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

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Wireless Networking &amp; 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 -&gt; Hi BER

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Lec #7: MAC Multi-Rate

• 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

Too few bits/s:

Less BER but lower throughput

Varying

with time

and space

How do we choose the rate of modulation

Some Basics
• Friss’ Equation:

• Bit error (p) for BPSK and QPSK :

Channel Bandwidth

SNR

Data Rate

Floor Noise

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

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

Large-scale variation with distance (Path loss)

8 Mbps

Path Loss

Mean Throughput (Kbps)

SNR (dB)

1 Mbps

Distance (m)

Distance (m)

Question

Which modulation scheme to choose?

SNR (dB)

SNR (dB)

2.4 GHz

2 m/s LOS

Distance (m)

Time (ms)

• Dynamically choose the best modulation scheme for the channel conditions

Desired

Result

Mean Throughput (Kbps)

Distance (m)

Is there a tradeoff ?

Rate = 10

B

C

E

A

D

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

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

Sender

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
• 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

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

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

Attempted to Increase

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)

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

SNR (dB)

Time (s)

Rate (Mbps)

Time (s)

RBAR

Rate (Mbps)

Time (s)

ARF

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

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