Opportunistic Use of Client Repeaters
to Improve Performance of WLANs
Victor Bahl1, Ranveer Chandra1, Patrick P. C. Lee2, Vishal Misra2,
Jitendra Padhye1, Dan Rubenstein2, Yan Yu3
1Microsoft
Research
2Dept of Computer Science, Columbia University
3Google Inc.
Dec 12, 2008
Outline

Rate anomaly problem

SoftRepeater design

Fairness requirements

Experimental results

Conclusions
2
Rate Anomaly of 802.11
Rate anomaly is well-known in WiFi
802.11 networks


Low-rate stations degrade
throughput of high-rate stations
54Mbps
AP
B
Why does rate anomaly exist?




Stations reduce data rates when
signal strength is poor (auto-rate)
Low-rate stations’ packets consume
more airtime
802.11 arbitrates transmissions on
per-packet basis
High-rate stations receive limited
airtime  throughput degrades
18Mbps
A
30
Throughput (Mbps)

54Mbps
20
10
A
B
0
A, B near AP
A
B
A far from AP
3
Limitations of Prior Solutions


What’s new? Rate anomaly is well-known, with many solutions
proposed.
Assumptions of prior solutions:




Require dedicated hardware (e.g., Cisco Aironet 1200 series APs)
Change MAC layer (e.g., Lee et al., Infocom ’04; Liu et al., JSAC ’05)
Construct ad-hoc mesh networks (e.g., Draves et al., Mobicom ’04)
Drawbacks of prior solutions:



More cost for hardware change
Not compatible with widely deployed infrastructure networks
Inflexible – solutions cannot be activated on demand
4
Our Solution: SoftRepeater


SoftRepeater: A practical, deployable system that addresses rate
anomaly
Main idea:

High-rate station (repeater) relays traffic for low-rate station (client)
traffic for A and B
AP

Key features:



traffic for A
B
A
client
repeater
Repeater is opportunistic - activated only when both repeater and client
receive “beneficial” throughput
No changes to 802.11 MAC and AP
Deployable in infrastructure and adhoc networks
5
Design Issues

How can we detect existence of rate anomaly occurring?

How do we formally define “beneficial” throughput?

How do we support multiple interfaces on a wireless card?



We need managed mode for communication between AP and repeater
We need adhoc mode for communication between repeater and client
What fractions of time should we give to managed/adhoc modes to
ensure “beneficial throughput”?
traffic for A and B
AP
traffic for A
B
repeater
A
client
6
Our Contribution



Propose a handshaking protocol for detecting rate anomaly and
reaching consensus on using SoftRepeater
Formalize a set of utility maximization problems for different
fairness requirements
Implement SoftRepeater on Windows XP; conduct extensive
testbed experiments and QualNet simulations
7
SoftRepeater Architecture

Built on VirtualWifi – allowing two virtual
interfaces for a wireless card:






Primary Virtual Interface – communication
between AP and repeater in managed mode
Repeater Virtual Interface – communication
between repeater and client in adhoc mode
Repeater Virtual Interface activated only
when beneficial to both repeater and client
Alternate between primary and repeater
interfaces with switching overhead < 40ms
Optional Network Coding Engine that
further boosts throughput, with slight
modifications to AP
Multiple radios can be supported (not in our
current experiments)
8
Detecting Rate Anomaly

Goal: Determine When SoftRepeater is beneficial

Key steps:

Collect information from nearby stations in promiscuous mode:






Number of packets transmitted
Average size of packets
RSSI
Data transmission rate
BSSID
Check utilization of medium. If neighbors send about the same number
of packets, but at a low rate, rate anomaly may exist.
9
Repeater Utility Function

Goal: capture throughput gain of both repeater and client

Define α: fraction of time spent in managed mode
AP

repeater
client
Assumptions:

Stations always have backlogged data to send (i.e., saturated case)



Implying equal channel access
Good approximation for file-transfer applications
Zero switching overhead



B
A
1 - α = fraction of time spent in adhoc mode
Can easily account for non-zero switching overhead
Intuition: if utility improved for both repeater and client, activate
SoftRepeater
10
Repeater Utility Function




B’s throughput: TB
A’s throughput: TA
AP
With SoftRepeater:


A
TA,B
TB
client
B
repeater
B’s Throughput: αTB/ 2
A’s throughput: min(αTB/ 2 , (1- α)TA,B)


TA
Without SoftRepeater:
TA,B = inferred throughput between A and B from RSSI measurement
If max-min fairness is used, repeater utility function becomes
T* = maxα min{αTB/ 2, min(αTB/ 2 , (1- α)TA,B)}

If T* > TA and T* > TB (better for both) activate SoftRepeater
11
Generalizing Repeater Utility Function

For different objectives:




For different settings:





Maximizing total throughput: starve client (bad)
Max-min fairness
Proportional fairness
In presence of interfering nodes
In presence of multiple clients
Multiple radios
Multiple wireless cards
Details in paper and tech report
12
Repeater Initiation Protocol

Goal: confirm and reach consensus on activating SoftRepeater

For now: simple 4-way handshake:




B broadcasts SoftRepeater offer
A infers data rate from A to B (from RSSI) and unicasts response
B picks clients to serve (if utility improved) and broadcasts final “Take
it or leave it” offer
A unicasts accept/reject
2. unicast response
4. unicast accept/reject
AP
1. broadcast offer
3. broadcast new offer
A
client
B
repeater
13
Testbed Experiments

SoftRepeater is implemented on Windows XP

Testbed experiments in office building



AP located at X
Repeater (node R) fixed at Y
Client (node C) moved
between Y, T, Z

Use 802.11a, with auto-rate feature enabled

Focus on Max-Min fairness
14
Experiment 1: Downlink UDP
rate anomaly scenario:
AP

R
UDP throughput improved by 200% with SoftRepeater
when rate anomaly exists
C
15
Experiment 2: Downlink TCP
rate anomaly scenario:
AP

R
C
TCP throughput improved by 50% with SoftRepeater when
rate anomaly exists, even communication alternates
between managed and adhoc modes
16
Experiment 3: UDP with 2 clients
rate anomaly scenario:
AP

R
UDP throughput improved with SoftRepeater when two
clients served
C1 C2
17
Qualnet Simulation: Effectiveness of
Repeater Initiation Protocol




AP in office 0
Client in office 9
Downlink UDP for both
repeater and client
SoftRepeater activated only when there is throughput gain
18
Qualnet Simulation: Multiple Clients



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AP in office 0
Repeater in office 3
N clients in office 9
Downlink UDP
SoftRepeater improves the baseline throughput by more than 65%.
19
Summary of Experimental Results


Main observation: throughput significantly improved for UDP/TCP
flows when rate anomaly exists
More experiments in paper/tech. report




Correctness of repeater initiation protocol
Extension with network coding
Various traffic scenarios
Qualnet simulation for more “complicated” scenarios (e.g., interfering
nodes, multiple repeaters/clients)
20
Conclusions



Propose SoftRepeater, a practical, deployable system that
addresses rate anomaly problem
Formulate different utility maximization problems for
SoftRepeater
Implement a prototype that demonstrates the improvement of
SoftRepeater
21
Questions: [email protected]
22
Security Issues

Security concerns:

Privacy


Greedy/malicious repeaters


End-to-end encryption (e.g., IPsec) can be used
Client monitors channel; quits if performance becomes worse after
SoftRepeater is used
Conclusion: Security is no worse than SoftRepeater-free networks
23