Wireless Mac Layer

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Wireless Ad hoc NetworkWireless Ad hoc Network

Associate Prof. Wei Liu ( )

Dept. of Electronics and Information Eng.Huazhong University of Science and Technology

2009.04


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Wireless Ad hoc network Lecture 3

Lecture 3Lecture 3

l Chapter 2. MAC Layer Protocolsl 2.1 Introduction

l 2.2 Important Issues and the Need

l 2.3 Classification of MAC Protocols

l 2.4 Summary


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Layering in Ad hoc networkLayering in Ad hoc network

Application Layer

Presentation Layer

Session Layer

Transport Layer

Network Layer

Data link layer

Physical Layer

Logical Link Control

Medium Access Control

Medium access control subMedium access control sub--layer deals with how tolayer deals with how tocontrol access to the shared mediumcontrol access to the shared medium


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MAC layerMAC layer

l The data-link layer

l involves the functions and procedures necessary totransfer data between two or more nodes of thenetwork.

l error correction, framing, physical addressing, andflow and error controls.

l MAC sub-layer

l is responsible for resolving conflicts among differentnodes for channel access.

l the MAC layer has a direct bearing on how reliablyand efficiently data can be transmitted between twonodes, it affects the quality of service (QoS) of thenetwork.


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Shared wireless mediaShared wireless media

l

Shared wireless media in ad hoc networksl Each node can only be a transmitter (TX) or a

receiver (RX) at a time, share the same frequencydomain to communicate

l Communication among mobile nodes is limited

within a certain transmission range.l Within such a range,only one transmission

channel is used, covering the entire bandwidth.

l Additional delay introduced in wireless senarcio,

packet delay is caused by the traffic load at theneighboring nodes, which is calledtrafficinterference.


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Wireless MACWireless MAC

l A MAC protocoll defines how each mobile unit can share the

limited wireless bandwidth resource in an efficient

manner

l

Design objectivel to provide an orderly and efficient use of the

common spectrum

l per-link connection establishment and per-link

connection cancellation

l to increase the overall network throughput whilemaintaining low energy consumption for packet

processing and communications


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Performance MetricsPerformance Metrics -- 11l Throughput and delay:

l Throughput is generally measured as the percentageof successfully transmitted radio-link level frames perunit time.

l Transmission delay is defined as the interval betweenthe frame arrival time at the MAC layer of a transmitterand the time at which the transmitter realizes that thetransmitted frame has been successfully received bythe receiver.

l Fairness:

l Generally, fairness measures how fair the channel

allocation is among the flows in the different mobilenodes.

l The node mobility and the unreliability of radiochannels are the two main factors that impact fairness.


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Performance MetricsPerformance Metrics -- 22

l Energy efficiency:

l Generally, energy efficiency is measured as the

fraction of the useful energy consumption (for

successful frame transmission) to the total energy

spent.

l Multimedia support:

l This is the ability of a MAC protocol to accommodate

traffic with different service requirements, such as

throughput, delay, and frame loss rate.


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Lecture 3Lecture 3



l 2.2.1 Need for Special MAC Protocols


l 2.4 Summary


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Several important issuesSeveral important issues

l Controlled wireless medium

l Industrial, Scientific and Medical (ISM) band

l a limited channel bandwidth much smaller than that of

wirednetworks

l Error prone channel

l an unreliable time-varying channe

l multiple access, signalfading,and noise and interference

l the effective throughput in wireless networks is significantly lower

l Mobile network topology

l MAC dealswith unidirectional links

l nodes are mostly rely on batteries, which requre energy

conservation design

l Security

l open media upon attacks


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Review: Typical MAC protocolsReview: Typical MAC protocols

l

ALOHAl Pure ALOHA, max channel utilization 18.4%

l Slotted ALOHA, double max channel utilization

l CSMA

l Carrier Sense Multiple Accessl Listen Before Talk, LBT

l CSMA-series


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Review:Review:CSMACSMA

l Carrier Sense Multiple Accessl Every station senses the carrier before

transmitting

l If channel appears free

l Transmit (with a certain probability)l Otherwise, wait for some time and try again

l Different CSMA protocols:

l Sending probabilities

l Retransmission mechanisms


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Review: Ingredients of CSMAReview: Ingredients of CSMA

l Carrier sense (CS)l Hardware capable of sensing whether transmission taking

place in vicinity

l Collision detection (CD)l Hardware capable of detecting collisions

l Collision avoidance (CA)l Protocol for avoiding collisions

l Acknowledgmentsl When collision detection not possible, link-layer

mechanism for identifying failed transmissions

l

Backoff mechanisml Method for estimating contention and deferring

transmissions


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Node ModelNode Model

l

Carrier sensing rangel nodes in the carrier sensing zone can sense a transmission, but

cannot decode packet correctly

l Transmission range

l nodes in transmission range can receive and decode packet

correctly.


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Hidden Terminal ProblemHidden Terminal Probleml CSMA for Ad Hoc Networks

l In CSMA, sender decides to transmit based on carrier strength in its vicinityl Collisions occur at the receiver

l Carrier strengths at sender and receiver may be different:

l Node B can communicate with A and C both

l A and C cannot hear each other

l When A transmits to B, C cannot detect the transmission using the

carrier sensemechanisml If C transmits, collision will occur at node B

A B C

Hidden Terminal


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Exposed Terminal ProblemExposed Terminal Problem

A B C

Exposed Terminal

D

l CSMA for Ad Hoc Networks

l In CSMA, sender decides to transmit based on carrier strength inits vicinity

l Collisions occur at the receiver

l Carrier strengths at sender and receiver may be different:

l Node B can communicate with A and C both

l

A and C cannot hear each otherl When A transmits to B, C cannot detect the transmission using

the carrier sensemechanism

l If C transmits, collision will occur at node B


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Lecture 3Lecture 3




l 2.3.1 Contention-Based MAC Protocols

l 2.3.2 Contention-Based MAC Protocols with

Reservation Mechanisms

l 2.3.3 MAC Protocols Using Directional Antennas

l 2.3.4 Multiple-Channel MAC Protocolsl 2.3.5 Power-Aware or Energy-Efficient MAC

Protocols

l 2.4 Summary


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Classification of MAC ProtocolsClassification of MAC Protocols


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Other ClassificationsOther Classifications

l

Energy-efficientl Qos-aware

l Equiped With directional antennas

l Support unidirectional links

l Support multiple channels

l single channel vs. multiple channels


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Lecture 3Lecture 3

l

Chapter 2. MAC Layer Protocolsl 2.1 Introduction



l 2.3.1 Contention-Based MAC Protocolsl 2.3.2 Contention-Based MAC Protocols with



l 2.3.4 Multiple-Channel MAC Protocols

l 2.3.5 Power-Aware or Energy-Efficient MAC

Protocols

l 2.4 Summary


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2.3.1 Contention2.3.1 Contention--Based MACBased MACl Random Access Protocols

l ALOHAl a node may access the channel as soon as it is ready

l ALOHA is more suitable under low system loads with alarge number of potential senders

l Slotted ALOHA

l introduces synchronized transmission time slots similarto TDMA

l nodes can transmit only at the beginning of a time slot,doubles the throughput as compared to the pure ALOHAscheme

l CSMA-based schemes furtherl reduce the possibility of packet collisions and improve

the throughput.


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Lecture 3Lecture 3

l

Chapter 2. MAC Layer Protocolsl 2.1 Introduction



l 2.3.1 Contention-Based MAC Protocolsl 2.3.2 Contention-Based MAC Protocols with



l 2.3.4 Multiple-Channel MAC Protocols

l 2.3.5 Power-Aware or Energy-Efficient MAC

Protocols

l 2.4 Summary


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2.3.2 Contention2.3.2 Contention--Based MAC ProtocolsBased MAC Protocolswith Reservation Mechanismswith Reservation Mechanisms

l Motivation

l To solve the hidden and exposed-terminal problems in CSMA

l use the request-to-send/clear-to-send (RTS/CTS) control packets

to prevent collisions

l Dynamic Reservationl The dynamic reservation approach involves setting up some sort

of a reservation prior to data transmission.

l sender-initiated protocol: a node that wants to send data takes

the initiative of setting up this reservation

l receiver-initiated protocol: the receiving node polls a potentialtransmitting node for data


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Multiple Access Collision AvoidanceMultiple Access Collision Avoidance

l

MACA (Multiple Access Collision Avoidance )l When node A wants to send a packet to node B,

node A first sends a Request-to-Send (RTS) to B

l On receiving RTS, node B responds by sending

Clear-to-Send (CTS), provided node B is able to

receive the packet

l When a node (such as C) overhears a CTS, it

keeps quiet for the duration of the transfer

l Transfer duration is included in RTS and CTS both


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MACA in ActionMACA in Action

l

If C also transmits RTS, collision at B

A B CRTS


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l

C knows the expected DATA length fromCTS

A B CCTS

Defers until DATA

completion


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l

Avoids the hidden terminal problem

A B CDATA


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l

CTS packets have fixed size

A B C DRTS

Defers until CTS


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l

C does not hear a CTS

A B C DCTS


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l

C is free to send to D; no exposed terminal

A B C DDATA


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l

Is C really free to send to D?

A B C DDATA RTS


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l

In fact, C increases its backoff counter!

A B C DDATA CTS


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IEEE 802.11 standardIEEE 802.11 standard

l 802.11 refers to a set of WLANs that was approved

by IEEE in 1997.

l Specifies the lowest two layers of the OSI model.

Standard Data ratePhysical

LayerOperatingFrequency

802.11 1-2 Mbps FHSS/DSSS 2.4 GHz

802.11b

11 Mpbs

Max.

DSSS/HR-

DSSS 2.4 GHz

802.11a

54 Mbps

Max. OFDM 5.5 GHz

802.11g 54 MbpsMax. OFDM 2.4 GHz


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Modes of operationModes of operation

1. Infrastructure-based:l The main most mature

technology for WLANs

l Most commonly used toconstruct Wi-Fi hotspots

l Costly for dynamicenvironments

2. Infrastructureless-based:l Also called Ad Hoc

mode

l Stations form anIndependent BasicService Set (IBSS)

l

Any stations within thesame transmissionrange can communicate


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IEEE 802.11 ArchitectureIEEE 802.11 Architecturel Physical Layer:

l infrared, FHSS, or DSSS in 1997l OFDM and HR-DSSS were added

in 1999

l MAC Layer:

1. Distributed Coordination Function

(DCF):l Provides the basic access method

to the 802.11 MAC protocol

l Based on CSMA/CA

l Uses random backoff timefollowing a busy signal

2. Point Coordination Function (PCF):

l Based on polling scheme

l Only used in infrastructure-based


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IEEE 802.11IEEE 802.11

l IEEE 802.11 was first designed for wirelessfixed networks

l Many problems occur when building ad hocnetworks with the IEEE 802.11 standard asthe lowest two layers

l Solutions for different problems were studied

l Until now, IEEE 802.11 doesnt functionwell in wireless ad hoc netwroks


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IEEE 802.11 Wireless MACIEEE 802.11 Wireless MAC

l Distributed and centralized MAC components

l Distributed Coordination Function (DCF)

l Point Coordination Function (PCF)

l DCF is suitable for multi-hop ad hoc networking

l DCF is a Carrier Sense Multiple Access/Collision

Avoidance (CSMA/CA) protocol


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DCF Basic Access MechanismDCF Basic Access Mechanism

l Uses CSMA/CA mechanism to check if medium is idle or busy.

l If idle -> wait forDistributed InterFrame Space(DIFS), then send

l If busy -> use the random backoff time

l Backoff timer is decreased if the channel is idle and reactivated if

busy


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Uses RTSUses RTS--CTSCTS

l Uses RTS-CTS exchange to avoid hidden terminal

problem

l Any node overhearing a CTS cannot transmit for the

duration of the transfer

l Any node receiving the RTS cannot transmit for theduration of the transfer

l To prevent collision with ACK when it arrives at the sender

l When B is sending data to C, node A will keep quite

A B C


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Uses ACK to achieve reliabilityUses ACK to achieve reliabilityl Wireless links are prone to errors. High packet loss rate

detrimental to transport-layer performance.

l Mechanisms are needed to reduce packet loss rateexperienced by upper layers

l

When node B receives a data packet from node A, node Bsends an Acknowledgement (Ack). This approach adopted inmany protocols

l If node A fails to receive an Ack, it will retransmit the packet

A B C


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Collision AvoidanceCollision Avoidance

l With half-duplex radios, collision detection is not possible

l CSMA/CA: Wireless MAC protocols often use collisionavoidancetechniques, in conjunction with a (physical orvirtual) carrier sensemechanism

l Carrier sense: When a node wishes to transmit a packet,it first waits until the channel is idle.

l Collision avoidance: Nodes hearing RTS or CTS staysilent for the duration of the corresponding transmission.

Once channel becomes idle, the node waits for arandomly chosen duration before attempting to transmit.


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Avoiding collisions (more)

idea: allow sender toreserve channel rather than randomaccess of data frames: avoid collisions of long data frames

l sender first transmits smallrequest-to-send (RTS) packets to BS

using CSMA

l RTSs may still collide with each other (but theyre short)

l BS broadcasts clear-to-send CTS in response to RTSl CTS heard by all nodes

l sender transmits data frame

l other stations defer transmissions

avoid dat a f r ame coll isions complet ely

using small r eser vat ion packet s!


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Collision Avoidance: RTS-CTS exchange

AP

A B

t ime

DATA (A)

reservation collision

def er


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IEEE 802.11IEEE 802.11

C FA B ED

RTS

Pretending a circular range

RTS = Request-to-Send


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IEEE 802.11IEEE 802.11

C FA B ED

RTS

NAV = 10

RTS = Request-to-Send

NAV = Network Allocation Vector,

remaining duration to keep quiet


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IEEE 802.11IEEE 802.11

C FA B ED

CTS

CTS = Clear-to-Send


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IEEE 802.11IEEE 802.11

C FA B ED

CTS

NAV = 8

CTS = Clear-to-Send


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IEEE 802.11IEEE 802.11

DATA packet follows CTS. Successful data

reception acknowledged usingACK.

C FA B ED

DATA


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IEEE 802.11IEEE 802.11

C FA B ED

ACK


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IEEE 802.11IEEE 802.11

C FA B ED

ACK

Reserved area


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IEEE 802.11IEEE 802.11

C FA B ED

DATA

Transmit range

Interference

range

Carrier senserange

FA


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CSMA/CACSMA/CA

l Physical carrier sense, and

l Virtual carrier sense using Network Allocation Vector

(NAV), a counter

l NAV is updated based on overheard

RTS/CTS/DATA/ACK packets, each of which

specified duration of a pending transmissionl Nodes stay silent when carrier sensed

(physical/virtual)

l Backoff intervalsused to reduce collision probability


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CSMA/CACSMA/CA

l Add carrier sense; C will sense Bs

transmission and refrain from sending RTS

A B C DDATA


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Backoff IntervalBackoff Interval

l When transmitting a packet, choose a backoffinterval in the range [0,cw]l cw is contention window

l Count down the backoff interval when

medium is idlel Count-down is suspended if medium becomes

busy

l When backoff interval reaches 0, transmitRTS


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DCF ExampleDCF Example

data

wait

B1 = 5

B2 = 15

B1 = 25

B2 = 20

data

wait

B1 and B2 are backoff intervals

at nodes 1 and 2cw = 31

B2 = 10


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Protocol OverheadProtocol Overhead

l The time spent counting down backoff

intervals is a part of MAC overhead

l Choosing a large cwleads to large backoff

intervals and can result in larger overhead

l Choosing a small cwleads to a largernumber of collisions (when two nodes count

down to 0 simultaneously)


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Binary Exponential Backoff in DCFBinary Exponential Backoff in DCF

l IEEE 802.11 DCF: contention window cwis chosen

dynamically depending on collision occurrence

l When a node fails to receive CTS in response to its

RTS, it increases the contention window

l cwis doubled (up to an upper bound)

l When a node successfully completes a data transfer,

it restores cwto Cwmin

l cwfollows a sawtooth curve


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MILD Algorithm in MACAWMILD Algorithm in MACAW

l MACAW (Medium Access Collision Avoidance

Wireless)

l When a node successfully completes a transfer,

reduces cwby 1

l In 802.11 cw is restored to cwmin

l In 802.11, cw reduces much faster than it increases

l MACAW: cw reduces slower than it increases

Exponential Increase Linear Decrease

l MACAW can avoid wild oscillations of cw when largenumber of nodes contend for the channel


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Contention Resolution OverheadContention Resolution Overhead

l Channel contention resolved using backoff

l Nodes choose random backoff interval from [0,

CW]

l Count down for this interval before transmission

l Backoff and (optional) RTS/CTS handshake

before transmission of data packet

Randombackoff Data Transmission/ACKRTS/CTS


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Inefficiency of IEEE 802.11Inefficiency of IEEE 802.11

l Backoff interval should be chosen

appropriately for efficiency

l Backoff interval with 802.11 far from optimum


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ObservationObservation

l Backoff and RTS/CTS handshake are

unproductive:

l Do not contribute to throughput

Random

backoffData Transmission/ACKRTS/CTS

Unproductive


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HomeworkHomework

l Reading paper

l Performance Issues with IEEE 802.11 in Ad hoc

Networking, IEEE Communication Magazine, July

2005

l Question

l Explain the working procedures of IEEE 802.11

protocol operation in Figure. 1

l What are the shortages of IEEE 802.11

performance, and how does the author prove

them in the simulation results?l What are the possible solutions?


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Thank you!Thank you!

LIU Wei, Ph.dAssociate Professor

ITEC Center, EI@HUSTMobile: 13986224922

Email: [email protected]

Homepage: http://itec.hust.edu.cn
mailto:[email protected]://itec.hust.edu.cn/http://itec.hust.edu.cn/mailto:[email protected]

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