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 uv 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]
27
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.
28
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.
29