Mac Maca Macaw

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Wireless Medium Access ControlProtocols

CS 851 Seminar

University of Virginia

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

Why Need MAC ?

Wireless medium is an open, shared, and broadcast medium

Multiple nodes may access the medium at the same time

Medium Access Control Protocol:

Define rules to force distributed nodes to access wireless medium in

an orderly and efficiently manner

Ideal MAC Protocol

Limited Delay

High Throughput

Fairness

Stability Scalability

Robustness against channel fading

Low power consumption

Support for multimedia

Some Background

CSMA/CD (carrier sense multiple access/ collision detection)

Every node senses the carrier before transmitting

If the carrier is not clear, the node defers transmission for a specified

period. Otherwise, transmits

While transmitting, the sender is listening to carrier and sender stops

transmitting if collision has been detected

Due to hidden & exposed terminal problem

Contention/collision will occur at receiver side

Carrier sense (send side) approach is inappropriate for wireless

networks

Wireless MAC Issues

Half-Duplex Operation

Time Varying Channel

Burst Channel Errors

Location Dependent Carrier SensingHidden Terminal

Exposed Terminal

Capture

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 transmissionusing the carrier sense mechanism

If C transmits to D, collision will occur at B

B CA D

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 not 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 not cause collision

B CA DX

Capture Effect

A and D transmit simultaneously to B, the signalstrength received by B from D is much higher thanthat from A, and D¶s transmission can be decodedwithout errors. This will result unfair sharing of bandwidth.

A D B C

PowerDifference

Of A and

D signals

Classification of Wireless MAC Protocols

Guaranteed

access

Wireless MAC Protocols

Distributed Mac

Protocols

Centralized MAC

Protocols

Random

access

Distributed MAC Protocols

Collision avoidance mechanisms

Collision avoidance with out-of-band signaling

Collision avoidance with in-band control messages

Two distributed random access protocols

DFWMAC: Distributed Foundation Wireless MAC (used in IEEE

802.11)

EY-NPMA: Elimination Yield-Nonpreemptive Priority Multiple

Access (used in HyperLan)

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 go through the base station

The arbitration and complexity are in base station

MACA: A New Channel Access

Method for Packet Radio

Phil Karn 1990

Goals , New Ideas, and Main

Contributions

Goals:

Try to overcome hidden & exposed terminal problems

New idea:

Reserve the channel before sending data packet

Minimize the cost of collision (control packet is much smaller than

data packet)

Main Contribution:

A three-way handshake MAC protocol : MACA

CSMA/CA MA/CA MACA

Fundamental Assumptions

Symmetry

A can hear from B B can hear from A

No capture

No channel fading Packet error only due to collision

Data packets and control packets are transmitted in the

same channel

Three-Way Handshake A sends Ready-to-Send (RTS)

B responds with Clear-to-Send (CTS)

A sends DATA PACK ET

RTS and CTS announce the duration of the data 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

CTS (10)

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

More Details for MACA

A sends out RTS and set a timer and waits for CTS

If A receives CTS before timer go to zero, OK! sends data packet

Otherwise, A assumes there is a collision at B

Double the backoff counter interval» Randomly pick up a timer from [1,backoff counter]

Send next RTS after timer go to zero

B sends out CTS, then set a timer and waits for data packet

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 receiveCTS. 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 toreduce interference

Note: no carrier sense

Hidden Terminal Problem Still Exists (1)

RTSDATARTS

Data packet still might suffer collision

Hidden Terminal Problem Still Exists (2)

RTSDATARTS

Data packet still might suffer collision

Exposed Terminal Problem Still Exists

Node C can not receive CTS

Summary

MACA did not solve hidden & exposed terminal problems

MACA did not provide specifications about parameters

What are RTS, CTS packet sizes ?

How to decide timers?

What is initial backoff window size?

A lot things need to do if using MACA

MACAW: A Media Access Protocol

for Wireless Lan¶s

V. Bharghavan, A. Demers, S. Shenker, and L. Zhang (Sigcomm 1994)

Goals, New Ideas, and Main

Contributions Goals:

This paper refined and extended MACA

New Idea: Information sharing to achieve fairness

Main Results:

Modified control messages

Four-way handshake (reliable, recover at MAC layer)

Five-way handshake (relieve exposed terminal problem)

RRTS (unfairness)

Modified back-off algorithms

Multiplicative increase and linear decrease (MILD)

Synchronize back-off counter using piggyback message

Multiple stream model (V-MAC)

Revisit Hidden Terminal Problem

Data packet still may suffer collision

To recover packet loss at transport layer is too slow

Recover at MAC layer is more fast

Need ACK from destination

Four-Way Handshake Sender sends Ready-to-Send (RTS)

Receiver responds with Clear-to-Send (CTS)

Sender sends DATA PACK ET

Receiver acknowledge with ACK

RTS and CTS announce the duration of the transfer

Nodes overhearing RTS/CTS keep quiet for that duration Sender will retransmit RTS 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

source

destination

ACKCTS(T)

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

Comparison with ACK and without

Revisit Exposed Terminal Problem

RTS/CTS/DATA/ACK can not solve exposed terminal

problem

When overhearing RTS, the node needs to wait longer

enough to allow the data packet being completely

transmitted even it does not overhear CTS

To relieve exposed terminal problem,

Let exposed terminal know the DATA packet does be transmitted

Extra message DS (data send)

Five Handshaking to let exposed terminal know how long

it should wait

Five-Way Handshake

Sender sends Ready-to-Send (RTS)

Receiver responds with Clear-to-Send (CTS)

Sender sends DATA SE NDI NG (DS)

Sender sends DATA PACK ET

Receiver acknowledge with ACK

RTS and CTS announce the duration of thetransfer

Nodes overhearing RTS/CTS keep quiet for that

duration

DATAACKCTS

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

Comparison with DS and without DS

RTSDATARTS

P1 B2P2

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%

CTSRTSDATARTS

Request for RTS (RRTS)

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

CTSRTSDATARTS

RRTS: Request for RTS

Why Uses RRTS Instead Of CTS ?

CTS or RTS packet size << data packet size

When nodes overhear CTS, they need to defer a time

period to allow the expected data packet transmission

When nodes overhear RRTS, they only need to defer a

time period to overhear the expected CTS

Uses CTS will cost long waiting

Comparison with RRTS and without RRTS (1)

Comparison with RRTS and without RRTS (2)

Multiple Stream Model (V-MAC)

Single stream model merges 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 propose a new MAC for

IEEE 802.11e in 2001

Single Stream MAC

Multiple Stream MAC

Why Multiple Stream MAC more fair Than Single

Stream MAC

When collision

all packets in single stream MAC are used a large backoff window

Different flow¶s packet in multiple stream MAC uses different backoff window

Comparison V-MAC and MAC

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

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}

Information Sharing in Backoff

Algorithms

When a node sends a packet, it embeds its current backoff

counter in the packet header. Other nodes which overhears the packet copy the value as itself backoff counter

Key idea: all nodes have the same backoff counter to achieve

fairness

Comparison BEB and BEB-Copy

Comparison BEB-COPY and MILD-Copy

Per-Destination Backoff

Evaluation of MACAW

TotalTroughput

MACA: 51.06MACAW: 70

37% higher

Every flow has the

same data rate

32 packet per second

Evaluation of MACAW

Open Problems

How to design a good backoff algorithm?

Adaptive MAC to achieve fairness in ad-hoc networks

Do upper layer operations need to tightly relate to MAC?

Reliable multicast MAC in ad-hoc networks

Mac Maca Macaw

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