Rate Control in Wireless Networks

ECE 256 1

Recall 802.11

 RTS/CTS + Large CS Zone  Alleviates hidden terminals, but trades off spatial reuse A B C CTS D E RTS F 2

Recall Role of TDMA

3

Recall Beamforming

Omni Communication No

a f b c Silenced Node d

Simultaneous Communication Directional Communication

e f a b c d

Ok Simultaneous Communication

4

Also Multi-Channel

 Current networks utilize non-overlapping channels  Channels 1, 6, and 11  Partially overlapping channels can also be used 5

Today

Benefits from exploiting channel conditions – Rate adaptation – Pack more transmissions in same time 6

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 7

802.11b – Transmission rates

Highest energy per bit 1 Mbps 2 Mbps 5.5 Mbps 11 Mbps Time Lowest energy per bit Optimal rate depends on SINR: i.e., interference and current channel conditions 8

Some Basics

 Friss’ Equation  Shannon’s Equation 

C = B * log 2 (1 + SINR)

Bit-energy-to-noise ratio

E b / N 0

Leads to BER

= SINR * (B/R)

How do we choose the rate of modulation Varying with time and space 9

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 10

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 11

Is there a tradeoff ?

B Rate = 10 C E A D 12

Is there a tradeoff ?

B Rate = 10 A C Rate = 20 D E What about length of routes due to smaller range ?

13

Any other tradeoff ?

Will carrier sense range vary with rate 14

Total interference

B Rate = 10 C E A Rate = 20 D Carrier sensing estimates energy in the channel.

Does not vary with transmission rate 15

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 … 16

A Rate-Adaptive MAC Protocol for Multi-Hop Wireless Networks

Gavin Holland

HRL Labs

Nitin Vaidya Paramvir Bahl UIUC Microsoft Research

MOBICOM’01 Rome, Italy

© 2001. Gavin Holland 17

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

18

Problem

Modulation schemes have different

error characteristics

8 Mbps 1 Mbps SNR (dB)

But, SINR itself varies With Space and Time 19

Impact

Large-scale variation with distance (Path loss)

8 Mbps Path Loss Distance (m) Distance (m) 1 Mbps

20

Impact

Small-scale variation with time (Fading)

Rayleigh Fading Time (ms) 2.4 GHz 2 m/s LOS

21

Question

Which modulation scheme to choose?

Distance (m) Time (ms) 2.4 GHz 2 m/s LOS

22

Answer

 Rate Adaptation  Dynamically choose the best modulation scheme for the channel conditions

Desired Result Distance (m)

23

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 24

Adaptation



At Which Layer ?

 Cellular networks  Adaptation at the

physical layer

 Impractical for 802.11 in WLANs Why?

25

Adaptation



At Which Layer ?

 Cellular networks  Adaptation at the

physical layer

 Impractical for 802.11 in WLANs Why?

RTS/CTS requires that the

rate be known in advance

Sender A CTS: 8 RTS: 10 Receiver B C 8



D 10

For WLANs, rate adaptation best handled at MAC 26

Who should select the data rate?

A B

27

Who should select the data rate?

 Collision is at the receiver  Channel conditions are only known at the receiver  SS, interference, noise, BER, etc.

A B

 The receiver is best positioned to select data rate 28

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 29

Lucent WaveLAN “Autorate Fallback” (ARF)

2 Mbps Effective Range 1 Mbps Effective Range A 2 Mbps DATA B

 Sender decreases rate after 

N

consecutive ACKS are lost  Sender increases rate after  

Y T

consecutive ACKS are received

or

secs have elapsed since last attempt 30

Performance of ARF

Time (s)

Dropped Packets

– – Time (s)

Failed to Increase Rate After Fade Attempted to Increase Rate During Fade

Slow to adapt to channel conditions Choice of

N , Y , T

may not be best for all situations 31

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) 32

Receiver-Based Autorate (RBAR) Protocol

1 Mbps 2 Mbps RTS (2) A DATA (1) 1 Mbps CTS (1) B D 2 Mbps C 1 Mbps

 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 33

Performance of RBAR

RBAR

Time (s) Time (s)

ARF

Time (s) 34

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 35

Implementation into 802.11

Frame Control Duration DA SA FCS Reservation Subheader (RSH) BSSID Sequence Control Body

 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

FCS

 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 WHY 36

Performance Analysis

 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)

1E-5 2 Mbps Threshold SNR (dB) 8 Mbps Threshold

37

Performance Results Single-Hop Network

38

Single-Hop Scenario

A Distance (m) B

39

Varying Node Speed

UDP Performance

RBAR ARF CBR source Packet Size = 1460

  

Mean Node Speed (m/s)

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 WHY?

40

Varying Node Speed

TCP Performance

RBAR ARF FTP source Packet size = 1460

  

Mean Node Speed (m/s)

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 41

No Mobility

UDP Performance

ARF RBAR CBR source Packet size = 1460 Distance (m) Distance (m)

   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 WHY?

42

No Mobility

UDP Performance

RBAR-P CBR source Packet size = 1460 Distance (m)

   RBAR-P – RBAR using a simple initial rate estimation algorithm  Previous rate used as estimated rate in RTS Better high-rate performance Why useful ?

Other initial rate estimation and rate selection algorithms are a topic of future work 43

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 44

Questions?

45

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 46

Motivation

 Consider the situation below   ARF? RBAR?

A B C 47

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?

Timeshare A C B C A B Throughput Fairness vs Temporal Fairness 48

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 49

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 50

OAR Idea

 Basic Idea   If bad channel, transmit minimum number of packets If good channel, transmit as much as possible D C A B A Data Data Data Data Data Data Data Data C 51

Why is OAR any better ?

 802.11 alternates between transmitters A and C  Why is that bad D C A B A Data Data Data Data C Data Data Data Data Is this diagram correct ?

52

Why is OAR any better ?

 Bad channel reduces SINR   Fewer packets can be delivered increases transmit time D C A B A Data Data Data C Data Data Data 53

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 !!

54

MAC Access Delay Simulation

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

55

OAR Protocol

802.11

802.11b

OAR Pkts 1 1 1 Rate 2 2 2 Channel Condition MEDIUM Pkts Rate 1 1 3 2 5.5

5.5

Pkts GOOD Rate 1 1 5 2 11 11

 Rates in IEEE 802.11b: 2, 5.5, and 11 Mbps  Number of packets transmitted by OAR ~ Tx Rate Base Rate 56

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] 57

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 58

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 59

Asymmetric Topology Results

 OAR maintains time shares of IEEE 802.11

 Significant gain over RBAR 60

OAR thoughts

 OAR does not offer benefits when  OAR may not be suitable for applications like  With TCP how can OAR get affected ?

61

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 62

Other Ideas (Briefly)

63

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

Channel Conditions SNR AP WLAN USERS TIME

64

The Protocol Overview

•

sender

MAD using Packet Concatenation (PAC)

DIFS SIFS SIFS SIFS GRTS SF DATA 0 DATA 1 user 1 CTS 1 user 2 user k CTS 2

…

CTS k DATA 2 SIFS ACK 0~2

Since at least one intermediate node is likely to have good channel condition, transmitter can transmit at a high data rate or concatenate Multiple 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.

65

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 66

Questions ?

67

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 68

Infrequent Traffic

UDP Performance

Pareto source Mean burst interval = 1 sec Packet size = 1460 Mean Burst Length (ms)

  Similar results for shorter burst intervals Similar results for TCP (see tech report) 69

Any other tradeoff ?

Can anyone think of yet another IMPORTANT tradeoff Hint: Related to the MAC Layer 70

Total interference

B Rate = 10 C E A Rate = 20 D More nodes free to transmit packets (A,B,E) Interference incident at receiver (D) increases 71

Performance Results Multi-Hop Network

72

Multi-Hop Network

UDP Performance

Mean Node Speed (m/s)

 Similar results for TCP

20 nodes CBR source Packet size = 1460

73

Multi-Hop Network

Sensitivity to RSH Loss

A Flow 1 B RSH C Flow 2 D

 

ARF RSH Loss Probability

Aggregate throughput is unaffected by RSH loss High loss probability results in only slight change in fairness 74

Multi-Hop Network

Sensitivity to RSH Loss

A Flow 1 B RSH C Flow 2 D

 

ARF RSH Loss Probability

Similar results Slightly more unfairness (vs. ARF) for no loss  (overall fairness problem due to MAC backoff by node A) 75

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 Highest energy per bit 1 Mbps 2 Mbps 5.5 Mbps 11 Mbps Lowest energy per bit 76