Divert: Fine-grained Path Selection for Wireless LAN

1
Divert Fine-grained Path Selection for Wireless
LAN

• Allen Miu, Godfrey Tan, Hari Balakrishnan, John
  Apostolopoulos
• MIT Computer Science and Artificial Intelligence
  Laboratory

Hewlett-Packard Laboratories
2
Todays wireless LAN
Backbone Network (wired)

AP1
APN-1
APN
Client
Client

• Clients communicates with one AP at a time
• Clients select AP based on long term signal
  quality

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Problems

• Communication suffer periods of high loss rate
• Link retransmission adds delay (1-100ms)
• Lowering link rate reduces bandwidth
• Impacts delay-sensitive, high-bandwidth apps
• e.g., VOIP, video-conferencing

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802.11b Experiment Setup

• Transmitters alternate broadcasts _at_ 11Mbps
• Receiver is moving (2mx2m area)
• Collect trace of two interleaved streams
• Examine loss behavior
• Within the same path
• Across different paths

A
B
Path A
Path B
15m
R

Ai
Bi
Ai1
Bi1
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Conditional frame loss probabilities in a mobile
environment
Losses are bursty within the same path
Losses have low loss correlation across
different paths
Lag after loss frame i
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Explanation of loss behavior

• Transmission depends on physical path
• Obstacles ? Attenuation
• Surfaces ? Multipath
• Mobility ? Rapid and deep fading
• Physical effects last for different durations
• Losses are time-correlated (bursty)
• Propagation environment is complex and dynamic
• Unlikely that all paths suffer simultaneously

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Use fine-grained path selection to reduce
transmission losses!

• Goal Switch communication quickly among nearby
  APs to avoid short-term channel outages (burst
  losses) in the current transmission path
• Challenges
• Architecture for fine-grained path selection
• Algorithm for fine-grained path selection

AP
Client
AP
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Divert architecture (Downlink)
DC

• Divert Monitor (DM)
• Monitors channel condition
• Divert Controller (DC)
• Performs path selection on a frame-by-frame
  basis
• Performs link-layer retransmissions

Wired Backbone
DM
DM

AP
AP
Client
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Divert architecture (Uplink)

• Divert Monitor (DM)
• Monitors channel condition
• Divert Controller (DC)
• Performs path selection on a frame-by-frame
  basis
• Performs link-layer retransmissions
• Uplink is optional

Wired Backbone

AP
AP
AP
DM
DC
Client
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Conventional WLANs have high path switch
signaling overhead

• Single Radio Client
• Must signal client to switch frequency
• Multiple Radio Client
• Expands cell size

M1
AP1
Cell 3
M2
AP2
Cell 1
Cell 2
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Divert avoids signaling overhead by deploying
secondary access points (SAP)

• SAP provides alternate paths
• No signaling overhead
• Compatible with cellular architecture

M1
AP1
Cell 3
SAP1
SAP2
M2
AP2
Cell 1
SAP2
Cell 2
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Practical path switching heuristic

• Selecting best path for each frame transmission
  is difficult
• Link conditions vary often and quickly
• Expensive to probe all transmission paths often

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Practical path switching heuristic

• Observation
• Losses are bursty
• Losses have low loss correlation across paths
• A simple heuristic is to
• Monitor channel quality in current path
• Switch path if current path has fallen to a bad
  state

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Practical path switching heuristic

• Per-client frame loss history window, H (frames)
• Switch paths when T frames lost within last H
  transmitted frames
• Can adapt H and T for
• Different channel conditions
• Relative loss rate differences among different
  transmission paths

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Testbed Setup
A AP B SAP R Receiver locations
15 m
A
R1
R2
R3
B

• Divert sends packet via A or B _at_ 11 Mbps link
  rate
• Load 1500 bytes _at_ 240pps for 5 min. (720,000
  frames)
• Disabled retransmissions
• Experiments with stationary and moving laptop
  receiver at R1, R2, R3

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Frame Loss Rates (at R3)
Legend H, T
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Burst Loss Length CCDF (at R3)
Mobile R3
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Frame Loss Rates (at R1)
Mobile R1
Legend H, T
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Burst Loss Length CCDF (at R1)
Mobile R1
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Related Work

• Physical layer spatial diversity techniques
• Diversity antenna/antenna arrays
• Distributed radio bridges for wireless LAN
  Leung 96

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Conclusion

• Fine-grained path selection can reduce frame loss
  and loss burstiness, especially for moving WLAN
  clients
• Design and implementation of a fine-grained path
  selection system on a wireless LAN (802.11b)

Code and traces will be made available soon
at http//nms.csail.mit.edu/divert