wireless & mobile networking cs 752/852 - spring 2011 tamer nadeem dept. of computer science lec...
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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
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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
Page 5 Spring 2011 CS 752/852 - Wireless and Mobile Networking
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
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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
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Is there a tradeoff ?
C EA
D
B Rate = 10
Page 8 Spring 2011 CS 752/852 - Wireless and Mobile Networking
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 ?
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Any other tradeoff ?
Will carrier sense range vary with rate
Page 10 Spring 2011 CS 752/852 - Wireless and Mobile Networking
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
Page 11 Spring 2011 CS 752/852 - Wireless and Mobile Networking
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
Page 13 Spring 2011 CS 752/852 - Wireless and Mobile Networking
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
Page 14 Spring 2011 CS 752/852 - Wireless and Mobile Networking
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
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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
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Impact
Small-scale variation with time (Fading)
SN
R (
dB
)
Time (ms)
Rayleigh Fading
2.4 GHz2 m/s LOS
Page 17 Spring 2011 CS 752/852 - Wireless and Mobile Networking
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 conditions
Mea
n T
hro
ug
hp
ut
(Kb
ps)
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
Page 20 Spring 2011 CS 752/852 - Wireless and Mobile Networking
• 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?
Page 21 Spring 2011 CS 752/852 - Wireless and Mobile Networking
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
Page 22 Spring 2011 CS 752/852 - Wireless and Mobile Networking
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
Page 23 Spring 2011 CS 752/852 - Wireless and Mobile Networking
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
Page 24 Spring 2011 CS 752/852 - Wireless and Mobile Networking
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
Page 25 Spring 2011 CS 752/852 - Wireless and Mobile Networking
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)
Page 26 Spring 2011 CS 752/852 - Wireless and Mobile Networking
• 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
Page 27 Spring 2011 CS 752/852 - Wireless and Mobile Networking
Performance of RBAR
Time (s)
SN
R (
dB
)
Time (s)
Ra
te (
Mbp
s)R
ate
(M
b ps)
Time (s)
RBAR
ARF
Page 28 Spring 2011 CS 752/852 - Wireless and Mobile Networking
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
PLCP Header
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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 BSSIDSequence
Control 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
BE
R
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
Mea
n T
hro
ug
hp
ut
(Kb
ps)
Distance (m)
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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?
Page 36 Spring 2011 CS 752/852 - Wireless and Mobile Networking
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
Page 37 Spring 2011 CS 752/852 - Wireless and Mobile Networking
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?
Page 38 Spring 2011 CS 752/852 - Wireless and Mobile Networking
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 ?
Page 39 Spring 2011 CS 752/852 - Wireless and Mobile Networking
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
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
Page 43 Spring 2011 CS 752/852 - Wireless and Mobile Networking
Motivation
• Consider the situation below
• ARF?
• RBAR?
AB C
Page 44 Spring 2011 CS 752/852 - Wireless and Mobile Networking
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
Page 45 Spring 2011 CS 752/852 - Wireless and Mobile Networking
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
Page 46 Spring 2011 CS 752/852 - Wireless and Mobile Networking
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
Page 47 Spring 2011 CS 752/852 - Wireless and Mobile Networking
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
Page 48 Spring 2011 CS 752/852 - Wireless and Mobile Networking
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 ?
Page 49 Spring 2011 CS 752/852 - Wireless and Mobile Networking
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
Page 50 Spring 2011 CS 752/852 - Wireless and Mobile Networking
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 !!
Page 51 Spring 2011 CS 752/852 - Wireless and Mobile Networking
MAC Access Delay Simulation
• Back to back packets in OAR decrease the average access delay
• Increase variance in time to access channel
Why?
Page 52 Spring 2011 CS 752/852 - Wireless and Mobile Networking
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
Page 53 Spring 2011 CS 752/852 - Wireless and Mobile Networking
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]
Page 54 Spring 2011 CS 752/852 - Wireless and Mobile Networking
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
Page 55 Spring 2011 CS 752/852 - Wireless and Mobile Networking
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)
Page 56 Spring 2011 CS 752/852 - Wireless and Mobile Networking
Asymmetric Topology Results
• OAR maintains time shares of IEEE 802.11
• Significant gain over RBAR
Page 57 Spring 2011 CS 752/852 - Wireless and Mobile Networking
OAR thoughts
• OAR does not offer benefits when
• OAR may not be suitable for applications like
• With TCP how can OAR get affected ?
Page 58 Spring 2011 CS 752/852 - Wireless and Mobile Networking
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
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
Page 72 Spring 2011 CS 752/852 - Wireless and Mobile Networking
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
Page 73 Spring 2011 CS 752/852 - Wireless and Mobile Networking
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.
Page 74 Spring 2011 CS 752/852 - Wireless and Mobile Networking
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