1 Wireless Medium Access Control Protocols CS 851 Seminar University of Virginia cs851-2/course.html.

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Wireless Medium Access Control Protocols

CS 851 Seminar

University of Virginia

www.cs.virginia.edu/~cs851-2/course.html

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What Are We Going to Learn?

Understand and appreciate fundamental principles in Wireless Medium Access Control Protocols Basic theory A real MAC protocol: IEEE 802.11b Open problems

Survey the state-of-the-art research and develop new ideas

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Why Need MAC ?

Wireless medium is a broadcast medium Multiple nodes may access the medium at the same time

Medium Access Control Protocol: Define rules to allow distributed nodes to communicate with each

other in an orderly and efficiently manner

B

CA

D 2 4 6 8 10 12 14 16

Th

rou

ghp

ut

Number of Nodes

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Ideal MAC Protocol

Limited Delay High Throughput Fairness Stability Robustness against channel fading Low power consumption Support for multimedia

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Wireless MAC Issues

Half-Duplex Operation Time Varying Channel Burst Channel Errors Location Dependent Carrier Sensing

Hidden Terminal Exposed Terminal

Capture

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Hidden Terminal Problem

Node B can communicate with A and C both A and C cannot hear each other When A transmits to B, C cannot detect the transmission

using the carrier sense mechanism If C transmits to D, collision will occur at B

B CA D

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Exposed Terminal Problem

Node C can communicate with B and D both Node B can communicate with A and C Node A cannot hear C Node D can nor hear B When C transmits to D, B detect the transmission using the

carrier sense mechanism and postpone to transmit to A, even though such transmission will nor cause collision

B CA DX

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Capture Problem

A and D transmit simultaneously to B, the signal strength received from D is much higher than that from A, and D’s transmission can be decoded without errors. This will result unfair sharing of bandwidth.

A D B C

PowerDifferenceOf A andD signals

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Classification of Wireless MAC Protocols

Guaranteed access

Wireless MAC Protocols

Distributed Mac Protocols

Centralized MAC Protocols

Random access

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Distributed MAC Protocols

Collision avoidance mechanisms Collision avoidance with out-of-band signaling Collision avoidance with control handshaking

Distributed Random access protocols DFWMAC (used in IEEE 802.11) EY-NPMA (used in HyperLan)

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Centralized MAC Protocols

Work for centralized wireless networks Base station has explicit control for who and when to access the

medium All nodes can hear from and talk to base station All communications must go through the base station

The arbitration and complexity are in base station Base station decides who and when to access the medium

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MACA: A New Channel Access Method for Packet RadioPhil Karn 1990

Main Contribution: Insights:

Carrier sense (send side) approach is inappropriate for wireless networks

Contention/collision will occur at receiver side

Proposed a three-way handshake MAC protocol : MACA

CSMA/CA MA/CA MACA

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Fundamental Assumptions

Symmetry A can hear from B B can hear from A

No capture No channel fading

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Three-Way Handshake Sender sends Ready-to-Send (RTS) Receiver responds with Clear-to-Send (CTS) Sender sends DATA PACKET RTS and CTS announce the duration of the transfer Nodes overhearing RTS keep quiet for some time to allow A to receive CTS Nodes overhearing CTS keep quiet for some time to allow B to receive data

packet

A

B

DATA

CTS (10)

CTS: Clear To Send RTS (10)RTS: Request To Send

C

D

E

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More Details for MACA A sends out RTS and set a timer

If A receives CTS before timer go to zero, OK Otherwise, A assumes there is a collision at B

• Double the backoff counter interval

• Randomly pick up a timer from backoff counter interval

• Send next RTS after time go to zero B sends out CTS, then set a timer

If data packet arrives before timer go to zero, OK Otherwise, B can do other things

C overhears A’s RTS, set a timer which is long enough to allow A to receive CTS. After the timer goes to zero, C can do other things

D overhears B’s CTS, set a timer which is long enough to allow B to receive data packet.

E overhears A’s RTS and B’s CTS, set a timer which is long enough to allow B to receive data packet.

RTS and CTS can also contain info to allow sender A to adjust power to reduce interference

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Hidden Terminal Problem Still Exists

A

RTSDATARTS

B

RTS

C

CTS

Data packet still might suffer collision

Four-way handshake (add ACK message)Sender will redo RTS/CTS/DATA/ACK if no ACK is received

If ACK is sent out, but not received by sender, after receiving new RTS, receiver returns ACK instead of CTS for new RTS

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Exposed Terminal Problem Still Exists

A

RTS

B

C

CTS

Node C can not receive CTS

DATA

RTSCTS

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MACAW: A Media Access Protocol for Wireless Lan’sV. Bharghavan, A. Demers, S. Shenker, and L. Zhang (Sigcomm 1994)

This paper refined and extended MACA Fixed RTS/CTS packet size as 30 bytes New back-off algorithms

Multiplicative increase and linear decrease (MILD) Synchronize back-off counter using piggyback message

Multiple stream model (V-MAC) Modified control messages

Four-way handshake Five-way handshake RRTS

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Four-Way Handshake

Sender sends Ready-to-Send (RTS) Receiver responds with Clear-to-Send (CTS) Sender sends DATA PACKET Receiver acknowledge with ACK RTS and CTS announce the duration of the transfer Nodes overhearing RTS/CTS keep quiet for that duration

source

destination

DATAACKCTS

CTS: Clear To Send RTSRTS: Request To Send

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Exposed Terminal Problem Still Exists

When B transmits, C is unable to hear CTS To relieve exposed terminal problem, let C know B

does transmit DATA Extra message DS

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Five-Way Handshake

Sender sends Ready-to-Send (RTS) Receiver responds with Clear-to-Send (CTS) Sender sends DATA SENDING (DS) Sender sends DATA PACKET Receiver acknowledge with ACK RTS and CTS announce the duration of the transfer Nodes overhearing RTS/CTS keep quiet for that duration

source

destination

DATAACKCTS

CTS: Clear To Send RTSRTS: Request To Send DSDS: Data Sending

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Unfairness Using RTS/CTS/DATA/ACK or RTS/CTS/DS/DATA/ACK might cause unfairness A sends data to B; D sends data to C A and D have enough data to send C can hears from B and D, but not A B can hear from A and C, but not D

A is in luck and gets the channel D sends RTS and times out Backoff window for D repeatedly doubles For the next transmission:

• A picks a random number from a smaller window

• Unequal probability of channel access

• Throughput for flow A B > 90 %

• Throughput for flow D C ~ 0%

A

CTSRTSDATARTS

B

RTS

DC

ACK

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Request for RTS (RRTS)

Try to solve unfairness by having C do the contending for D

A

CTSRTSDATARTS

B

RRTS

RRTS: Request for RTS

RTS

DC

ACK

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Request for RTS (RRTS)

CSMA/CA with binary exponential backoff

IEEE 802.11 data transmission is accomplished via a four-way handshake

source

destination

DATACTS

CTS: Clear To Send RTSRTS: Request To Send

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Backoff Algorithms When collision occurs, node A pick up a random number T from

[1,Bo], then retransmits RTS after T time unit How to determine Bo

Upper bound Bo_max Low bound Bo_Min After each collision Bo_new = Fun_inc(Bo_old) After each successful transmission Bo_new = Fun_dec(Bo_old)

Binary exponential backoff (BEB) algorithm Fun_inc(Bo_old)=min{2*Bo_old, Bo_max} Fun_dec(B_old)=Bo_min

Multiplicative increase linear decease (MILD) Fun_inc(Bo_old)=min{1.5*Bo_old, Bo_max} Fun_dec(B_old)=max{Bo_old -1, Bo_min}

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Multiple Stream Model (V-MAC)

Single stream model merge traffic from different flows into a mixed stream and uses a single MAC

Multiple stream model uses multiple MAC (one flow one MAC) to achieve fairness

This idea was used by Intersil Company to proposed a new MAC for IEEE 802.11e

MA

CNode

Single Stream MAC

MAC

NodeMAC

MAC

Multiple Stream MAC

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Open Problems

Adaptive MAC to achieve fairness in ad-hoc networks Does upper layer operation need to depend on MAC?

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Dual Busy Tone Multiple Access (DBTMA) : A Multiple Access Control Scheme for Ad Hoc

Networks

Z. Haas and J. Deng

IEEE Trans. on Communications June, 2002

This paper completely solves hidden and exposed terminal problems

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DBTMA Two narrow-bandwidth tones

BTt

• Send out by the node which has data to send – Increase the probability of successful RTS reception

• Stop after sensing BTr BTr

• Send out by the node which received RTS– Acknowlesdge RTS reception– Provides continuous protection for the transmitted data packet

• Stop when completely receives the data packet All nodes sensing any busy tone are not allowed to send RTS Any node no sensing any busy tone is allowed to transmit

A

B

RTSRTSDATA

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Finite State Machine of DBTMA

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Related Works

BTMA :ok RI-BTMA ?? MACA:done MACAW : done FAMA : FAMA-NPS, FAMA-NCS :ok IEEE 802.11 MAC:done WCD: ok

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Example When A has data to send

Senses BTt and BTr

• If both are clear– Turns on BTt– Sends RTS– Sets a timer for BTr and enters WF_BTR state

» If BTr is sensed, waites for tmw, then sends data packet» Otherwise, timer goes to zero, A goes to 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 if no any busy tone signal is sen

When B receives RTS, B turns on BTr and sets a timer 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

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Time Diagram of DBTMA

RTS

RTS

DATA

DATA

A

B

BTr of B

BTt of A

tmw

receiver ander transmittebetween thdelay n propagatio maximum 2

timewaitingMandatory t mw

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Performance Analysis

Assumptions: A lot of nodes and all nodes

are in the same broadcast domain

No channel fading, capture effect

Packet collisions are the only reason for packet errors

Data processing time and transmit/receive turn around time are negligible

Bandwidth consumption of busy tones is negligible compared with data channel

/1)1()6(P

P t throughpuChannel

t5.0T periodbusy failed Average

6t periodion transmisssuccessfulA

Pion transmissRTS successful ofy Probabilit

ratemean with afficPoisson tr a generately collective nodes All

2 t timewaitingMandatory

tdelay detection Busy tone

τdelayn propagatio way one aximum

ion time transmissRTS on timeTransmissiPacket DATA

s

s

df

d

)t(s

wm

d

d

fsd TPt

e

M

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Performance Analysis (cont)

/1)1()6(P

P t throughpuChannel

t5.0T periodbusy failed Average

6t periodion transmisssuccessfulA

Pion transmissRTS successful ofy Probabilit

ratemean with afficPoisson tr a generately collective nodes All

2 t timewaitingMandatory

tdelay detection Busy tone

τdelayn propagatio way one aximum

ion time transmissRTS on timeTransmissiPacket DATA

s

s

df

d

)t(s

wm

d

d

fsd TPt

e

M

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Channel Throughputs of DBTMA

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Channel Throughput

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Comparisons of Channel Throughput

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Performance of Different Length of Control Packet

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Network Utilization of DBTMA in Multi-Hop Networks

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