Distributed Channel Management

1
Distributed Channel Management
in
Uncoordinated Wireless Environments
Arunesh Mishra Vivek Shrivastava
Dheeraj Agrawal Suman Banerjee
Samrat Ganguly
University of Wisconsin, Madison
Presented by T-S Kim
2
Channel Assignment in Hotspots

• Hotspots are uncoordinated and dense
• Unsatisfactory and Unpredictable performance
• Efficient use of 802.11 channels is important
• Channel Assignment Problem
• Mapping of channels to APs
• Can affect performance significantly

Hotspot locations in Manhattan
3
Take Home Message - 1
B
A
APs on same channel
D
C
3 channels, 4 APs

• Fairness in throughput among APs is important
  when assigning channels in hotspots
• Static channel assignment methods cause
  unfairness in hotspot environments

4
Take Home Message -2
APs on same channel

• Channel Hopping can improve the fairness
  properties of existing static channel assignment
  methods

5
Take Home Message - 3

• Through careful modeling of interference, it is
  possible to use partially overlapped channels
• Refer to our paper in Sigmetrics 06
• Partially Overlapped Channels Not Considered
  Harmful
• Channel Hopping can take advantage of partially
  overlapped channels

6
Talk Outline

• Fairness in Hotspots
• Channel Hopping improves fairness
• Designing MAXchop
• Evaluation

7
Proper Usage of 802.11 Channels

• Nodes share a single channel
• Throughput scales with the number of
  non-overlapping channels

3 Channels
Throughput
1 channel
3 channels
8
Channel Assignment for Hotspots

• Hotspots typically consist of single APs
• An AP and all its associated clients use one
  channel
• Interfering hotspots need to be on a different
  channel

9
Channel Assignment for Hotspots
Channel 6
Channel 1
Channel 11

• 2.4 GHz has 3 non-overlapping channels 1, 6 and
  11
• APs can select such channels by scanning
• Least Congested Channel Search

10
Channel Assignment for Hotspots
Channel 6
Channel 1
Channel 11
Channel ?

• What if there are more hotspots ?
• Typical in todays wireless landscape

11
Fairness is key for Hotspots

• Each hotspot wants to maximize throughput for its
  users

12
Fairness is key for Hotspots

• Operating in unlicensed band, no hotspot should
  have greater priority on the total bandwidth over
  another, irrespective of the number of clients.
• Providing proportional fairness in these
  environment will require additional coordination
  between APs/clients across different management
  domains, which is not too practical.

• Thus, a fair division of the wireless bandwidth
  is important
• Among APs (and not just users)

13
Static Methods are unfair !
2.4 GHz ISM Band
Ch 1
Ch 6
Ch 11

• Only 3 non-overlapping channels in 2.4 GHz band

14
Static Methods are unfair !

• Resulting interference graph is dense !

15
Static Methods are unfair !
APs on same channel

• Channel assignment essentially becomes graph
  coloring

16
Static Methods are unfair !
11
1
11
1
6
6
6
11
APs on same channel

• Even optimal centralized solutions are unfair

17
Talk Outline

• Fairness in Hotspots
• Channel Hopping improves fairness
• Designing MAXchop
• Evaluation

18
Channel Hopping improves Fairness
Time
Time slot

• Network as a whole cycles through multiple
  different channel assignments

19
Channel Hopping improves Fairness
Time

• No single AP suffers for long

20
Channel Hopping improves Fairness
Time

• Long term throughput of each AP gets averaged
  over multiple different channel assignments

21
Channel Hopping improves Fairness
Time

• The potential to improve fairness even over the
  best static channel assignments

22
Talk Outline

• Fairness in Hotspots
• Channel Hopping improves fairness
• Designing MAXchop
• Evaluation

23
Designing MAXchop
24
Designing MAXchop

• Beacon frames contain sequences of channels or
  hopping sequences
• Client-AP Channel switch is synchronized with
  beacons
• Hopping sequences evolve in a distributed manner
• Collectively the sequences converge for the
  network as a whole

25
Designing MAXchop - Practical Considerations

• Channel Switch overhead
• 20 ms for Prism cards
• 5 ms for Atheros
• 200 us for Intel (not tested)
• Packet loss during switch
• Hardware transmit is disabled during switch

Data
Ack
26
Talk Outline

• Fairness in Hotspots
• Channel Hopping improves fairness
• Designing MAXchop
• Evaluation

27
Simulation Parameters
San Francisco City

• Hotspot data for the city of San Francisco from
  wigle.net
• Partitioned into 12 topologies
• Two sets of simulations
• One sample topology for detailed analysis
• Statistical properties over the remaining
  topologies

Red dots indicate AP locations
28
Simulation Parameters

• NS-2 packet level simulations
• Augmented with support for bit-level errors
• Auto-rate fallback
• Power levels similar to commodity APs
• Algorithms
• Commodity algorithm Least Congested Channel
  Search
• Each AP scans and selects the channel that offers
  the least amount of congestion
• Channel Hopping MAXchop

29
Simulation Sample Topology

• Concentrated dense pockets of 27 APs
• Representative of hotspot interference in urban
  areas

30
Simulation Results Sample Topology
UDP Throughput
Least Congested Channel Search (LCCS)
MAXchop

• UDP Throughput, full throttle.
• 20 improvement in aggregate throughput

31
Simulation Results Statistical Properties

• Twelve topologies chosen from wigle.net
• Representative of the dense and variable
  deployment patterns

32
Simulation Results Statistical Properties

• UDP traffic, full throttle
• Randomized Compaction (RaC) Infocom 06
• Centralized, assumes full coordination and
  roaming
• Acts as an upper bound

33
Simulation Results Statistical Properties

• RaC optimizes for max-min fair throughput
• MAXchop does nearly as well as RaC

34
Experiments

• Implementation
• Standard Linux platform
• User-level daemon does channel switching
• Study performance of TCP/UDP traffic
• Experiment topology designed to mimic dense
  hotspots

35
Experiment Results UDP traffic
Non-overlapped channels
Partially overlapped channels
MAXchop
MAXchop
LCCS
Manual

• With partially overlapped channels, MAXchop
  improves both fairness and throughput
• See Partially Overlapped Channels Not Considered
  Harmful, Sigmetrics 06.

36
Key Insights

• Fairness is important for hotspots
• All static channel assignment schemes will be
  unfair
• Because of the dense nature of the hotspots
• Even optimally computed ones
• Channel hopping improves fairness
• Uses any existing channel assignment method
• Channel hopping can take advantage of partially
  overlapped channels

37
Your Questions

• Thank you for listening !