CMAP: Harnessing Exposed Terminals in Wireless Networks

1
CMAP Harnessing Exposed Terminalsin Wireless
Networks

• Mythili Vutukuru
• Joint work with Kyle Jamieson and Hari
  Balakrishnan

2
The Problem
X

• Which transmissions concurrently?
• Increase throughput by maximizing concurrency.

3
Todays Solution CSMA
Energy gt carrier sense threshold

• Exposed terminal problem.

4
Key Insight

• Existing solutions rules to predict which
  concurrent transmissions increase throughput.
• Instead, watch and discover which concurrent
  transmissions increase throughput.

5
CMAP Conflict Map

• Map of conflicting transmissions based on
  empirical evidence.
• Built in distributed, online manner.
• Exposed terminals 2x gain on CSMA.

u ? v x ? y NO!
6
Roadmap

• CMAP Design
• Conflict maps
• ACK Backoff Policy
• Implementation
• Evaluation

7
What is a conflict?
u
x
No Conflict.
Conflict.
y
50 loss

• Conflict throughput lower when concurrent.
• Loss rate threshold to decide conflicts.

8
Discovering Conflicts
u
q
p
x
When u transmits to me, x causes interference.
y

• Loss rate of u?v when x is concurrent
• gt50 then infer conflict at v.
• Conflict entries timed out periodically.

9
Populating the Conflict Map
Conflict map
Do not transmit to v when x ? anyone.
Do not transmit to anyone when u ? v.
u
x
z
y
When u transmits to me, x causes interference.
10
Channel Access Decisions

• Nodes always overhear channel.
• Consult conflict map before transmission.
• Carrier sense always disabled!

11
Roadmap

• CMAP Design
• Conflict maps
• ACK Backoff Policy
• Implementation
• Evaluation

12
Windowed ACKs
X

• Sliding window of packets at sender.

13
Backoff Policy
u
x
Do not transmit to v when x ? anyone.
u must hear x.
y

• Cannot defer when hidden terminals.
• Exponential backoff.
• When loss rate in ACKs gt threshold.

14
Roadmap

• CMAP Design
• Conflict maps
• ACK Backoff Policy
• Implementation
• Evaluation

15
Implementation Challenges

• At receiver Identify colliding senders.
• At sender Identify ongoing transmissions.

Trailer
MAC
PHY
16
Implementation Options

• Software radios Partial Packet Recovery.
  Jamieson and Balakrishnan, SIGCOMM 2007
• Commodity hardware separate header and trailer
  packets.

Header pkt
Trailer pkt
17
Prototype Implementation
Click Kernel Module
Conflict Maps
ACKs Backoff
MAC
PHY
CSMA, ACKs Backoff disabled.
MadWifi Driver Atheros 802.11 card
18
Roadmap

• CMAP Design
• Conflict maps
• ACK Backoff Policy
• Implementation
• Evaluation

19
Evaluation

• 50-node 802.11a indoor testbed.
• Does CMAP improve throughput by increasing
  concurrency?

20
Two Senders In Range

• Senders in range.
• 1400-byte UDP _at_ 6 Mbps.
• 50 unique sets of four nodes.
• CMAP, CSMA, no CS no acks.

• Exposed terminals.

• Interfering transmissions.

21
Two Senders In Range
No CS better.
CSMA better.
22
Two Senders In Range
Ideal is max of CSMA No CS
23
Two Senders In Range
CMAP traces ideal curve.
24
Multiple Concurrent Senders

• Tree-based mesh networks.

• AP-client networks.

25
Multiple Concurrent Senders
AP-client 20-47 better over CSMA.
Mesh 52 better over CSMA.
26
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.
• CMAPs gains hold across multiple bit-rates.

27
Related Work
RTS
X
CTS

• RTS but no CTS ? exposed. Karn, Shukla et al.
• Offline training to identify exposed terminals.
  Mittal and Belding

28
Limitations

• Losses until conflict map entries populated.
• Unequal pkt sizes ? longer to detect conflicts.
• Cannot detect conflicts when interfering nodes
  headers cannot be decoded.

29
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.