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Transcript of Wireless Networking & Mobile Computing CS 752/852 - Spring 2012 Tamer Nadeem Dept. of Computer...
Wireless Networking & Mobile Computing
CS 752/852 - Spring 2012
Tamer NadeemDept. of Computer Science
Lec #7: MAC Multi-Rate
Page 2 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
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
Page 3 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
Some Basics
Floor NoiseData Rate
Received Power
Channel Bandwidth
• Bit error (p) for BPSK and QPSK :
SNR
• Friss’ Equation:
Varyingwith timeand space
How do we choose the rate of modulation
Page 4 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
802.11b – Transmission rates
Page 5 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
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
Page 6 Spring 2012 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
Page 7 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
Impact
Large-scale variation with distance (Path loss)
SN
R (
dB
)
Distance (m) Distance (m)
Mea
n T
hro
ug
hp
ut
(Kb
ps)
Path Loss
1 Mbps
8 Mbps
Page 8 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
QuestionS
NR
(d
B)
SN
R (
dB
)
Time (ms)Distance (m)
2.4 GHz2 m/s LOS
Which modulation scheme to choose?
Page 9 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
Answer Rate Adaptation
• Dynamically choose the best modulation scheme for the channel conditions
Mea
n T
hro
ug
hp
ut
(Kb
ps)
Distance (m)
DesiredResult
Page 10 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
Is there a tradeoff ?
C EA
D
B Rate = 10
Page 11 Spring 2012 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
Page 12 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
• Cellular networks
• Adaptation at the physical layer
• Impractical for 802.11 in WLANs
• For WLANs, rate adaptation best handled at MAC
Adaptation At Which Layer ?
D
C
BACTS: 8
RTS: 10
10
8Sender Receiver
RTS/CTS requires that the rate be known in advanceRTS/CTS requires that the rate be known in advance
Why?
Page 13 Spring 2012 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.
• The receiver is best positioned to select data rate
A
B
Page 14 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
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
• …
Page 15 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
Lucent WaveLAN “Autorate Fallback” (ARF)
• Sender decreases rate after
• N consecutive ACKS are lost
• Sender increases rate after
• Y consecutive ACKS are received or
• T secs have elapsed since last attempt
BADATA2 Mbps
2 MbpsEffective Range
1 MbpsEffective Range
Page 16 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
Performance of ARF
Time (s)Ra
te (
Mbp
s)S
NR
(d
B)
Time (s)
– Slow to adapt to channel conditions
– Choice of N, Y, T may not be best for all situations
Attempted to IncreaseRate During Fade
Dropped Packets
Failed to IncreaseRate After Fade
Page 17 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
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)
Page 18 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
• 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
Receiver-Based Autorate (RBAR) Protocol
C
BA CTS (1)
RTS (2)
2 Mbps
1 Mbps
D
1 MbpsDATA (1)
2 Mbps
1 Mbps
Page 19 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
Performance of RBAR
Time (s)
SN
R (
dB
)
Time (s)
Ra
te (
Mbp
s)R
ate
(M
b ps)
Time (s)
RBAR
ARF
Page 20 Spring 2012 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
Page 21 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
Implementation into 802.11
Page 22 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
Implementation into 802.11
PLCP Header
Page 23 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
Implementation into 802.11
• 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
FrameControl
Duration DA SA BSSIDSequence
Control Body FCSFCS
Reservation Subheader (RSH)
WHY