Transcript Routing in Sensor Networks: Directed Diffusion and other proposals
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
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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
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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 ?
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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
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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
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Impact
Small-scale variation with time (Fading)
Rayleigh Fading Time (ms) 2.4 GHz 2 m/s LOS
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Question
Which modulation scheme to choose?
Distance (m) Time (ms) 2.4 GHz 2 m/s LOS
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Answer
Rate Adaptation Dynamically choose the best modulation scheme for the channel conditions
Desired Result Distance (m)
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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 24
Adaptation
At Which Layer ?
Cellular networks Adaptation at the
physical layer
Impractical for 802.11 in WLANs Why?
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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
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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.
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
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Performance Results Single-Hop Network
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Single-Hop Scenario
A Distance (m) B
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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?
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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?
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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 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 ?
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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 !!
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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
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 ?
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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 62
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
Channel Conditions SNR AP WLAN USERS TIME
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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.
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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 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
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Multi-Hop Network
UDP Performance
Mean Node Speed (m/s)
Similar results for TCP
20 nodes CBR source Packet size = 1460
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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