Transcript Presentation - Networks and Mobile Systems

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
Today’s wireless LAN
Backbone Network (wired)
AP1
…
APN-1
Client
APN
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

Can we use multiple transmission paths (APs)
to help reduce losses in a wireless LAN?
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802.11b Experiment Setup

A
B

Path A
Path B

~15m
R

…
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
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




Physical effects last for different durations


Obstacles  Attenuation
Surfaces  Multipath
Mobility  Rapid and deep fading
Losses are time-correlated (bursty)
Propagation environment is complex and
dynamic

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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
AP
Client
AP

Challenges:


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Architecture for fine-grained path selection
Algorithm for fine-grained path selection
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Divert architecture (Downlink)

DC
Wired Backbone

DM
AP
DM
… DM
AP
AP
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)


Wired Backbone

…
AP
AP
Divert Monitor (DM)
Divert Controller (DC):

AP

DM
DC
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
Monitors channel
condition
Performs path
selection on a
frame-by-frame basis
Performs link-layer
retransmissions
Uplink is optional
Client
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Conventional WLANs have high path switch
signaling overhead
 Single Radio Client

M1
Cell 3

Multiple Radio Client

AP1
Cell 1
M2
Must signal client to
switch frequency
Expands cell size
AP2
Cell 2
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Divert avoids signaling overhead by
deploying secondary access points (SAP)
M1
Cell 3
AP1
SAP provides alternate paths
 No signaling overhead
 Compatible with cellular
architecture
SAP1
Cell 1
SAP2
M2
AP2
SAP2
Cell 2
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Practical path switching heuristic

Selecting best path for each frame
transmission is difficult


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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:


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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


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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 @ 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
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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
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