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
Too few bits/s:
Less BER but lower throughput
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CS 752/852 - Wireless Networking and Mobile Computing
Some Basics
• Friss’ Equation:
Received Power
• Bit error (p) for BPSK and QPSK :
Channel Bandwidth
SNR
Floor Noise
Data Rate
Varying
with time
and space
How do we choose the rate of modulation
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CS 752/852 - Wireless Networking and Mobile Computing
802.11b – Transmission rates
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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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CS 752/852 - Wireless Networking and Mobile Computing
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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Impact
Large-scale variation with distance (Path loss)
SNR (dB)
Path Loss
Distance (m)
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Mean Throughput (Kbps)
8 Mbps
1 Mbps
Distance (m)
CS 752/852 - Wireless Networking and Mobile Computing
Question
SNR (dB)
SNR (dB)
Which modulation scheme to choose?
2.4 GHz
2 m/s LOS
Distance (m)
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Time (ms)
CS 752/852 - Wireless Networking and Mobile Computing
Answer  Rate Adaptation
Dynamically choose the best modulation scheme for the channel
conditions
Mean Throughput (Kbps)
•
Desired
Result
Distance (m)
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CS 752/852 - Wireless Networking and Mobile Computing
Is there a tradeoff ?
B
A
Rate = 10
C
E
D
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CS 752/852 - Wireless Networking and Mobile Computing
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
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Adaptation  At Which Layer ?
• Cellular networks
• Adaptation at the physical layer
Why?
• Impractical for 802.11 in WLANs
RTS/CTS requires that the rate be known in advance
C
Receiver
Sender
RTS: 10
A
8
B
CTS: 8
D
10
• For WLANs, rate adaptation best handled at MAC
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CS 752/852 - Wireless Networking and Mobile Computing
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
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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
• …
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CS 752/852 - Wireless Networking and Mobile Computing
Lucent WaveLAN “Autorate Fallback” (ARF)
A
2 Mbps
Effective Range
2 Mbps
DATA
B
• Sender decreases rate after
• N consecutive ACKS are lost
1 Mbps
Effective Range
• Sender increases rate after
• Y consecutive ACKS are received or
• T secs have elapsed since last attempt
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SNR (dB)
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
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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)
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Receiver-Based Autorate (RBAR) Protocol
1 Mbps
C
2 Mbps
RTS (2)
1 Mbps
CTS (1)
A
DATA (1)
B
1 Mbps
• RTS carries sender’s estimate of best rate
D
2 Mbps
• 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
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SNR (dB)
Performance of RBAR
Time (s)
RBAR
Time (s)
ARF
Time (s)
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CS 752/852 - Wireless Networking and Mobile Computing
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
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Implementation into 802.11
PLCP Header
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Implementation into 802.11
Frame
Control
Duration
DA
SA
FCS
BSSID
Sequence
Control
Body
FCS
Reservation Subheader (RSH)
• 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
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WHY
CS 752/852 - Wireless Networking and Mobile Computing
Questions
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CS 752/852 - Wireless Networking and Mobile Computing