rice university origins and solutions of proportional unfairness in ieee 802.11 multi-hop wireless...

Rice University

Origins and Solutions of Proportional Unfairnessin IEEE 802.11 Multi-hop Wireless Networks

Jingpu Shi

PhD Thesis Defence

Department of Electrical and Computer Engineering

Rice University

November, 2007

November 2007 ECE Department, Rice UniversityJingpu Shi

2

Motivation

• Gaining popularity. e.g., mesh networks in Philadelphia, San Francisco, Corpus Christi ...

• IEEE 802.11 widely deployed as the MAC.

• However, IEEE 802.11 incurs unfairness problem.

• Multi-hop network: not all nodes hear each other's transmissions.


3

Thesis Objective

• To investigate unfairness in 802.11 multi-hop networks and propose solutions to improve fairness. Specifically, to

– demonstrate existence of unfairness.

– analyze the origins of unfairness.

– analytically model the origins of unfairness.

– propose solutions to improve fairness.

– evaluate the proposed solutions.


4

Fairness Background

• There are many definitions for fairness, we consider proportional fairness.• Proportional fairness.

– Proposed by Kelly in 1997.– Definition: A vector of rates x is proportionally fair if it is feasible and if for any other feasible vector

x*, the aggregate of proportional changes is zero or negative:

0*

r

rr

rx

xx

Example:

x2

x1

x2 + 3x1 = 0

p1

p2

p3

– It is a maximizer of the strictly concave log utility function: r

rx )log(


5

Outline

• Analysis and modelling of unfair MAC contention.

• Analysis and modelling of joint effect of MAC and congestion control on unfairness.

• Solutions to improve fairness.

• Conclusion.


6

Prior work

• [Bianchi 00] [Kumar 05 ] [Sharma 06] etc.• Single clique, same channel view

This work

• Different channel view,• Topological factors.

• [Bharghavan 94] [Kanodia 02] etc.some unfair scenarios, but not modelled.

• Analytically modelled.

Objective: understand and model unfair MAC contention in multi-hop 802.11 networks where nodes have incomplete channel information.

Unfair MAC Contention


7

SNR > CSthresold

Basic Two-flow Four-node Layout

A

ba

BAB

Ab

aB

ab

• Senders A, B receivers a, b.

• A sender must be connected to its respective receiver.

• Dotted line: two nodes are connected (SNR is above carrier sense threshold.)

• Nodes from one flow may hear nodes from the other flows.

• 12 possible combinations.


8

12 Possible Scenarios, 3 Categories

A

ba

BA

ba

BA

ba

BA

ba

BAb

A

ba

B

ab

A

ba

BAb

A

ba

BABA

ba

BAB

A

ba

B

ab

A

ba

BA

ba

B

ab

Ab

aB

A

ba

B AB AB AB

AB

Ab Ab

aB

aB aBaB

ab

ab ab

aB aB

Asymmetric Incomplete state (AIS)

Sender Connected

(SC)

Symmetric Incomplete State (SIS)


9

Analysis of AIS

• Assume there is no capture effect.• The two flows have different view of the channel. • And experience different collision probabilities.

t

A a

B b

R C DATA A R C DATA A R C DATA A R C DATA A

Bb's view: medium is idle.

R

Aa's view: medium is busy.

R R R


10

Modeling AIS

• The channel “private view” of a node:

… …Successful Idle Collisio

n

t

Busy state

• Modeled as a renewal-reward process.

Throughput (pkt/s) =P [event Ts occurs]

Average duration of event

• Compute probability of each state.

• Generality is validated in [Garetto Infocom 06].


11

Model Validation: Simulation Set Up

Simulator Ns2 Ver. 2.1b7a

Data rate 11Mbps

Basic rate 2Mbps

Data size Varying

SIFS, DIFS, EIFS 10,50,364 us

Time slot 20 us

PLCP length 192 bits @ 1 Mbps

(CWmin, CWmax) (31,1023)

Short, Long retry limit 7,4


12

Validation – Model vs. Simulation

0

200

400

600

800

1000

200 400 600 800 100012001400

Pa

cke

t Thr

oug

hpu

t (p

kt/s

)

Data Payload Size (bytes)

0

200

400

600

800

1000

200 400 600 800 100012001400

ns - Flow Bmodel - Flow B

ns - Flow Amodel - Flow A

With RTS/CTS Without RTS/CTS

ns - Flow Amodel - Flow A

ns - Flow Bmodel - Flow B


13

Validation – Model vs. Experiments

0

200

400

600

800

1000

Pa

cke

t Thr

oug

hpu

t (p

kt/s

)

0

200

400

600

800

1000

With RTS/CTS Without RTS/CTS

Flow B Flow A

model

TFA

Flow B Flow A

model

TFA

TFA network, SMC 2532-b 802.11b 200 mW power, 1500 Bytes, 11Mbps


14

Analysis of SIS

• Long-term fair.– Due to symmetric topology.

• Short-term unfair.– Once a flow loses, it is harder and harder for it to win back, until it resets its contention status.

A a

Bb


15

Modeling SIS

• The two flows closely coupled.

• Propose a two-dimensional Markov model with the state being { backoff stage of A, backoff stage of B }.

• Computing the transition probability is the key.

Two key steps:

– Simplifying assumption to remove memory property: replace uniform distribution with geometric distribution.

– Handle hidden terminals.


16

SIS: Model vs. Simulation

Case Throughput

(pkt/s)

Collision probability

Time scale of unfairness (ms)

RTS/CTS

7 stages

218

216

0.25

0.25

235

223

RTS/CTS

9 stages

229

230

0.11

0.09

982

1156

Basic access

4 stages

125

107

0.69

0.75

15

15

Basic access

7 stages

222

220

0.37

0.38

59

60

ns

model

System’s bi-stability:With large probability, the system is in one of the two stable states:(1) Flow Aa captures the channel.(2) Flow Bb captures the channel.


17

• Summary of analysis of unfair MAC contention.

– Systematically and analytically studied two-flow scenarios.

– Nodes can contend unfairly under IEEE 802.11 due to incomplete channel information.

– The AIS class incurs long-term unfairness.

– The SIS class incurs short-term unfairness.


18

• Objective: to understand what is the joint effect of TCP and MAC on fairness.

• Prior Work:– [Gerla Infocom 05, Sivakumar MobiHoc 03]

• TCP poor performance due to TCP's large congestion window.

• Solution: TCP should not inject more than what can be served. E.g. limit or fix TCP window to a small value.

• This thesis:– Will show that window flow control itself (even with very small window) on top

of 802.11 can lead to unfairness.– Analytically model the unfairness scenario.

TCP and MAC Joint Effect


19

Basic Scenario

A B GW

• Two upload TCP flows (Many variations discussed in the thesis).

• The basic topology.– (A,B) (B,GW) are in range, A and GW are hidden terminals to each other.– Important topology in mesh networks.

A B GW

Two-hop TCP

One-hop TCP

Abstraction, not necessarily a real line topology.


20

Outline of Basic Scenario Analysis

• Analyze the case with RTS/CTS on. – Same analysis applies to RTS/CTS off.

• Analyze the MAC behaviour first, then add window flow control.

• Mathematically model the scenario.

• Propose and evaluate a solution.


21

MAC Bi-stability

DATA Aggregate ACKA B GW

• GW is also contending.• Consider A's and GW's traffic first, add B's traffic later.• Variation of the SIS class.• Bi-stable state: either A transmits and GW is in high backoff, or GW transmits and A is in high backoff.• Success state and fail state.

CW=2CWmin CW=22CWmin

CW=2CWmin CW=CWminCW=22CWmin

CW=2kCWmin

CW=CWmin CW=CWmin CW=CWmin CW=CWmin

A's RTS

GW's RTS

A's RTS

GW's RTS


22

MAC Bi-stability

• Take B into consideration: B is in range of both A and GW, so B's packets interleave with A's and GW's packets.

• Either pair (A,B) transmit and GW is in high backoff, or pair (B,GW) transmit and A is in high backoff.

A B GW

TCP ACK

TCP DATA

A trafficGW traffic B traffic

Multiple packet burst (GW,B) Multiple packet burst (A,B)


23

Impact of TCP on Bi-stability

• Two nested transport loops, the inner and the outer loop.

GWBA

DATA

DATA

ACK

GWBA

ACK

DATA

DATA

Inner loopOuter loop

GWBAACK

DATA

• Transport loops change the duration of bi-stability.

• (A, B) burst, GW in fail state, the burst size is limited by:

– TCP window size.

• (GW, B) burst, A in fail state,self-sustaining loop:

– TCP ACK are generated.


24

Two-hop Node’s Severe MAC and TCP Penalty

A B GW

TCP DATA

TCP ACKCW=2CWmin

CW=22CWmin

CW=2CWmin CW=CWminCW=22CWmin

A's MAC penalty

A

B

A

Pkt loss Pkt loss Pkt loss Pkt loss

Timeout

Timeout Timeout

A's TCP penalty


25

Modelling the Basic Topology

• Only model sliding window mechanism– Will show sliding window along can induce unfairness.

• Assume geometric backoff counter distribution.

• Six-dimensional Markov Model– Eight channel states: 1 idle, 1 collision, 6 success transmission states.– Possible switching times: mini-slot boundaries.– System state:

{ Q1,Q2,Q3,Qg,Wa,Wg } where Qg = Q4+Q5, Wa and Wg are minimum contention window of A and GW, respectively.


26

Compute Transition Probability and Solve the Model

{ Q1,Q2,Q3,Qg,Wa,Wg } { Q1 -1,Q2+1,Q3,Qg,0,Wg }

The transition probability is :f

gba eee )1)(1( where e is the success probability f is the transmission duration of node A in time slots.

A example: when a success transmission state occurs on link 1:


27

Solution to Improve Fairness

• Unfairness is caused by collision between A and GW.

• To improve fairness – We need to decrease the steady state probability of those system states

where

Q1 > 0 and Qg > 0

– This can be done by increasing CWmin at node B.

A B GWQ1 Qg


28

Reduced Backlog at A and GW With Increased CWmin at B

A B GW

• Indeed, the probability that both A and GW are backlogged is reduced.

Default CWmin

Default CWmin

IncreaseCWmin

Q1Qg


29

Model Validation

• Validate the model in three levels:

• How well does it agree with ns to validate the geometric distribution assumption?

• Can the model capture the trend of throughput changes when CWmin at node B increases in real networks ?

• Can the solution motivated by the model improve fairness in real networks ?


30

Default CWmin

VaryingCWmin

Default CWmin

Model Vs. Simulation

• ns-2.28, 11M bps, RTS/CTS on, 1500 bytes, TCP window fixed.

• Model agrees with simulation results.

A B GW


31

Model Vs. Real Experiment

• Model captures the trend of throughput changes when CWmin increases.

• Motivates solution: to increase CWmin of GW’s one-hop neighbor.

• Mirror Mesh

• Linux Operating System (kernel 2.6).• Atheros wireless card (Madwifi

v.0.9.2 driver).• Data rate fixed to 11Mbps.• RTS/CTS on.


32

Two or More Branches

• Same analysis for basic scenario applies to two or more branches

– Two hop flow(s) contend unfairly with one-hop flow(s)

• Solution: to increase CWmin of GW’s one-hop neighbors

A B GW

TCP ACK

TCP DATA

C

TCP DATA

TCP ACK


33

Solution Validation: Mirror Mesh Setup

GWB

A


34

Validation For Basic Topology With RTS/CTS On

Increase CWmin

A B GW

CWmin at 1st hop nodes

• Severe unfairness with default CWmin, log utility = -3.8• Improved fairness with larger CWmin at 1st hop nodes, log

utility = -1.23• Maximum log utility = 3.2


35

Validation For Two Branches RTS/CTS ON

Increase CWmin Increase CWmin

B->GW

A->GW

C->GW

CWmin at 1st hop nodes

• Severe unfairness with default CWmin, log utility = -3.8.• Improved fairness with larger CWmin at 1st hop nodes, log

utility = -1.23.• Maximum log utility = 3.2.


36

1st hop CWmin = 128

Validation for Long Hop Chain RTS/CTS ON

2 1 GW34

Increase CWmin

• NS simulation.• Severe unfairness with default CWmin, log utility = -11.9763.• Improved fairness with larger CWmin, log utility = -6.1721.• Maximum log utility = 3.6931.


37

• Summary– Analysis for the basic topology.

• Identified the two-hop topology where unfairness manifests.

• Analyzed the joint effect of MAC and window flow control on unfairness

• Modelled the basic topology.

– Solution: minimum contention window policy.• Increase the CWmin of 1st hop node(s).


38

Utilizing multiple channels to improve fairness

Asynchronous Multi-channel Coordination Protocol (AMCP)


39

Motivation and Challenges

• Multiple channels add capacity to the network.

• Multiple channels provide opportunity to improve throughput of the flows that would receive little throughput due to unfair contention in single channel case.

• Challenges: – Single-channel unfair contention.– Multi-channel coordination problems.

• Multi-channel hidden terminal problem.

• Missing receiver problem.


40

AMCP General Description

• Asynchronous protocol, one radio per node.

• One common control channel, multiple data channels.

• Reserve common channel and data channel differently.

• Nodes only contend for channels clear of traffic.

• Self-learning channel hopping.


41

Related Work

• SSCH [ Bahl MobiCom04], MMAC [So MobiHoc 04]

• All existing protocols are designed to improve aggregate throughput.


42

Thesis Contributions

– Identified topologies where unfair contention manifests.

– Explained how MAC independently or jointly with window flow control induce unfairness, when nodes don't have complete channel information.

– Proposed analytical models to study unfairness in multi-hop networks.

• A model that allows different nodes to have different channel view.• A two-dimensional Markov model to capture the coupling between two flows.• A six-dimensional Markov model to capture the joint effect of MAC and

window flow control on fairness.

– Designed protocol solutions to improve fairness.• A simple minimum contention window policy for mesh networks.• AMCP to utilize multiple channels.


43

Publications

1. M. Garetto, J. Shi and E. Knightly, "Modeling Media Access in Embedded Two-Flow Topologies of Multi-hop Wireless Networks," in Proceedings of ACM MobiCom'05, Cologne, Germany, August 2005.

2. J. Shi, T. Salonidis and E. Knightly, "Starvation Mitigation Through Multi-Channel Coordination in CSMA-based Wireless Networks," in Proceedings of ACM MobiHoc'06, Florence, Italy, May 2006.

3. J. Shi, O. Gurewitz, V. Mancuso, J. Camp and E. Knightly, "Measurement and Modeling of the Origins of Starvation in Congestion Controlled Mesh Networks," Proceedings of Infocom'08, Phoenix, AZ, April, 2008.


44

Thanks !

rice university origins and solutions of proportional unfairness in ieee 802.11 multi-hop wireless...

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