Divert: Fine-grained Path Selection for Wireless LAN

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

6/9/2004 MobiSys 2004 2

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

Today’s wireless LAN

…AP1

Client

APN-1 APN

Backbone Network (wired)

Client

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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

Can we use multiple transmission paths (APs)to help reduce losses in a wireless LAN?

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802.11b Experiment Setup Transmitters alternate

broadcasts @ 11Mbps Receiver is moving

(2mx2m area) Collect trace of two

interleaved streams Examine loss behavior

Within the same path Across different paths

AB

R

Path APath B

~15m

…

Ai Bi Ai+1Bi+1

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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

AP

Client

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Divert architecture (Downlink)

DMAP

DC

AP…DM

AP

DM

Wired Backbone

Divert Monitor (DM) Monitors channel

condition Divert Controller (DC):

Performs path selection on a frame-by-frame basis

Performs link-layer retransmissions

Client

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Divert architecture (Uplink)

AP AP…

AP

Wired Backbone

DCDM

Client

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

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Conventional WLANs have high path switch signaling overhead

AP1

AP2

M1

M2Cell 1

Cell 2

Cell 3

Single Radio Client Must signal client to

switch frequency Multiple Radio Client

Expands cell size

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AP2M2

SAP1

SAP2

SAP2

AP1

M1

Divert avoids signaling overhead by deploying secondary access points (SAP)

Cell 1

Cell 2

Cell 3

SAP provides alternate paths No signaling overhead Compatible with cellular

architecture

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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

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

A – AP B – SAP R – Receiver locations

A

B

R1

R2 R3

~15 m

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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)

Legend:{ H, T }

Mobile R1

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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 athttp://nms.csail.mit.edu/divert