots available for transmission
• W = Bmax + B’min
– Example• B’min = 1• W = 2
1 0 0 1 1 0 1 …
Modeling Burstiness
• Modeling link bursts– Bmax, B’min per link
• Bmax = Maximum No. of time slots where transmission can fail
• B’min = Minimum No. of time slots available for transmission
• W = Bmax + B’min
– Example• B’min = 1• W = 3• Bmax = 2
1 0 0 1 1 0 1 …
Modeling Burstiness
• Different from existing models– The β Factor [Srinivasan et al.]
• Models burstiness based on the distribution of burst lengths
– Our model only cares about the maximum burst length
X X X X X X X X X …
Empirical Study
• 21 Days-long • Indoor testbed• 48 Tmote Sky nodes • 3.6 M packets/link• 200 packets/sec• Compute Bmax, B’min, PRR of every link
Empirical Study
• Verified:– Some links are very bursty– Some links are not bursty
• Bmax is not predicted by PRR– Some highly reliable links
(PRR>0.99) still have very large bursts
Outline
• Modeling link burstiness• E2E latency bounds• Evaluation
E2E Latency Bound
• Min Latency Bound: NP-Hard• Greedy solution: Principles
– Routing : Least burst routing
N1 N2 N3
N4 N5N6 N7
N8 N9N10
Source
Destination
N1 N2 N3
N4 N5N6 N7
N8 N9N10
Source
Destination
Bmax
BmaxBmax Bmax
Bmax
Bmax
Bmax
Bmax
Bmax
Bmax
Bmax
BmaxBmax
Bmax
E2E Latency Bound
• Greedy solution : Principles– Routing: Least burst route– Schedule packet transmission
• Allocating time slots:– How many time slots to allocate per link?
» Allocate Bmaxi+1 contiguous time slots, for i-th link
– Can we do even better?» Yes ! Overlap some streams’ time slot allocation
N1 N2 N3
N4 N5N6 N7
N8 N9N10
Source
Destination
E2E Latency Bound
• What is overlapping?– Assume Link L(1,2) has Bmax=2, B’min=4
– 2 Streams: S1, S2
• Why do we need overlapping ?– W/O overlapping: Avg LB = (3 + 6)/2 = 4.5– W/ overlapping: Avg LB = (3 + 4) /2 = 3.5
N1 N2
1 2 3 4 5 6
S1 S1 S1
S2 S2 S2
Schedule w/o overlapping
1 2 3 4
S1 S1 S1
S2 S2 S2
Schedule w/ overlapping
Prioritizing rule
Time Slots
E2E Latency Bound
• How much to overlap?– Assume Link L(1,2) has Bmax=2, B’min=4
– 2 Streams: S1, S2
– Overlap at most B’min number of streams
N1 N2
1 2 3
S1 S1 S1
S2 S2 S2
Complete overlapping: Doesn’t work !
1 2 3 4 5 6
S1 S1 S1
S2 S2 S2
S3 S3 S3
S4 S4 S4
Time Slots
Time Slots
E2E Latency Bound
• How much to overlap?– Assume Link L(1,2) has Bmax=2, B’min=4
– 2 Streams: S1, S2
– Overlap at most B’min number of streams
N1 N2
1 2 3
S1 S1 S1
S2 S2 S2
Complete overlapping: Doesn’t work !
1 2 3 4 5 6
S1 S1 S1
S2 S2 S2
S3 S3 S3
S4 S4 S4
Time Slots
Time Slots
E2E Latency Bound
• How much to overlap?– Assume Link L(1,2) has Bmax=2, B’min=4
– 2 Streams: S1, S2
– Overlap at most B’min number of streams
N1 N2
1 2 3
S1 S1 S1
S2 S2 S2
Complete overlapping: Doesn’t work !
1 2 3 4 5 6
S1 S1 S1
S2 S2 S2
S3 S3 S3
S4 S4 S4
Time Slots
Time Slots
E2E Latency Bound Summary
• Greedy solution : Principles– Routing: Least burst routing– Allocating time slots:
• How many time slots to allocate per link?– Bmaxi+1 contiguous time slots
– Without complete overlapping
• How much to overlap?– Overlap at most B’mini streams’ time slot allocation
• How to handle interference?– Use IM to avoid interference
Outline
• Modeling link burstiness• E2E latency bounds• Evaluation
Evaluation
• Experimental Setup– Same testbed as empirical study
• 48 Tmote Sky nodes
– Same packet transmission rate• 200 packets/sec
– RBS style time-synchronization
• Effect of Bmax– B’min = 1– Multiplying factor: K
• Allocate Bmaxi*K + 1 time slots, for i-th link
– As K increases• Average LB increase linearly • E2E DMR becomes 0
at K = 0.6 !• Allocate Bmaxi*0.6 + 1
time slots -> save 12.4% latency
– K allows us to control E2E DMR and LB !
• Avg. LB increases linearly
Evaluation
12.4%
Evaluation
• Effect of B’min– As B’min increases
• LB decreases• Then starts to increases again !
– Minimize average LB by an intelligent selection of B’min.
Contributions
• New model of link burstiness– Estimates maximum consecutive packet loss – Not captured by β factor or PRR
• New scheduling algorithms for E2E latency bounds
• Empirical evaluation– 21 day link characterization– Testbed evaluation of LB miss ratio with 10
simultaneous streams
Conclusions
• Can provide reasonable estimate of latency bounds– Not a guarantee– The “K” parameter helps control the trade-off between
miss ratio and latency
• One important step to combine wireless networking with real-time control
Questions?
Backup Slides
One Final Issue…
• Change in burst behavior?– Packet Recovery
• Each node queues un-transmitted packets.• Transmits later if free slot available.
– Link Adaptation• Each node keeps a record when it fails to transmit• Sends this report to B.S. periodically• B.S. reschedules by doubling/ halving the allocated
time slots• LB expands/shrinks dynamically
Stationarity
• Can we assume that Bmax is stationary?– Can classify links:
• Bursty links had highly variable Bmax• Non-bursty links were more consistent
– Why? Due to physical dynamics
– Must ensure that measurement period captures all physical dynamics
• No stronger requirement than any model
IM
• Characterizing Interference:– Define an Interference Matrix, IM
– Measurement based on PRR
otherwise 0,
range ceinterferen in are Lj link and Li linkif 1,j)IM(i,
Ni1 Ni2
Nj1 Nj2
Li
Lj
L1 L3
top related