Wireless Networking & Mobile Computing
CS 752/852 - Spring 2012
Lec #5: Advanced MAC Schemes
Dual Busy Tone & Collision Notification
Tamer Nadeem
Dept. of Computer Science
Dual Busy Tone Multiple Access
(DBTMA) : A Multiple Access Control
Scheme for Ad Hoc Networks *
(Z. Haas and J. Deng)
• This paper completely solves hidden and exposed terminal problems
* Slides adapted from Z. Haas
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Key Idea & Goals & Main Results
• Key idea:
Continuously protect data packet transmission
Use out-band channels to distribute information
• Goals
Solve hidden & exposed terminal problems
• Main Results
DBTMA: two out-of-band busy tones & RTS
Completely solve hidden & exposed terminal problems
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Related Works

BTMA (Busy Tone Multiple Access, F. A. Tobagi & L. Kleinrock 1975):
 Using two channels: data channel & control channel
 A control center - basestation
 When base station senses the transmission of a terminal, it broadcasts a busy tone
signal to all terminals, keeping them (except the current transmitter) from accessing
the channel

RI-BTMA (Receiver-Initiated Busy Tone Multiple Access, C. Wu & V. O. K. Li
1987)





Time is slotted (similar to slotted ALOHA & need time clock synchronization)
A packet preamble is sent to intended receiver by the transmitter
Receiver sets up an out-of-band busy tone and waits for the data
When sensing busy tone, transmitter sends the data packet
FAMA (Floor Acquisition Multiple Access, C. L. Fuller & J.J Garecia-Luna-Aceves 1995)
 FAMA-NPC (NPC = on-persistent packet sensing)
o MACA
 FAMA-NCS (NCS non-persistent carrier sensing)
o Sensing carrier before sending RTS
• If clear, sends RTS
• Otherwise, waiting a random time, sensing carrier again
o CTS is more larger than RTS
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DBTMA
• Two narrow-bandwidth tones
• BTt (Transmitter Busy Tone)
• Set up by the node which has data to send
• Stop when completing transmitting RTS
• BTr (Receiver Busy Tone)
• Set up by the node which receives RTS
• Stop when completely receives the data packet
• All nodes sensing any busy tone are not allowed to send RTS
• Any node sensing no busy tone is allowed to transmit
C
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A
B
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Functionalities of Busy Tones
• BTr (set up by receiver)
Notifying the RTS sender that RTS has been received and channel
has been acquired
Announcing to its neighbor nodes that it is receiving data packet and
they should refrain from accessing the channel
• BTt (set up by sender)
Providing protection for the RTS packet
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Seven DBTMA Operation States
• IDLE
Node with on packets to send stays in IDLE state
• CONTEND
Node has data to send but it is not allowed to send RTS, it stays in CONTEND state
• S_RTS
Node sending RTS is in S_RTS state
• S_DATA
Node sending data is in S_DATA state
• WF_BTR
RTS packet sender waiting for the ACK from its intended receiver is in WF_BTR
state
• WF_DATA
Receiver waiting for DATA is in WF_DATA state
• WAIT
Node send out RTS and senses BTr and waits a mandatory time, it is WAIT state
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Finite State Machine of DBTMA
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More Details for DBTMA
• When A has data to send
• Senses BTt and BTr
• If both are clear
• Turns on BTt
• Sends out RTS and enters S_RTS state
• Turns off BTt at the end of RTS transmission and gets out S_RTS state
• Sets a timer for expected BTr and enters WF_BTR state
• If BTr is sensed, enters WAIT state and waits for tmw, then enters S_DATA state
and sends data packet
• Otherwise, timer goes to zero, A goes to IDLE state
• Enters IDLE state
• Otherwise
• Sets a random timer and goes to CONTENT state
• If BTt or BTr is still sensed when timer goes to zero, A goes to IDLE state
• Otherwise, A turns on BTt and enters S_RTS state and sends out RTS if no any
busy tone signal is sensed
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More Details for DBTMA
• When B receives RTS, B turns on BTr and sets a timer for expected data
packet and enters WF_DATA state
• If B has not received data packet before timer goes to zero
•
B turns off BTr and goes to IDLE state
Otherwise, B receives data packet and turns off its BTr when completely getting the data
packet
When BTr sensed by any Other Node which is in S_RTS state, the node aborts
it RTS and goes to IDLE state
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Time Diagram of DBTMA
BTt of A
A
B
tmw
RTS
RTS
DATA
DATA
BTr of B
RTS
C
τ
t mw  Mandatory waiting time
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 2 maximum propagatio n delay between th e transmitt er and receiver
 2
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Channel Throughputs of DBTMA
(Single Broadcast Region)
Capacity = 1 Mbps
Data packet = 4096 b
RTS = 200 b
20 nodes in 50 by 50 m^2
Radio transmission
range = 35m
Maximum propagation
delay = 0.12 μs
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Impact of Busy Tone Detection Delay
BTt of A
A
B
tmw
RTS
RTS
DATA
DATA
BTr of B
C
τ
Busy Tone Detection Delay
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Performance Analysis
(single broadcast domain case)
• Assumptions:
• A lot of nodes and all nodes
are in the same broadcast
DATA Packet Transmissi on time  
domain
• No channel fading, capture
effect
• Packet collisions are the
only reason for packet
errors
RTS transmiss ion time  
Maximum one way propagatio n delay  τ
Busy tone detection delay  t d
Mandatory waiting time  t wm  2
All nodes collective ly generate a Poisson tr affic with mean rate 
Probabilit y of successful RTS transmiss ion Ps  e  ( t d  )
A successful transmiss ion period      t d  6
• Data processing time and
transmit/receive turn around Average failed busy period Tf      0.5t d
time are negligible
Ps
• Bandwidth consumption of
busy tones is negligible
compared with data
channel
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Channel throughpu t 
Ps (    t d  6 )  (1  Ps )T f  1 / 
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Channel Throughput (ad-hoc network)
Capacity = 1 Mbps
Data packet = 4096 b
RTS = 200 b
Radio transmission
range = 2 km
Propagation
delay = 6.7 μs
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Comparisons of Channel Throughput
Capacity = 256 kbps Data packet = 4096 b RTS = 200 b
Each node are 6 km from each other Propagation delay = 20 μs
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Comparison of Different Length
of Control Packet
Full connected network
Every node randomly
choose its destination for
each generated data
packet
Capacity = 1 Mbps
Data packet size =4096 b
20 nodes in 50 by 50 m^2
Radio transmission
range = 35 m
Propagation delay = 0.12 μs
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Network Utilization of DBTMA
in Multi-Hop Networks
50 nodes in 400 by 400 m^2
FAMA-NCS 2.4
RI-BTMA 4.8
Radio transmission range = 100
RTS size = 200 b
MACA 2.2
Modified DBTMA 4.2
Packet size = 4096 b
Capacity = 1 Mbps
μs
Propagation delay = 0.33
Packet arrival at each node
is Poisson distributed
DBTMA 5.7
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Each node randomly selects
a neighbor as the destination
of each packet
CS 752/852 - Wireless Networking and Mobile Computing
Summary
• DBTMA does solve hidden & exposed terminal problems
• DBTMA is based on the idea presented in RI-BTMA
• Some idea
Using some kind of out-of-band control channel to propagate some info to
achieve some performance targets
19
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Towards Collision Detection in
Wireless Networks*
(Souvik Sen, Naveen Santhapuri,
Romit Roy Choudhury, Srihari Nelakuditi)
* Slides adapted from Souvik Sen
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Collision in Wireless Networks
T1
R
t0
T2
t1
Collision
time
ACK Timeout
Retransmit
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2
Collision in Wireless Networks
T1
R
t0
T2
t1
Collision
time
ACK Timeout
Not Efficient!
Retransmit
T1 should have stopped
right after collision
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Collision in Wired Networks
T1
R
T2
Collision
Ethernet BUS

Transmitter aborts transmission on collision
✦ Transmitter senses the signal while transmitting
✦ If (sensed != transmitted), abort
Collision Detection (CSMA/CD)
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2
Is CSMA/CD Beneficial in Wireless?
Dont
Transmit!
Collision
Detected
T2
T3
T1
R2
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R1
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Collision
R3
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2
Is CSMA/CD Beneficial in Wireless?
Dont
Transmit!
Collision
Abort Tx!
Detected
T2
T3
T1
R2
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R1
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Collision
R3
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2
Is CSMA/CD in Wireless Beneficial?
Channel
free now
Collision
Detected
T2
T3
T1
R2
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R1
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R3
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2
Is CSMA/CD in Wireless Beneficial?
Lets
Transmit!
Collision
Detected
T2
T3
T1
R2
R1
R3
CSMA/CD frees the channel for
other transmissions
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2
Can we imitate CSMA/CD on Wireless?
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Practical Requirements?
1. Transmitter cannot detect collision
• Receiver needs to detect it
Collision!
Rx
Tx
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2
Practical Requirements?
1. Transmitter cannot detect collision
• Receiver needs to detect it
2. Receiver needs to convey
• collision notification to the transmitter
Collision!
Rx
Tx
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3
Practical Requirements?
1. Transmitter cannot detect collision
• Receiver needs to detect it
2. Receiver needs to convey
• collision notification to the transmitter
3. Transmitter needs an additional antenna
• To receive notification
Collision!
Rx
Tx
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Overview
If Collision,
Notify Tx
If Notification,
Abort Tx
S=S1
Notify Collision (S2)
PHY
Tx
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MAC
PHY
Data Transmission (S1)
Spring 2012
CrossLayer
CrossLayer
MAC
Rx
CS 752/852 - Wireless Networking and Mobile Computing
3
Overview
If Collision,
Notify Tx
If Notification,
Abort Tx
S=S1+S2
S=S1
Notify Collision (S2)
PHY
Tx
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MAC
PHY
Data Transmission (S1)
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CrossLayer
CrossLayer
MAC
Rx
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3
Two Key Challenges
1. Find Notification on
Listening Antenna
2. Detect Collision
in real time
If Collision,
Notify Tx
If Notification,
Abort Tx
S=S1+S2
Notify Collision (S2)
PHY
Tx
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MAC
PHY
Data Transmission (S1)
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CrossLayer
CrossLayer
MAC
Rx
CS 752/852 - Wireless Networking and Mobile Computing
3
1. Find Notification on
Listening Antenna
2. Detect Collision
in real time
CSMA/CN key idea: Correlation
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Challenge 1: Detecting Notification
• Hard to decode notification on same channel
• Self-signal too strong
MAC
PHY
• Let Tx and Rx share a unique signature
• Tx correlates with shared signature
• Detects collision notification, aborts
Observe: No decoding, just correlate
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Challenge 1: Detecting Notification
Correlation
Self Signal
Notification Signature
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Correlation
Challenge 1: Detecting Notification
Sample Number
Whenever there is a notification,
there is a jump in correlation
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Collision Correlation, Notification, and Abort
T2
T1
Data
Data
Sign(R1)
R1
R
Correlate (Sign(R1))
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Sign(R2)
R2
Collision
CS 752/852 - Wireless Networking and Mobile Computing
3
Collision Correlation, Notification, and Abort
Notification
!
Stop Tx
Corr (Sign(R1))
T1
T2
Sign(R1)
Data
Data
Sign(R1)
R1
R
Correlate (Sign(R1))
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Sign(R2)
R2
Collision
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4
Performance Evaluation
• 7 node USRP testbed
• Zigbee CC2420 PHY
• Max data rate: 250Kbps
• Signature size: 5 bytes
• Compare with 802.11-like and PPR
• PPR detects interfered portion of received packet
• Transmitter sends only the interfered portion
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4
MAC
Notification Detection at Tx
PHY
Notification Signal << Self Signal
How weak can the notification signal be?
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How weak the notification signal be?
}✔
18 dB
Signal
power
Self
Signal
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Notification
Signal
CS 752/852 - Wireless Networking and Mobile Computing
4
How weak the notification signal be?
}
✘
Signal
power
Self
Signal
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18 dB
Spring 2012
Notification
Signal
CS 752/852 - Wireless Networking and Mobile Computing
4
MAC
Interference Detection at Rx
PHY
• Interference detection accuracy of 93%
• Receiver should detect interference quickly
• Quicker detection
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Faster Tx abortion
CS 752/852 - Wireless Networking and Mobile Computing
4
Interference Detection: Speed
Bytes after interferer started
CSMA/CN predicts collision within 7 bytes
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Testbed Experimentation
• One link doing CSMA/CN
• CSMA/CN link has an exposed and hidden terminal
• Whenever CSMA/CN link fails due to interference
• CSMA/CN link stops
• Exposed terminal transmits reducing channel wastage
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Testbed Throughput
PPR continues to transmit under
collision, worse than CSMA/CN
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Traced Based Evaluation
50%
Throughput in Kbps
Upto 50% gain in per link throughput
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Summary
• CSMA/CN imitates CSMA/CD in wireless
• Rx uses correlation to detect interference
• Tx uses correlation to detect notification
• Others can utilize freed-up channel
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5
Questions
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