1

Rate Control in Wireless Networks

ECE 256

2

Recall 802.11

RTS/CTS + Large CS Zone Alleviates hidden terminals, but trades off spatial reuse

C

F

A B

E

D

CTS

RTS

3

Recall Role of TDMA

4

Recall Beamforming

Omni CommunicationOmni Communication Directional CommunicationDirectional Communication

a b c

d

Silenced NodeSilenced Node

a b c

d

e

f

Simultaneous CommunicationNo Simultaneous CommunicationOk

f

5

Also Multi-Channel

Current networks utilize non-overlapping channels Channels 1, 6, and 11

Partially overlapping channels can also be used

6

Today

Benefits from exploiting channel conditions– Rate adaptation– Pack more transmissions in same time

7

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

8

802.11b – Transmission rates

Optimal rate depends on SINR:i.e., interference and current channel conditions

Time

1 Mbps

2 Mbps

5.5 Mbps

11 Mbps

Highest energy per bit

Lowest energy per bit

9

Some Basics Friss’ Equation

Shannon’s Equation

Bit-energy-to-noise ratio

C = B * log2(1 + SINR)

Eb / N0 = SINR * (B/R)

Leads to BERVaryingwith timeand space

How do we choose the rate of modulation

10

Static Rates

SINR

time

# Estimate a value of SINR # Then choose a corresponding rate that would transmit packets correctly (i.e., E b / N 0 > thresh) most of the times# Failure in some cases of fading live with it

11

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

12

Is there a tradeoff ?

C EA

D

B Rate = 10

13

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 ?

14

Any other tradeoff ?

Will carrier sense range vary with rate

15

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

16

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 …

17

© 2001. Gavin Holland

A Rate-Adaptive MAC Protocol forMulti-Hop Wireless Networks

Gavin HollandHRL Labs

Nitin Vaidya Paramvir Bahl UIUC Microsoft Research

MOBICOM’01 Rome, Italy

18

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

19

Modulation schemes have different error characteristics

Problem

BER

SNR (dB)

1 Mbps

8 Mbps

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

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Impact

Large-scale variation with distance (Path loss)

SN

R (

dB

)

Distance (m) Distance (m)

Mean

Th

rou

gh

pu

t (K

bp

s)

Path Loss

1 Mbps

8 Mbps

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Impact

Small-scale variation with time (Fading)

SN

R (

dB

)

Time (ms)

Rayleigh Fading

2.4 GHz2 m/s LOS

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Question

Which modulation scheme to choose?

SN

R (

dB

)

SN

R (

dB

)

Time (ms)Distance (m)

2.4 GHz2 m/s LOS

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Answer Rate Adaptation

Dynamically choose the best modulation scheme for the channel conditionsM

ean

Th

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bp

s)

Distance (m)

DesiredResult

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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

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Cellular networks Adaptation at the physical layer

Impractical for 802.11 in WLANs

Adaptation At Which Layer ?

Why?

26

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?

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Who should select the data rate?

A

B

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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

29

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

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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

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Performance of ARF

Time (s)Rate

(M

bp

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

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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)

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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

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Performance of RBAR

Time (s)

SN

R (

dB

)

Time (s)

Rate

(M

bp

s )R

ate

(M

bp

s )

Time (s)

RBAR

ARF

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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

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Implementation into 802.11

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 BSSIDSequenceControl Body FCSFCS

Reservation Subheader (RSH)

WHY

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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

BER

SNR (dB)

1E-5

2 MbpsThreshold

8 MbpsThreshold

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Performance Results

Single-Hop Network

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Single-Hop Scenario

A B

Mean

Th

rou

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pu

t (K

bp

s)

Distance (m)

40

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

Mean

Th

rou

gh

pu

t (K

bp

s)

Mean Node Speed (m/s)

RBAR

ARF

WHY?

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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 = 1460M

ean

Th

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t (K

bp

s)

Mean Node Speed (m/s)

RBAR

ARF

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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

Mean

Th

rou

gh

pu

t (K

bp

s)

Mean

Th

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pu

t (K

bp

s)

WHY?

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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

Mean

Th

rou

gh

pu

t (K

bp

s)

Why useful ?

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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

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Questions?

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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

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Motivation

Consider the situation below ARF? RBAR?

AB C

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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

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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

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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

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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

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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 ?

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Why is OAR any better ?

Bad channel reduces SINR increases transmit time Fewer packets can be deliveredA BCD

A

C

Data

Data

Data

Data

Data

Data

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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 !!

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MAC Access Delay Simulation

Back to back packets in OAR decrease the average access delay Increase variance in time to access channel Why?

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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

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Simulations

Three Simulation experiments1. Fully connected networks: all nodes in radio range of

each other• Number of Nodes, channel condition, mobility, node

location

2. Asymmetric topology3. Random topologies

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

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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

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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)

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Asymmetric Topology Results

OAR maintains time shares of IEEE 802.11Significant gain over RBAR

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OAR thoughts

OAR does not offer benefits when

OAR may not be suitable for applications like

With TCP how can OAR get affected ?

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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

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Other Ideas(Briefly)

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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

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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.

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Summary

Rate control can be useful When adapted to channel fluctuations (RBAR) When opportunistically selecting transmitters (OAR) When utilizing node diversity

Benefits maximal when Channel conditions vary widely in time and space

Correlation in fluctuation can offset benefits OAR and Diversity-based MAC may show negligible gain

Several more research possibilities with rate control

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Questions ?

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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

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Infrequent TrafficUDP Performance

Similar results for shorter burst intervals Similar results for TCP (see tech report)

Delivery

Rati

o (

MA

C R

x/T

x)

Mean Burst Length (ms)

Pareto sourceMean burst interval = 1 secPacket size = 1460

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Any other tradeoff ?

Can anyone think of yet another IMPORTANT tradeoff

Hint: Related to the MAC Layer

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Total interference

C EA

D

B

Rate = 20

Rate = 10

More nodes free to transmit packets (A,B,E)Interference incident at receiver (D) increasesMore nodes free to transmit packets (A,B,E)Interference incident at receiver (D) increases

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Performance Results

Multi-Hop Network

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Multi-Hop NetworkUDP Performance

Similar results for TCP

20 nodesCBR sourcePacket size = 1460M

ean

Th

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bp

s)

Mean Node Speed (m/s)

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A B C DFlow 1 Flow 2

Multi-Hop NetworkSensitivity to RSH Loss

Aggregate throughput is unaffected by RSH loss High loss probability results in only slight change in

fairness

Mean

Th

rou

gh

pu

t (K

bp

s)

RSH Loss Probability

Mean

Th

rou

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pu

t (K

bp

s)

ARF

RSH

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A B C DFlow 1 Flow 2

Multi-Hop NetworkSensitivity to RSH Loss

Mean

Th

rou

gh

pu

t (K

bp

s)

RSH Loss Probability

Mean

Th

rou

gh

pu

t (K

bp

s)

ARF

RSH

Similar results Slightly more unfairness (vs. ARF) for no loss

(overall fairness problem due to MAC backoff by node A)

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802.11b – Transmission rates

Different modulation methods for transmitting data. Binary/Quadrature Phase

Shift Keying Quadrature Amplitude

Modulation

Each packs different number of data in modulation.

The highest speed has most dense data and is most vulnerable to noise.

Time

1 Mbps

2 Mbps

5.5 Mbps

11 Mbps

Highest energy per bit

Lowest energy per bit