CMAP: Harnessing exposed terminals in Wireless Networks
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Transcript CMAP: Harnessing exposed terminals in Wireless Networks
CMAP:
Harnessing Exposed Terminals
in Wireless Networks
Mythili Vutukuru
Joint work with Kyle Jamieson and Hari Balakrishnan
The Problem
x
u
z
X
v
y
Which transmissions concurrently?
Increase throughput by maximizing
concurrency.
2
Today’s Solution: CSMA
u
x
z
v
y
Energy > carrier
sense threshold
Exposed terminal problem.
3
Key Insight
Existing solutions: rules to predict which
concurrent transmissions increase throughput.
Instead, watch and discover which concurrent
transmissions increase throughput.
4
CMAP: Conflict Map
Map of conflicting transmissions based on
empirical evidence.
Built in distributed, online manner.
u
x
u v & x y NO!
z
v
y
Exposed terminals – 2x gain on CSMA.
5
Roadmap
CMAP Design
• Conflict maps
• ACK & Backoff Policy
Implementation
Evaluation
6
What is a “conflict”?
u
x
v
NoConflict.
Conflict.
y
50% loss
Conflict – throughput lower when concurrent.
Loss rate threshold to decide conflicts.
7
Discovering Conflicts
q
p
u
When u transmits to me,
x causes interference.
x
v
y
Loss rate of uv when x is concurrent
>50% then infer conflict at v.
Conflict entries timed out periodically.
8
Populating the Conflict Map
Conflict map
Do not transmit to v
when x anyone.
Do not transmit to anyone
when u v.
u
z
x
v
y
When u transmits to me,
x causes interference.
9
Channel Access Decisions
Nodes always overhear channel.
Consult conflict map before transmission.
Carrier sense always disabled!
10
Roadmap
CMAP Design
• Conflict maps
• ACK & Backoff Policy
Implementation
Evaluation
11
Windowed ACKs
u
z
X
x
y
Sliding window of packets at sender.
12
Backoff Policy
Do not transmit to v
when x anyone.
u must hear x.
u
x
v
y
Cannot defer when hidden terminals.
Exponential backoff.
When loss rate in ACKs > threshold.
13
Roadmap
CMAP Design
• Conflict maps
• ACK & Backoff Policy
Implementation
Evaluation
14
Implementation Challenges
1. At receiver: Identify colliding senders.
Trailer
2. At sender: Identify ongoing transmissions.
MAC
PHY
15
Implementation Options
Software radios – Partial Packet Recovery.
[Jamieson and Balakrishnan, SIGCOMM 2007]
Commodity hardware – separate header and
trailer packets.
Header pkt
Trailer pkt
16
Prototype Implementation
Conflict Maps
ACKs & Backoff
MadWifi Driver
Atheros 802.11 card
Click Kernel
Module
MAC
PHY
CSMA, ACKs &
Backoff disabled.
17
Roadmap
CMAP Design
• Conflict maps
• ACK & Backoff Policy
Implementation
Evaluation
18
Evaluation
50-node 802.11a indoor testbed.
Does CMAP improve throughput by
increasing concurrency?
19
Two Senders In Range
Senders in range.
1400-byte UDP @ 6 Mbps.
50 unique sets of four nodes.
CMAP, CSMA, no CS no acks.
Interfering transmissions.
Exposed terminals.
20
Two Senders In Range
CSMA
No CS, No Acks
1
No CS better.
CDF
0.8
0.6
0.4
0.2
CSMA better.
0
0
2
4
6
Aggregate Throughput (Mbits/s)
8
10
21
Two Senders In Range
CSMA
No CS, No Acks
Ideal
1
CDF
0.8
0.6
0.4
Ideal is max of
CSMA & No CS
0.2
0
0
2
4
6
Aggregate Throughput (Mbits/s)
8
10
22
Two Senders In Range
Ideal
CMAP
1
CDF
0.8
0.6
0.4
CMAP traces ideal
curve.
0.2
0
0
2
4
6
Aggregate Throughput (Mbits/s)
8
10
23
Multiple Concurrent Senders
AP-client networks.
Tree-based mesh
networks.
24
Aggregate throughput (Mbits/s)
Multiple Concurrent Senders
18
16
14
12
10
8
CSMA
AP-client – 20-47% better over CSMA.
CMAP
6
Mesh – 52% better over CSMA.
4
2
0
3
4
5
Number of concurrent senders
6
25
More results in the paper
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.
26
Related Work
RTS
x
u
X
z
CTS
y
RTS but no CTS → exposed. [Karn, Shukla et al.]
Offline training to identify exposed terminals.
[Mittal and Belding]
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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.
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