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Wireless Networking & Mobile Computing CS 752/852 - Spring 2012. Lec #7: MAC Multi-Rate. Tamer Nadeem Dept. of Computer Science. 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

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Wireless Networking & Mobile Computing CS 752/852 - Spring 2012

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Wireless networking mobile computing cs 752 852 spring 2012

Wireless Networking & Mobile ComputingCS 752/852 - Spring 2012

Lec #7: MAC Multi-Rate

Tamer Nadeem

  • Dept. of Computer Science


What is data rate

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


Some basics

Varying

with time

and space

How do we choose the rate of modulation

Some Basics

  • Friss’ Equation:

Received Power

  • Bit error (p) for BPSK and QPSK :

Channel Bandwidth

SNR

Data Rate

Floor Noise


802 11b transmission rates

802.11b – Transmission rates


Static rates

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


Adaptive rate control

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


Impact

Impact

Large-scale variation with distance (Path loss)

8 Mbps

Path Loss

Mean Throughput (Kbps)

SNR (dB)

1 Mbps

Distance (m)

Distance (m)


Question

Question

Which modulation scheme to choose?

SNR (dB)

SNR (dB)

2.4 GHz

2 m/s LOS

Distance (m)

Time (ms)


Answer rate adaptation

Answer  Rate Adaptation

  • Dynamically choose the best modulation scheme for the channel conditions

Desired

Result

Mean Throughput (Kbps)

Distance (m)


Is there a tradeoff

Is there a tradeoff ?

Rate = 10

B

C

E

A

D


Design issues

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


Adaptation at which layer

RTS: 10

C

8

A

B

CTS: 8

D

10

Adaptation  At Which Layer ?

  • Cellular networks

    • Adaptation at the physical layer

  • Impractical for 802.11 in WLANs

  • For WLANs, rate adaptation best handled at MAC

Why?

RTS/CTS requires that the rate be known in advance

Receiver

Sender


Who should select the data rate

A

B

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


Bigger picture

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


Lucent wavelan autorate fallback arf

DATA

2 Mbps

A

B

2 Mbps

Effective Range

1 Mbps

Effective Range

Lucent WaveLAN “Autorate Fallback” (ARF)

  • Sender decreases rate after

    • N consecutive ACKS are lost

  • Sender increases rate after

    • Y consecutive ACKS are receivedor

    • T secs have elapsed since last attempt


Performance of arf

Performance of ARF

SNR (dB)

  • Slow to adapt to channel conditions

  • Choice of N, Y,T may not be best for all situations

Time (s)

Dropped Packets

Rate (Mbps)

Time (s)

Failed to Increase

Rate After Fade

Attempted to Increase

Rate During Fade


Rbar approach

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)


Receiver based autorate rbar protocol

1 Mbps

C

2 Mbps

RTS (2)

1 Mbps

CTS (1)

A

B

DATA (1)

1 Mbps

D

2 Mbps

Receiver-Based Autorate (RBAR) Protocol

  • 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


Performance of rbar

Performance of RBAR

SNR (dB)

Time (s)

Rate (Mbps)

Time (s)

RBAR

Rate (Mbps)

Time (s)

ARF


Question to the class

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


Implementation into 802 11

Implementation into 802.11


Implementation into 802 111

Implementation into 802.11

PLCP Header


Implementation into 802 112

Implementation into 802.11

Sequence

Control

Frame

Control

Duration

DA

FCS

SA

BSSID

FCS

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

  • 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

Reservation Subheader (RSH)

WHY


Wireless networking mobile computing cs 752 852 spring 2012

Questions


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