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A Rate-Adaptive MAC A Rate-Adaptive MAC Protocol for Multi-hop Protocol for Multi-hop

Wireless NetworksWireless Networks 황 태 호

taeo@keti.re.kr

Gavin Holland Texas A&M University

Nitin Vaidya Texas A&M University Department of Electrical and Computer Engineering Co-

Director, Illinois Center for Wireless Systems Research Professor

Paramvir Bahl Microsoft Research

ACM SIGMOBILE July 2001, Rome, Italy

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Introduction - 1Introduction - 1 in WLAN (IEEE 802.11)

Devices can transmit at 11 Mbps, with 54 Mbps Number of encoded bits per symbol Data rate Modulation in mobile wireless networks

path loss, fading, interference SNR, BER variations Support Multi-Modulation scheme

BPSK QPSK QAM16 QAM64 QAM256 Tradeoff emerges between modulation schemes.

The higher the data rate, the higher the BER Figure 1, Figure 2

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Introduction - 2Introduction - 2

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Rate AdaptationRate Adaptation Dynamically switching data rates to match the

channel conditions Two Aspect

Channel quality estimation Measuring Signal Strength, Symbol error rate, etc Prediction of future quality

Rate selection Channel Quality Prediction Threshold selection

Minimize the delay between prediction and selection

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Previous Work on rate adaptionPrevious Work on rate adaption Ref. [19]. Dual Channel Slotted ALOHA

Separate control channel Receiver feedback to sender

Ref. [15]. Auto Rate Fallback(ARF, 802.11) Lucent’s WaveLAN II The sender selects the best rate based on previous tx data.

Ref [9]. Adaptive Transmission Protocol Selects based on cached per-link information Separate transmit receive tables

Maintained by exchanging control packet(RTS/CTS)

Cellular network Channel quality estimation by the receiver Rate selection by the sender using the feedback Reside at the physical layer (symbol-by-symbol)

Improper to MAC based on contention access

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MotivationMotivation ARF Protocol

Receiver Channel Quality Estimation Rate Selection

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Overview of IEEE 802.11Overview of IEEE 802.11 Src sends a data packet to Dst Transmission using one of basic rate set

All node can demodulatethe RTS/CTS packets

Virtual carrier sense RTS includes DRTS

CTS includes DCTS

NAV Network Allocation Vector The aggregate duration

of time that medium is presumed to be busy

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Receiver-Based Autorate (RBAR) Receiver-Based Autorate (RBAR) ProtocolProtocol The receiver selects the appropriate rate for the data packet during

the RTS/CTS exchange More accurate rate selection Smaller overhead for the channel quality estimation

In control packet Instead of DRTS ,DCTS modulation rate and packet size

Src chooses a data rate based on some

heuristic method Send RTS

Dst Estimate the channel condition Send CTS

Node A, B Calculates the duration Update NAV

Reservation SubHeader (RSH) in the MAC header of the data packet

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Incorporation of RBAR into 802.11Incorporation of RBAR into 802.11 Data Packet

Header Check Sequence

RTS/CTS Rate and Length

PLCP header RSH rate

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Simulation EnvironmentSimulation Environment NS-2

Extensions from the CMU Monarch project for modeling mobile ad hoc networks

Number of traffic generators PHY/MAC/Networking stacks

Addition Detailed MAC and PHY models Modulation and rate adaption Rayleigh fading simulator Interfaces Intersil Prism II chipset

IEEE 802.11, DSSS radio,

Observation Hot the individual rate adaption protocols reacted to the

changing channel conditions

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Simulation – ARF modelSimulation – ARF model Rate selection

If no ACKs for two consecutive data packets, DOWN Rate

If received ACKs for ten consecutive data packets, UP Rate and timer cancelled

If timer expired, UP Rate

Relatively insensitive tochoice of timeout

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Simulation – RBARSimulation – RBAR Rate selection

Simple threshold based technique Estimate : SNR of RTS Select : (BER) ≤ 1E-5 , highest data rate

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Simulation – Error Model 1Simulation – Error Model 1 Jake’s method

Simulation of Rayleigh fading A finite number of oscillators with Doppler shifted

frequencies

Instantaneous gain

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)()()( tjxtxt sc

Simulation – Error Model 2Simulation – Error Model 2 Log-distance path loss model

Friis free space propagation model

Noise model

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n : path loss exponent

k : Boltzmann’s constantT : temperature (in Kelvin)BT : bandwidth

Simulation – Error Model 3Simulation – Error Model 3 Computed Bit Error Rate

BPSK , QPSK

M-ary QAM

Eb/N0 : bit energy to noise ratio

For gain, Coherence time

For noise, Adjusting SNR

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Simulation – Network ConfigurationSimulation – Network Configuration Configuration 1

Two node One of the nodes was fixed position, the other traveled

along a direct-line path (300m)

Configuration 2 20 nodes Random waypoint mobility Random speed : 2, 4, 6, 8, 10 m/s 1500 x 300 m2

DSR (Dynamic source routing) Protocol Average of 30 times

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Performance EvaluationPerformance Evaluation Overhead of RSH

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Slow Changing Channel ConditionsSlow Changing Channel Conditions Configuration 1 0 ~ 300m, by 5m 60s, Tx UDP packets(1460 bytes)

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Fast Changing Channel ConditionsFast Changing Channel Conditions Experiment 1

Configuration 1 Mean node speed : 2, 4, 6, 8, 10 m/s Single UDP Connection Performance improvement

from 6% (10m/s) to 20% (2m/s)

Experiment 2 Single TCP Connection

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Fast Changing Channel Conditions Fast Changing Channel Conditions

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Impact of Variable Traffic SourcesImpact of Variable Traffic Sources Configuration 1 Bursty data sources Pareto distribution

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Multi-hop PerformanceMulti-hop Performance Configuration 2

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Future work & ConclusionFuture work & Conclusion Basic Access mode in 802.11

Not used the RTS/CTS protocol

Hybrid scheme conditional RTS/CTS When ACKs are lost When Long packet size

Proposed RBAR Optimizing performance WLAN

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