Wireless & Mobile Networking CS 752/852 - Spring 2011 Tamer Nadeem Dept. of Computer Science Lec #7: Medium Access Control – V Data Rate Control Slides.

Wireless & Mobile Networking

CS 752/852 - Spring 2011

Tamer NadeemDept. of Computer Science

Lec #7: Medium Access Control – VData Rate Control

Slides adapted from Romit Roy Choudhury

(Duke)

Page 2 Spring 2011 CS 752/852 - Wireless and Mobile Networking

What is Data Rate ?

Number of bits that you transmit per unit time

under a fixed energy budget

Too many bits/s:

Each bit has little energy -> Hi BER

Too few bits/s:

Less BER but lower throughput


Some Basics

Floor NoiseData Rate

Received Power

Channel Bandwidth

• Bit error (p) for BPSK and QPSK :

SNR

• Friss’ Equation:

Varyingwith timeand space

How do we choose the rate of modulation


802.11b – Transmission rates


Static Rates

SINR

time

# Estimate a value of SINR # Then choose a corresponding rate that would transmit packets correctly most of the times# Failure in some cases of fading live with it


Adaptive Rate-Control

SINR

time

# Observe the current value of SINR# Believe that current value is indicator of near-future value# Choose corresponding rate of modulation# Observe next value# Control rate if channel conditions have changed


Is there a tradeoff ?

C EA

D

B Rate = 10


Is there a tradeoff ?

C EA

D

B

Rate = 20

Rate = 10

What about length of routes due to smaller range ?What about length of routes due to smaller range ?


Any other tradeoff ?

Will carrier sense range vary with rate


Total interference

C EA

D

B

Rate = 20

Rate = 10

Carrier sensing estimates energy in the channel.Does not vary with transmission rate

Carrier sensing estimates energy in the channel.Does not vary with transmission rate


Bigger Picture

• Rate control has variety of implications

• Any single MAC protocol solves part of the puzzle

• Important to understand e2e implications

• Does routing protocols get affected?

• Does TCP get affected?

• …

• Good to make a start at the MAC layer

• RBAR

• OAR

• Opportunistic Rate Control

• …

© 2001. Gavin Holland

A Rate-Adaptive MAC Protocol forMulti-Hop Wireless Networks

Gavin Holland

HRL Labs

Nitin Vaidya Paramvir Bahl

UIUC Microsoft Research

MOBICOM’01 Rome, Italy


Background

• Current WLAN hardware supports multiple data rates

• 802.11b – 1 to 11 Mbps

• 802.11a – 6 to 54 Mbps

• Data rate determined by the modulation scheme


Modulation schemes have different error characteristics

Problem

BE

R

SNR (dB)

1 Mbps

8 Mbps

But, SINR itself variesWith Space and TimeBut, SINR itself variesWith Space and Time


Impact

Large-scale variation with distance (Path loss)

SN

R (

dB

)

Distance (m) Distance (m)

Mea

n T

hro

ug

hp

ut

(Kb

ps)

Path Loss

1 Mbps

8 Mbps


Impact

Small-scale variation with time (Fading)

SN

R (

dB

)

Time (ms)

Rayleigh Fading

2.4 GHz2 m/s LOS


Question

Which modulation scheme to choose?

SN

R (

dB

)

SN

R (

dB

)

Time (ms)Distance (m)

2.4 GHz2 m/s LOS


Answer Rate Adaptation

• Dynamically choose the best modulation scheme for the channel conditions

Mea

n T

hro

ug

hp

ut

(Kb

ps)

Distance (m)

DesiredResult


Design Issues

• How frequently must rate adaptation occur?

• Signal can vary rapidly depending on:

• carrier frequency

• node speed

• interference

• etc.

• For conventional hardware at pedestrian speeds, rate adaptation is feasible on a per-packet basis

Coherence time of channel higher than transmission time


• Cellular networks

• Adaptation at the physical layer

• Impractical for 802.11 in WLANs

• For WLANs, rate adaptation best handled at MAC

Adaptation At Which Layer ?

D

C

BACTS: 8

RTS: 10

10

8Sender Receiver

RTS/CTS requires that the rate be known in advanceRTS/CTS requires that the rate be known in advance

Why?


Who should select the data rate?

• Collision is at the receiver

• Channel conditions are only known at the receiver

• SS, interference, noise, BER, etc.

• The receiver is best positioned to select data rate

A

B


Previous Work

• PRNet

• Periodic broadcasts of link quality tables

• Pursley and Wilkins

• RTS/CTS feedback for power adaptation

• ACK/NACK feedback for rate adaptation

• Lucent WaveLAN “Autorate Fallback” (ARF)

• Uses lost ACKs as link quality indicator


Lucent WaveLAN “Autorate Fallback” (ARF)

• Sender decreases rate after

• N consecutive ACKS are lost

• Sender increases rate after

• Y consecutive ACKS are received or

• T secs have elapsed since last attempt

BADATA2 Mbps

2 MbpsEffective Range

1 MbpsEffective Range


Performance of ARF

Time (s)Ra

te (

Mbp

s)S

NR

(d

B)

Time (s)

– Slow to adapt to channel conditions

– Choice of N, Y, T may not be best for all situations

Attempted to IncreaseRate During Fade

Dropped Packets

Failed to IncreaseRate After Fade


RBAR Approach

• Move the rate adaptation mechanism to the receiver

• Better channel quality information = better rate selection

• Utilize the RTS/CTS exchange to:

• Provide the receiver with a signal to sample (RTS)

• Carry feedback (data rate) to the sender (CTS)


• RTS carries sender’s estimate of best rate

• CTS carries receiver’s selection of the best rate

• Nodes that hear RTS/CTS calculate reservation

• If rates differ, special subheader in DATA packet updates nodes that overheard RTS

Receiver-Based Autorate (RBAR) Protocol

C

BA CTS (1)

RTS (2)

2 Mbps

1 Mbps

D

1 MbpsDATA (1)

2 Mbps

1 Mbps


Performance of RBAR

Time (s)

SN

R (

dB

)

Time (s)

Ra

te (

Mbp

s)R

ate

(M

b ps)

Time (s)

RBAR

ARF


Question to the class

• There are two types of fading

• Short term fading

• Long term fading

• Under which fading is RBAR better than ARF ?

• Under which fading is RBAR comparable to ARF ?

• Think of some case when RBAR may be worse than ARF


Implementation into 802.11



PLCP Header



• Encode data rate and packet length in duration field of frames

• Rate can be changed by receiver

• Length can be used to select rate

• Reservations are calculated using encoded rate and length

• New DATA frame type with Reservation Subheader (RSH)

• Reservation fields protected by additional frame check sequence

• RSH is sent at same rate as RTS/CTS

• New frame is only needed when receiver suggests rate change

FrameControl

Duration DA SA BSSIDSequence

Control Body FCSFCS

Reservation Subheader (RSH)

WHY


• Ns-2 with mobile ad hoc networking extensions

• Rayleigh fading

• Scenarios: single-hop, multi-hop

• Protocols: RBAR and ARF

• RBAR

• Channel quality prediction:

• SNR sample of RTS

• Rate selection:

• Threshold-based

• Sender estimated rate:

• Static (1 Mbps)

Performance Analysis

BE

R

SNR (dB)

1E-5

2 MbpsThreshold

8 MbpsThreshold


Performance Results

Single-Hop Network


Single-Hop Scenario

A B

Mea

n T

hro

ug

hp

ut

(Kb

ps)

Distance (m)


CBR sourcePacket Size = 1460

Varying Node SpeedUDP Performance

• RBAR performs best

• Declining improvement with increase in speed

• Adaptation schemes over fixed

• RBAR over ARF

• Some higher fixed rates perform worse than lower fixed rates

Mea

n T

hro

ug

hp

ut

(Kb

ps)

Mean Node Speed (m/s)

RBAR

ARF

WHY?


Varying Node SpeedTCP Performance

• RBAR again performs best

• Overall lower throughput and sharper decline than with UDP

• Caused by TCP’s sensitivity to packet loss

• More higher fixed rates perform worse than lower fixed rates

FTP sourcePacket size = 1460

Mea

n T

hro

ug

hp

ut

(Kb

ps)

Mean Node Speed (m/s)

RBAR

ARF


No MobilityUDP Performance

• RSH overhead seen at high data rates

• Can be reduced using some initial rate estimation algorithm

• Limitations of simple threshold-based rate selection seen

• Generally, still better than ARF

Distance (m) Distance (m)

CBR sourcePacket size = 1460

RBARARF

Mea

n T

hro

ug

hp

ut

(Kb

ps)

Mea

n T

hro

ug

hp

ut

(Kb

ps)

WHY?


No MobilityUDP Performance

• RBAR-P – RBAR using a simple initial rate estimation algorithm

• Previous rate used as estimated rate in RTS

• Better high-rate performance

• Other initial rate estimation and rate selection algorithms are a topic of future work

Distance (m)

CBR sourcePacket size = 1460

RBAR-P

Mea

n T

hro

ug

hp

ut

(Kb

ps)

Why useful ?


RBAR Summary

• Modulation schemes have different error characteristics

• Significant performance improvement may be achieved by MAC-level adaptive modulation

• Receiver-based schemes may perform best

• Proposed Receiver-Based Auto-Rate (RBAR) protocol

• Implementation into 802.11

• Future work

• RBAR without use of RTS/CTS

• RBAR based on the size of packets

• Routing protocols for networks with variable rate links


Questions?

Three scenarios:1)Both nodes use 11 Mb/s2)Both nodes use 1 Mb/s3)n1 uses 11 Mb/s, n2 uses 1 Mb/s

Fast user’s throughput is severely degraded whereas

Slow user achieves throughput even larger than it expects

Multi-rate Anomaly

Multi-rate Anomaly [Heusse03]

OAR: An Opportunistic Auto-Rate Media Access Protocol for Ad Hoc Networks

B. Sadeghi, V. Kanodia, A. Sabharwal, E. Knightly

Rice University

Slides adapted from Shawn Smith


Motivation

• Consider the situation below

• ARF?

• RBAR?

AB C


Motivation

• What if A and B are both at 56Mbps, and C is often at 2Mbps?

• Slowest node gets the most absolute time on channel?

AB C

A

BC

Timeshare

Throughput Fairness vs Temporal Fairness


Opportunistic Scheduling

Goal

• Exploit short-time-scale channel quality variations to increase throughput.

Issue

• Maintaining temporal fairness (time share) of each node.

Challenge

• Channel info available only upon transmission


Opportunistic Auto-Rate (OAR)

• In multihop networks, there is intrinsic diversity

• Exploiting this diversity can offer benefits

• Transmit more when channel quality great

• Else, free the channel quickly

• RBAR does not exploit this diversity

• It optimizes per-link throughput


OAR Idea

• Basic Idea

• If bad channel, transmit minimum number of packets

• If good channel, transmit as much as possible

A BCD

A

C

Data Data Data Data

Data Data Data Data


Why is OAR any better ?

• 802.11 alternates between transmitters A and C

• Why is that bad

A BCD

A

C

Data

Data

Data

Data

Data

Data

Data

Data

Is this diagram correct ?


Why is OAR any better ?

• Bad channel reduces SINR increases transmit time

• Fewer packets can be delivered

A BCD

A

C

Data

Data

Data

Data

Data

Data


OAR Protocol Steps

• Transmitter estimates current channel

• Can use estimation algorithms

• Can use RBAR, etc.

• If channel better than base rate (2 Mbps)

• Transmit proportionally more packets

• E.g., if channel can support 11 Mbps, transmit (11/2 ~ 5) pkts

• OAR upholds temporal fairness

• Each node gets same duration to transmit

• Sacrifices throughput fairness the network gains !!


MAC Access Delay Simulation

• Back to back packets in OAR decrease the average access delay

• Increase variance in time to access channel

Why?


OAR Protocol

• Rates in IEEE 802.11b: 2, 5.5, and 11 Mbps

• Number of packets transmitted by OAR ~ Rate Base

RateTx

Pkts Rate Pkts Rate Pkts Rate

802.11 1 2 1 2 1 2

802.11b 1 2 1 5.5 1 11

OAR 1 2 3 5.5 5 11

Protocol

Channel Condition

BAD MEDIUM GOOD


Simulations

• Three Simulation experiments

1. Fully connected networks: all nodes in radio range of each other

• Number of Nodes, channel condition, mobility, node location

2. Asymmetric topology

3. Random topologies

• Implemented OAR and RBAR in ns-2 with extension of Ricean fading model [Punnoose et al ‘00]


Fully Connected Setup

• Every node can communicate with everyone

• Each node’s traffic is at a constant rate and continuously backlogged

• Channel quality is varied dynamically


Fully Connected Throughput Results

• OAR has 42% to 56% gain over RBAR

• Increase in gain as number of flows increases

• Note that both RBAR and OAR are significantly better than standard 802.11 (230% and 398% respectively)


Asymmetric Topology Results

• OAR maintains time shares of IEEE 802.11

• Significant gain over RBAR


OAR thoughts

• OAR does not offer benefits when

• OAR may not be suitable for applications like

• With TCP how can OAR get affected ?


OAR thoughts

• OAR does not offer benefits when

• Neighboring nodes do not experience diverse channel conditions

• Coherence time is shorter than N packets

• With TCP can OAR get affected ?

• Back-to-back packets can increase TCP performance

• However, bottleneck bandwidth can get congested quick

• Also, variance of RTT can increase

Multi-user Data Rate Adjustment Algorithm for Enterprise Networks

Nazif Tas, Tamer Nadeem, Ashok AgrawalaINFOCOM 2011

Previous Work

Data Rate Adjustment MechanismsAgreement between sender and the transmitter on rate to use.

Sender-based: statistical.Easy, resource efficient.ARF, AARF, CARA, AMRR…

Receiver-based: receiver chooses data rate and notifies the sender.More accurate, unnecessary resource usage.RBAR

Multi-rate Anomaly SolutionsInfrastructure-based solutions

AP selection, traffic scheduling, etc [Wang, Ji, Im04, …]

MAC Layer modifications Packet bursting, minimum CW adjustment, etc [Mai09, Yah-hong07, …]

Higher Layer adjustments TCP window adjustment, traffic slow down [Yoo08, Kashibuchi09, …]

Requires holistic view of the system

Requires modifications in the standards

Can we lessen the effects of Multi-Rate anomaly efficiently in a distributed & seamless manner with no modifications in the standards?

Requires non-seamless cross layer support

• IEEE 802.11 DCF

• CSMA/CA: carrier sensing

• Binary Exponential Backoff

Binary Exponential Backoff (BEB)

IEEE 802.11 BEB

Fairness

[Tan04]

: throughput seen by data rate d users in a network with ns number of slow users and nf number of fast users.

FastSlow

Multi-rate Anomaly

Baseline Fairness Criteria

Bianchi Model with Retry Limit

For data rate di,

k: retry limit

Can we use arbitrary limits to mitigate Multi-rate Anomaly?

Our extension supports arbitrary retry limits per user groups

Multi-user Data Rate Adjustment Algorithm (MORAL)

Automatically adjust the retry limits of each user such that•Improve fairness: Take the fair throughput share without hurting others.

•Distributed computation: No centralized decision making.

•Standard Coherence: No packet modification, restructuring or protocol alteration.

“The first thing to note is that [happiness] is a relative quality. We experience it differently according to our circumstances. What makes one person glad may be a source of suffering to another”

Two step operation:1.Collect information about the overall network.2.Adjust the retry limit accordingly

MAC

Observe

Decide

Act

LALA

MORAL Details:Heuristic Principles

After each transmission cycle, we have two pieces of information:

ccurrent: number of transmissions this cycle for the current nodeSuccessful transmission (ccurrent =1)Failed transmission(ccurrent =0)

ccalculated: Fair number of transmissions for the current node

Experiments I

Default MORAL

11, 1 Mb/s – 20 Users Each

1 Mb/s

11 Mb/s

60%

61.3%

Experiments II

11, 5.5, 2, 1 Mb/s– 10 Users Each

Default MORAL

31.8%

43.0%

Experiments III

93.8%

30 1 Mb/s, 10 11 Mb/s

1 Mb/s

11 Mb/s

1 Mb/s

11 Mb/s

Default

MORAL

20.8%

48.0%

Experiments IV

11, 1 Mb/s – 20 Users Each

23.9%

40.2%

Summary

MORAL is an effective MAC layer link adaptation mechanism which lessens the effects of multi-rate anomaly and promises

- Better fairness- Increased throughput

MORAL is- Fully compatible with the current standards- Totally distributed- Highly adaptable- Easily deployable in any IEEE 802.11 compliant devices


Other Ideas

(Briefly)


Exploiting Diversity in Rate Adaptation

• Yet another idea exploits multiple user diversity

• Among many intermediate nodes, who has best channel

• Use that node as forwarding node

• Forwarding node can change with time

• Due to channel fluctuations at different time and space

WLAN

Channel ConditionsSNR

TIMEUSERS

AP


The Protocol Overview

SIFS SIFS SIFS SIFSDIFS

GRTS

ACK 0~2

CTS 1

CTS k

CTS 2

…

sender

user 1

user 2

user k

DATA 0 DATA 1 DATA 2SF

• MAD using Packet Concatenation (PAC)

Since at least one intermediate node is likely to have good channelcondition, transmitter can transmit at a high data rate or concatenateMultiple packets

• Choosing subset of neighbor-group is important• Coherence time of channel must be greater than packet chain• Group needs to really have independent channel gain

• Correlated channel gains can lead to performance hit.


What lies ahead ?

• Routing based on rate-control

• Choosing routes that contain high-rate links

• ETX metric proposed from MIT accomodates link character

• Opportunistic routing from MIT again – takes neighbor diversity into account (best paper Sigcomm 2005)

• Fertile area for a project …

• Dual of rate-control is power control

• One might be better than the other

• Decision often depends on the scenario open problem

• Directional antennas for DD link for data/ack

• Rate control can be introduced Not been studied yet

… many many more


Questions ?

Wireless & Mobile Networking CS 752/852 - Spring 2011 Tamer Nadeem Dept. of Computer Science Lec #7: Medium Access Control – V Data Rate Control Slides.

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