cmap: harnessing exposed terminals in wireless networks
DESCRIPTION
CMAP: Harnessing Exposed Terminals in Wireless Networks. Mythili Vutukuru Joint work with Kyle Jamieson and Hari Balakrishnan. u. v. x. y. z. The Problem. Which transmissions concurrently? Increase throughput by maximizing concurrency. X. u. v. y. z. x. Today’s Solution: CSMA. - PowerPoint PPT PresentationTRANSCRIPT
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CMAP: Harnessing Exposed
Terminalsin Wireless Networks
Mythili Vutukuru
Joint work with Kyle Jamieson and Hari Balakrishnan
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The Problemu
v
x
y
Which transmissions concurrently? Increase throughput by maximizing
concurrency.
Xz
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u
v yz
Exposed terminal problem.
Today’s Solution: CSMA
x
Energy > carrier sense threshold
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Key Insight Existing solutions: rules to predict which
concurrent transmissions increase throughput.
Instead, watch and discover which concurrent transmissions increase throughput.
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CMAP: Conflict Map Map of conflicting transmissions based on
empirical evidence. Built in distributed, online manner.
Exposed terminals – 2x gain on CSMA.
u
v yz
x u v & x y NO!
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Roadmap CMAP Design
• Conflict maps
• ACK & Backoff Policy
Implementation
Evaluation
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What is a “conflict”?u
v y
x
Conflict – throughput lower when concurrent. Loss rate threshold to decide conflicts.
No Conflict.Conflict.
50% loss
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Discovering Conflictsu
v y
x
Loss rate of uv when x is concurrent
>50% then infer conflict at v. Conflict entries timed out periodically.
When u transmits to me, x causes interference.
pq
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Populating the Conflict Map
u
v y
x
When u transmits to me, x causes interference.
Do not transmit to v when x anyone.
Do not transmit to anyone when u v.
Conflict map
z
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Channel Access Decisions Nodes always overhear channel.
Consult conflict map before transmission.
Carrier sense always disabled!
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Roadmap CMAP Design
• Conflict maps
• ACK & Backoff Policy
Implementation
Evaluation
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Windowed ACKs
Sliding window of packets at sender.
u
yz
xX
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Backoff Policy
Cannot defer when hidden terminals. Exponential backoff. When loss rate in ACKs > threshold.
u
v y
xDo not transmit to v when x anyone.
u must hear x.
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Roadmap CMAP Design
• Conflict maps
• ACK & Backoff Policy
Implementation
Evaluation
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Implementation Challenges1. At receiver: Identify colliding senders.
2. At sender: Identify ongoing transmissions.
PHYMAC
Trailer
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Implementation Options Software radios – Partial Packet Recovery.
[Jamieson and Balakrishnan, SIGCOMM 2007]
Commodity hardware – separate header and trailer packets.
Header pkt Trailer pkt
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Prototype Implementation
MadWifi DriverAtheros 802.11 card
Conflict Maps ACKs & Backoff
CSMA, ACKs & Backoff disabled.
Click Kernel Module
PHYMAC
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Roadmap CMAP Design
• Conflict maps
• ACK & Backoff Policy
Implementation
Evaluation
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Evaluation
50-node 802.11a indoor testbed. Does CMAP improve throughput by
increasing concurrency?
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Two Senders In Range Senders in range. 1400-byte UDP @ 6 Mbps. 50 unique sets of four nodes. CMAP, CSMA, no CS no acks.
Exposed terminals. Interfering transmissions.
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Two Senders In Range
0
0.2
0.4
0.6
0.8
1
0 2 4 6 8 10
Aggregate Throughput (Mbits/s)
CD
F
CSMA No CS, No Acks
CSMA better.
No CS better.
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Two Senders In Range
0
0.2
0.4
0.6
0.8
1
0 2 4 6 8 10
Aggregate Throughput (Mbits/s)
CD
F
CSMA No CS, No Acks Ideal
Ideal is max of CSMA & No CS
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Two Senders In Range
0
0.2
0.4
0.6
0.8
1
0 2 4 6 8 10
Aggregate Throughput (Mbits/s)
CD
F
Ideal CMAP
CMAP traces ideal curve.
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Multiple Concurrent Senders
AP-client networks. Tree-based mesh networks.
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Multiple Concurrent Senders
0
2
4
6
8
10
12
14
16
18
3 4 5 6
Number of concurrent senders
Agg
rega
te th
roug
hput
(Mbi
ts/s
)
CSMACMAP
AP-client – 20-47% better over CSMA.
Mesh – 52% better over CSMA.
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Hidden terminals – backoff ensures CMAP similar to CSMA.
CMAP without windowed ACKs gets only half the gains → windowed ACKs useful.
CMAP’s gains hold across multiple bit-rates.
More results in the paper
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Related Work
RTS but no CTS → exposed. [Karn, Shukla et al.]
Offline training to identify exposed terminals. [Mittal and Belding]
u
yz
xRTS
CTS
X
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Limitations Losses until conflict map entries populated.
Unequal pkt sizes → longer to detect
conflicts.
Cannot detect conflicts when interfering
node’s headers cannot be decoded.
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Contributions MAC to improve throughput by increasing
concurrency. Key idea: watch and discover conflicts. Experiments show increased throughput.
• 2x improvement over CSMA with exposed terminals.
• ~50% improvement in AP and mesh networks.