# Wireless Networking & Mobile Computing CS 752/852 - Spring 2012 - PowerPoint PPT Presentation

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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 ComputingCS 752/852 - Spring 2012

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)

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

• 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

• 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

SNR (dB)

Time (s)

Rate (Mbps)

Time (s)

RBAR

Rate (Mbps)

Time (s)

ARF

### Question to the class

• There are two types of 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