Physical and Data Link Layer

7/29/2019 Physical and Data Link Layer

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COMPUTER

NETWORKS

ByK.EUGINE RAJAP/SCAD Engg College


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UNIT IPHYSICAL LAYER


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DATACOMMUNICATIONS

Sharing or conveying ofinformation is calledCommunication.

Data communications systemdepends on four fundamentalcharacteristics;

1. Delivery

2. Accuracy

3.Timeliness


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Delivery:-The system mustdeliver data to the correctdestination.

Accuracy:-The system mustdeliver the data accurately.

Timeliness:-The system mustdeliver data in a timely manner.

Jitter:- Jitter refers to the

variation in the packet arrival


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omponents o DataCommunication

Message

Sender

Receiver

Transmission medium


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Data Representation

Information today comes indifferent forms such as text,

numbers, images, audio,and video.


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Data Flow

Communication between twodevices can be simplex, half-duplex, or full-duplex.


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Simplex:- The communication isunidirectional, as on a one-waystreet. Only one of the two deviceson a link can transmit.

Half-Duplex:- Each station cantransmit and receive, but not at thesame time. When one device issending, the other can only receive,

and vice versa.Full-Duplex:- Both stations can

transmit and receive data

simultaneously. The full-duplex


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NETWORKSA network is a set of devices (often

referred to as nodes) connected bycommunication links.


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Physical TopologyMesh:

In a mesh topology, every devicehas a dedicated point-to-point linkto every other device.

S T l


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Star Topology:In a star topology, each device

has a dedicated point-to-pointlink only to a central controller,usually called a hub.


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Bus Topology:A bus topology, on the

other hand, is multipoint.One long cable acts as abackbone to link all the

devices in a network.


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Ring Topology:

In a ring topology, each device hasa dedicated point-to-pointconnection with only the twodevices on either side of it.


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Hybrid Topology:


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Network Models

Computer networks are createdby different entities.

The two best-known standards

are the OSI model and theInternet model.

C t i f N t k


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Categories of Networks

Local Area Network

A local area network (LAN) is usuallyprivately owned and links the devicesin a single office, building, or campus


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Wide Area NetworkA wide area network (WAN) provides

long-distance transmission of data,image, audio, and video informationover large geographic areas that maycomprise a country, a continent, oreven the whole world.


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Metropolitan AreaNetworksA metropolitan area network (MAN)

is a network with a size between aLAN and a WAN. It normally covers

the area inside a town or a city.Global Area Networks

A network that composed of

different interconnected computernetwork which covers thegeographical area is called GAN.

GAN operates from 1.5Mbps to


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OSI MODEL

It was first introduced in the late1970s.

Open system is a set of

protocols that allows any twodifferent systems tocommunicate regardless of their

underlying architecture.The OSI model is not a protocol;it is a model for understanding

and designing a network


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OSI MODEL


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Peer-to-Peer Processes


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MODEL

Physical LayerThe physical layer coordinates thefunctions required to carry a bitstream over a physical medium


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Physical characteristics ofinterfaces and medium.

Representation of bits.

Data rate.

Synchronization of bits.

Line configuration.Physical topology.

Transmission mode.


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Data Link Layer

It makes the physical layerappear error-free to the upperlayer (network layer).


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Responsibilities of the datalink layerFraming.Physical addressing.

Flow control.Error control.

Access control.

N t k L


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Network LayerThe network layer is responsible

for the source-to-destinationdelivery of a packet, possiblyacross multiple networks. Whereasthe data link layer oversees the

delivery of the packet between twosystems on the same network.

Responsibilities of the


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Responsibilities of thenetwork layerLogical addressing.

-The physical addressingimplemented by the data link layerhandles the addressing problemlocally.

Routing.

-When independent networks orlinks are connected to create internetworks (network of networks) or alarge network, the connectingdevices (called routers or switches)


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Transport LayerThe transport layer is responsible

for process-to-process delivery ofthe entire message.

R ibiliti f th


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Responsibilities of thetransport layer

Service-point addressing.

Segmentation and reassembly.

Connection control.

Flow control.

Error control.


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Session LayerIt establishes, maintains, and

synchronizes the interaction amongcommunicating systems.

Respons t es o t e


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Dialog control.

Synchronization.

Respons t es o t eSession layer


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Presentation Layer

The presentation layer isconcerned with the syntax andsemantics of the informationexchanged between two systems.

Respons t es o t e


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Translation.

Encryption.

Compression.

Respons t es o t ePresentation layer


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Application LayerThe application layer enables the user,

whether human or software, to accessthe network.

It provides user interfaces and supportfor services such as electronic mail,remote file access and transfer, shareddatabase management, and other typesof distributed information services.

Respons t es o t e


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Network virtual terminal.

File transfer, access, andmanagement.

Mail services.

Directory services.

Respons t es o t eApplication layer

Comparison of Protocol


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Comparison of ProtocolTCP/IP PROTOCOL SUITE

The TCPIIP protocol suite wasdeveloped prior to the OSI model.

ys ca an a a n


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ys ca an a a nLayersAt the physical and data link

layers, TCPIIP does not define anyspecific protocol.

It supports all the standard and

proprietary protocols. A network ina TCPIIP internetwork can be alocal-area network or a wide-areanetwork.

N t k L


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Network LayerAt the network layer (or, more

accurately, the internetwork layer),TCP/IP supports the InternetworkingProtocol.

IP, in turn, uses four supportingprotocols: ARP, RARP, ICMP, andIGMP.


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Transport Layer

Traditionally the transport layerwas represented in TCP/IP by twoprotocols: TCP and UDP.

The User Datagram Protocol (UDP)is a process-to-process protocolthat adds only port addresses,

checksum error control, and lengthinformation to the data from theupper layer.

The Transmission Control Protocol


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Application Layer

The application layer in TCPIIP isequivalent to the combinedsession, presentation, and

application layers in the OSImodel.


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ADDRESSINGFour levels of addresses are used

in an internet employing the TCP/IPprotocols: physical (link) addresses,logical (IP) addresses, portaddresses, and specific addresses

Relationship of layers and


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Relationship of layers andaddresses in TCPIIP

Ph i l Add


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Physical Addresses

The physical address, also knownas the link address, is the addressof a node as defined by its LAN or

WAN.It is included in the frame used

by the data link layer.

It is the lowest-level address.


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Logical Addresses

Logical addresses are necessaryfor universal communications thatare independent of underlyingphysical networks.

Physical addresses are notadequate in an internetworkenvironment where differentnetworks can have differentaddress formats.

P Add


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Port Addresses

The IP address and the physical

address are necessary for aquantity of data to travel from asource to the destination host.

However, arrival at the destinationhost is not the final objective ofdata communications on the

Internet.A port address in TCPIIP is 16 bits

in length.


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Specific Addresses

Some applications have user-friendly addresses that aredesigned for that specific

address.Examples include the e-mail

address and Universal Resource

Locator

TRANSMISSION MEDIA


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TRANSMISSION MEDIATransmission media are actually

located below the physical layerand are directly controlled by thephysical layer.

You could say that transmissionmedia belong to layer zero.

In telecommunications,

transmission media can be dividedinto two broad categories: guidedand unguided.

Guided media include twisted-pair-

Twisted Pair Cable


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Twisted-Pair CableA twisted pair consists of two

conductors (normally copper), eachwith its own plastic insulation,twisted together.

One of the wires is used to carrysignals to the receiver, and theother is used only as a groundreference. The receiver uses the

difference between the two.

Shielded Twisted Pair


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Shielded Twisted-PairCableThe most common twisted-pair

cable used in communications isreferred to as unshielded twisted-pair

Coaxial Cable


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Coaxial CableCoaxial cable (or coax) carries

signals of higher frequency rangesthan those in twisted pair cable, inpart because the two media areconstructed quite differently.

Instead of having two wires, coaxhas a central core conductor ofsolid or stranded wire (usually

copper) enclosed in an insulatingsheath.

Fiber-Optic Cable


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Fiber-Optic CableA fiber-optic cable is made of glass

or plastic and transmits signals inthe form of light.

Total internal Reflection.

UNGUIDED MEDIA


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UNGUIDED MEDIAWIRELESS

Unguided media transportelectromagnetic waves withoutusing a physical conductor.

This type of communication isoften referred to as wirelesscommunication.

Electromagnetic spectrum forwireless communication


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Types of Wireless system

RADIO waves

-Ground wave

-Sky wave

Microwaves

-Cellular Communication

-Ultra High FrequencyCommunication

-Satellite Communication


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Radio Wave


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Microwave

WIT HIN


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WIT HINA network is a set of connected

devices. Whenever we have

multiple devices, we have theproblem of how to connect them tomake one-to-one communicationpossible.

A better solution is switching.

A switched network consists of a

series of interlinked nodes, calledswitches.

Figure shows a switched


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Figure shows a switchednetwork

Taxonomy of switched


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Taxonomy of switchednetworks

-


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NETWORKSA circuit-switched network is made

of a set of switches connected byphysical links, in which each link isdivided into n channels.


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DATAGRAM NETWORKSIn data communications, we need

to send messages from one endsystem to another.

If the message is going to passthrough a packet-switched network,it needs to be divided into packetsof fixed or variable size.

The allocation is done on a firstcome, first-served basis.

When a switch receives a packet,no matter what is the source or

In this example all four packets (or


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In this example, all four packets (ordatagrams) belong to the samemessage, but may travel differentpaths to reach their destination.

VIRTUAL-CIRCUIT


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VIRTUAL CIRCUITNETWORKS

A virtual-circuit network is a cross

between a circuit-switched networkand a datagram network. It hassome characteristics of both.


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Data Transmission

Telephone networks were originallycreated to provide voicecommunication.

The need to communicate digitaldata resulted in the invention of thedial-up modem.

With the advent of the Internetcame the need for high-speeddownloading and uploading.

The telephone companies added a


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DIALUP MODEMS

Traditional telephone lines cancarry frequencies between 300and 3300 Hz, giving them a

bandwidth of 3000 Hz.In general, we can say that the

signal bandwidth must be smaller

than the cable bandwidth.The effective bandwidth of a

telephone line being used for

data transmission is 2400 Hz,

Modem


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ModemModem stands for modulator/

demodulatorA modulator creates a bandpass

analog signal from binary data. A

demodulator recovers the binarydata from the modulated signal.


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LINEDSL is developed to provide higher-

speed access to the Internet.DSL technology is a set of

technologies, each differing in the

first letter (ADSL, VDSL, HDSL, andSDSL).

The set is often referred to as xDSL,

where xcan be replaced by A, V, H,or S.

ADSL(ASYMMETRIC DIGITAL


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ADSL(ASYMMETRIC DIGITALSUBSCRIBER LINE)

ADSL, like a 56K modem, provideshigher speed (bit rate) in thedownstream direction (from theInternet to the resident) than in the

upstream direction (from theresident to the Internet).

ADSL is an asymmetric

communication technology designedfor residential users; it is not suitablefor businesses.

ADSL


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ADSLUsing Existing Local Loops

One interesting point is that ADSLuses the existing local loops.

The twisted-pair local loop is

actually capable of handlingbandwidths up to 1.1 MHz, but thefilter installed at the end office of

the telephone company where eachlocal loop limits the bandwidth to 4kHz (sufficient for voicecommunication).

ADSL


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ADSLDiscrete Multitone Technique

ADSL modem uses the standardcalled the discrete multitonetechnique (DMT) which combines

QAM and FDM.Each system can decide on its

bandwidth division. Typically, an

available bandwidth of 1.104 MHz isdivided into 256 channels.

Each channel uses a bandwidth of

4.312 kHz, as shown in Figure.

ADSL


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ADSL


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ADSL System

Telephone Company Site:


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p p yDSLAMInstead of an ADSL modem, a

device called a digital subscriberline access multiplexer (DSLAM) isinstalled that functions similarly.

In addition, it packetizes the datato be sent to the Internet (ISPserver).

ADSL Lite


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ADSL LiteA new version of ADSL technology

called ADSL Lite or Universal ADSL orsplitter less ADSL.

This technology allows an ASDL Litemodem to be plugged directly into atelephone jack and connected to thecomputer.

The splitting is done at the telephonecompany.

ADSL Lite uses 256 DMT carriers with8-bit modulation.

HDSL


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HDSLThe high-bit-rate digital subscriber

line (HDSL) was designed as analternative to the T-l line (1.544Mbps).

The T-1line uses alternate markinversion (AMI) encoding, which isvery susceptible to attenuation athigh frequencies.

This limits the length of a T-l line to3200 ft (1 km).

For longer distances, a repeater is

SDSL


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SDSLIt provides full-duplex

symmetric communicationsupporting up to 768 kbps ineach direction.

SDSL, which provides symmetriccommunication, can beconsidered an alternative to

ADSL.

ADSL provides asymmetric

communication, with a

S


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VDSL

The very high-bit-rate digitalsubscriber line (VDSL), analternative approach that is similar

to ADSL, uses coaxial, fiber-optic,or twisted-pair cable for shortdistances.

It provides a range of bit rates (25to 55 Mbps) for upstreamcommunication at distances of

3000 to 10,000 ft. The downstream

Comparison of xDSL


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Comparison of xDSLtechnologies

CABLE TV NETWORKS


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CABLE TV NETWORKS

The cable TV network startedas a video service provider,but it has moved to the

business of Internet access.In this section, we discusscable TV networks per se; infuture we discuss how thisnetwork can be used to

provide high-speed access to Traditional Cable


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NetworksCable TV started to distribute

broadcast video signals tolocations with poor or noreception in the late 1940s.

It was called communityantenna TV (CATV) because anantenna at the top of a tall hill

or building received the signalsfrom the TV stations anddistributed them, via coaxial

Traditional cable TVk


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networkCommunication in the traditional

cable TV network is unidirectional

Hybrid Fiber-Coaxial


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y(HFC) Network

The second generation of cable

networks is called a hybrid fiber-coaxial (HFC) network.

The network uses a combinationof fiber-optic and coaxial cable.

The transmission medium from

the cable TV office to a box,called the fiber node, is opticalfiber; from the fiber node

through the neighborhood and

Hybridfiber-coaxial (HFC)


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Hybridfiber coaxial (HFC)network

Hybridfiber-coaxial (HFC)


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Hybridfiber coaxial (HFC)network

The regional cable head (RCH)normally serves up to 400,000subscribers.

The RCHs feed the distributionhubs, each of which serves up to40,000 subscribers.

Communication in an HFC cableTV network can be bidirectional.

Each coaxial cable serves up to

1000 subscribers.

CABLE TV FOR DATA


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TRANSFERCable companies are now

competing with telephonecompanies for the residentialcustomer who wants high-speed

data transfer.DSL technology provides high-data-

rate connections for residential

subscribers over the local loop.However, DSL uses the existing

unshielded twisted-pair cable, which

is very susceptible to interference.

Division of coaxial cable


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Division of coaxial cableband by CATV

Downstream Video Band


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Downstream Video Band

-The downstream video band occupies

frequencies from 54 to 550 MHz. Sinceeach TV channel occupies 6 MHz, thiscan accommodate more than 80channels.

Downstream Data Band

-The downstream data (from the

Internet to the subscriber premises)occupies the upper band, from 550 to750 MHz.

Upstream Data Band

Cable modem (CM)


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Cable modem (CM)


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UNIT 2

DATA LINK LAYER

Data Link Control


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Data Link ControlThe two main functions of the

data link layer are data linkcontrol and media accesscontrol.

Data link control functionsinclude framing, flow and error

control, and softwareimplemented protocols thatprovide smooth and reliable

transmission of frames between

FRAMING


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Data transmission in the physical

layer means moving bits in the formof a signal from the source to thedestination.

The physical layer provides bitsynchronization to ensure that thesender and receiver use the samebit durations and timing.

The data link layer, on the otherhand, needs to pack bits into frames,so that each frame is distinguishable

from another.

The simple act of inserting a letter


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The simple act of inserting a letterinto an envelope which serves as the

delimiter.Each envelope defines the sender

and receiver addresses like postal

system.Framing in the data link layer

separates a message from one

source to a destination, or fromother messages to otherdestinations,.

The destination address defines

Framing size


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gFrames can be of fixed orvariable size.

Fixed-Size Framing

In fixed-size framing, there is no

need for defining the boundariesof the frames; the size itself canbe used as a delimiter.

Variable-Size Framing

We need a way to define the

end of the frame and the

Variable size framing


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gTwo approaches were used.

1. Character-Oriented Protocols2. Bit-oriented approach

Character-Oriented Protocols

. Data to be carried are 8-bitcharacters from a coding systemsuch as ASCII.

.The header, which normally carriesthe source and destinationaddresses and other control

information and the trailer which

-Protocols


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ProtocolsTo separate one frame from the

next, an 8-bit flag is added.

Character-oriented framing wasl h l t t


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popular when only text wasexchanged by the data link layers.

If we send other types ofinformation such as graphs, audio,and video.

The pattern used for the flag couldalso be part of the information.

To fix this problem, a byte-stuffingstrategy was added to character-oriented framing.

In byte stuffing (or character

Bit Oriented Protocols


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Bit-Oriented Protocols

It uses a special 8-bit pattern flag01111110 as the delimiter todefine the beginning and the endof the frame.

A stuffing of single bit (instead ofI byte) to prevent the patternfrom looking like a flag is usedhere.

FLOW AND ERROR


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CONTROL

Data communication requires atleast two devices workingtogether, one to send and theother to receive.

Even system needs acoordination for an intelligibleexchange to occur.

The most importantresponsibilities of the data linklayer are flow control and error

Flow Control


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Flow Control

Flow control coordinates theamount of data that can be sentthrough the data link layer.

Flow control refers to a set ofprocedures used to restrict theamount of data that the sendercan send before waiting foracknowledgment.

Error Control


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Error Control

Error control is both errordetection and errorcorrection.

Error control in the datalink layer is based on

automatic repeat request,which is theretransmission of data.

PROTOCOLSThe data link layer can combine


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The data link layer can combineframing, flow control, and error

control to achieve the delivery ofdata from one node to another.

All the protocols we discuss are

unidirectional.In a real-life network, the data link

protocols are implemented as

bidirectional.In these protocols the flow and error

control information such as ACKs and

NAKs is included in the data frames

NOISELESS CHANNELS


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NOISELESS CHANNELS

Assume an ideal channel inwhich no frames are lost.

We introduce two protocols

for this type of channel.

The first is a protocol that

does not use flow control; thesecond is the one that does.

Simplest Protocol


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pIt has no flow or error control.

it is a unidirectional protocol.Design

-As there is no need for flow control

the data link layer at the sender sitegets data from its network layer,makes a frame out of the data, and

sends it.-The data link layer at the receiversite receives a frame from its physical

layer extracts data from the frame

Design


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Design

Flow diagram


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Flow diagram

Stop-and-Wait Protocol


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If data frames arrive at the

receiver site faster than theycan be processed, the framesmust be stored until their use.

Normally, the receiver does nothave enough storage space,especially if it is receiving data

from many sources.This may result in either thediscarding of frames or denial of



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To prevent the receiver frombecoming overwhelmed withframes, we somehow need to tellthe sender to slow down.

There must be feedback from thereceiver to the sender.

Stop-and-Wait Protocol becausethe sender sends one frame,stops until it receivesconfirmation from the receiver

Design


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Design

Flow diagram


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Flow diagram

NOISY CHANNELS


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NOISY CHANNELS

Although the Stop-and-WaitProtocol gives us an idea of howto add flow control to itspredecessor, noiseless channelsare nonexistent.

We can ignore the error (as wesometimes do), or we need toadd error control to our protocols.

We discuss three protocols in thissection that use error control.

Stop-and-Wait Automatic


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Repeat Request

Stop-and-Wait Automatic RepeatRequest (Stop-and Wait ARQ),adds a simple error controlmechanism to the Stop-and-WaitProtocol.

Design


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Design


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Go-Back-N AutomaticR R


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Repeat Request

To improve the efficiency oftransmission (filling the pipe),multiple frames must be intransition while waiting foracknowledgment.

In this protocol we can sendseveral frames before receivingacknowledgments.

Frames from a sending stationare numbered sequentially.

Design


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Flow diagram


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e ect ve RepeatAutomatic Repeat


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Automatic RepeatRequest

Go-Back-N ARQ simplifies theprocess at the receiver site.

The receiver keeps track of only

one variable, and there is noneed to buffer out-of-orderframes.

In a noisy link a frame has ahigher probability of damage,which means the resending ofmultiple frames.

e ect ve RepeatAutomatic Repeat


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Automatic RepeatRequest

Instead of sending N frameswhen just one frame is damaged;only the damaged frame isresent.

This mechanism is calledSelective Repeat ARQ.

Design


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Flow diagram


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Piggybacking


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The three protocols we discussed in

this section are all unidirectional:although control information such asACK and NAK frames can travel inthe other direction.

In real life, data frames are normallyflowing in both directions.

So control information needs toflow in both directions.

A technique called piggybacking is

used to improve the efficiency of the

es gn o p ggy ac ng nGo-Back-NARQ


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Q

HDLCi h l l i k l ( )


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High-level Data Link Control (HDLC)

is a bit-oriented protocol forcommunication over point-to-pointand multipoint links.

It implements the ARQ mechanisms.HDLC provides two common

transfer modes that can be used in

different configurations.Normal response mode (NRM) and

Asynchronous balanced mode

(ABM)

Normal Response Mode


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p

In normal response mode (NRM),the station configuration isunbalanced.

We have on primary station andmultiple secondary stations.

A primary station can send

commands; a secondary stationcan only respond.

Normal Response Mode


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p

sync ronous a anceModeI h b l d


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In asynchronous balanced

mode (ABM), the configurationis balanced. The link is point-to-point, and each station can

function as a primary and asecondary .

Frames


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Flag field. The flag field is an 8-bitsequence with the bit pattern01111110 h id ifi b h h


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01111110 that identifies both the

beginning and the end of a frameand serves as a synchronization.

Address field. The second field of

an HDLC frame contains theaddress of the secondary station.

Primary station contains to address

& secondary creates a fromaddress.

Control field. The control field is a1 2 b t t f th f


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1- or 2-byte segment of the frame

used for flow and error control.Information field. The information

field contains the user's data from

the network layer or managementinformation.

FCS field. The frame check

sequence (FCS) is the HDLC errordetection field. It can contain eithera 2- or 4-byte ITU-T CRC.

Control Field


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I-frames are designed to carryuser data from the network layer.


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use da a o e e o ayeIn addition, they can include flow

and error control information

Supervisory frames are used forflow and error control wheneverpiggybacking is either impossibleor inappropriate.

S-frames do not have informationfields.Unnumbered frames are used toexchange session management

MULTIPLE ACCESSThe data link layer has two


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The data link layer has twosublayers.

The upper sublayer is responsiblefor data link control, and the lowersublayer is responsible for

resolving access to the sharedmedia.


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The upper sublayer that isresponsible for flow and errorcontrol is called the logical linkcontrol (LLC) layer; the lowersublayer that is mostlyresponsible for multiple accessresolution is called the media

access control (MAC) layer.

MULTIPLE ACCESSPROTOCOL


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PROTOCOL

RANDOM ACCESSThe access to the medium from


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The access to the medium frommany entry is called contention.

It is controlled with a contentionprotocol.

In random access method eachstation has the right to the mediumwithout being controlled by anyother station.

If more than one station tries tosend, there is an accessconflict(collision and distortion).

ALOHA


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ALOHA, the earliest randomaccess method, was developed atthe University of Hawaii in early1970.

It was designed for a radio(wireless) LAN, but it can be usedon any shared medium.

Pure ALOHA


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The original ALOHA protocol iscalled pure ALOHA.

This is a simple, but elegant

protocol.The idea is that each station

sends a frame whenever it has a

frame to send.

Pure ALOHA


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Some of these frames collidebecause multiple frames are incontention for the shared channel


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contention for the shared channel.

We need to resend the frames thathave been destroyed duringtransmission.

The pure ALOHA protocol relies onacknowledgments from thereceiver.

If the acknowledgment does notarrive after a time-out period, thestation assumes that the frame has

been destroyed and resends the

A collision involves two or morestations


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stations.

If all these stations try to resendtheir frames after the time-out, theframes will collide again.

Pure ALOHA has a second methodto prevent congesting the channelwith retransmitted frames.

After a maximum number ofretransmission attempts Kmax' astation must give up and try later.


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Slotted ALOHAPure ALOHA has a vulnerable time


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Pure ALOHA has a vulnerable timeof 2 x Tfr.

This is so because there is no rulethat defines when the station cansend.

A station may send soon afteranother station has started or soonbefore another station has finished.

Slotted ALOHA was invented toimprove the efficiency of pureALOHA.

Slotted ALOHA


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arr er ense Mu t p eAccess (CSMA)


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To minimize the chance of collision

and, therefore, increase theperformance, the CSMA methodwas developed.

The chance of collision can bereduced if a station senses themedium before trying to use it.

Carrier sense multiple access(CSMA) requires that each stationfirst listen to the medium (or check

the state of the medium) before

Persistence MethodsWhat should a station do if the


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What should a station do if thechannel is busy?

What should a station do if thechannel

is idle?Three methods have been devised

to answer these questions:

the I-persistent method

the non-persistent method and

the p-persistent method.


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I-Persistent


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The I-persistent method issimple and straightforward.

In this method, after the station

finds the line idle, it sends itsframe immediately.

Nonpersistent


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In the nonpersistent method,a station that has a frame to sendsenses the line.

If the line is idle, it sendsimmediately.

If the line is not idle, it waits a

random amount of time and thensenses the line again.

p-Persistent


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The p-persistent method isused if the channel has time slotswith a slot duration equal to orgreater than the maximum

propagation time.

The p-persistent approachcombines the advantages of the

other two strategies.

It reduces the chance of collisionand improves efficiency.

Flow diagram


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arr er ense Mu t p eAccess with CollisionD i (CSMA/CD)


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Detection (CSMA/CD)The CSMA method does not specify

the procedure following a collision.

Carrier sense multiple access with

collision detection (CSMA/CD)augments the algorithm to handlethe collision.

In this method, a station monitorsthe medium after it sends a frameto see if the transmission was

successful

Collision and abortion inCSMA/CD


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CSMA/CD

wCSMA/CD


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Access with CollisionAvoidance (CSMA/CA)The basic idea behind CSMA/CD


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Avoidance (CSMA/CA)The basic idea behind CSMA/CD

is that a station needs to be ableto receive while transmitting todetect a collision.

When there is no collision, thestation receives one signal: itsown signal.

When there is a collision, thestation receives two signals: its

own signal and the signal

The signal from the second

Carrier Sense MultipleAccess with Collision


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The signal from the second

station needs to add asignificant amount of energy tothe one created by the first

station.In a wired network, the receivedsignal has almost the same

energy as the sent signalbecause either the length of thecable is short or there are

Avoidance (CSMA/CA)

Carrier Sense MultipleAccess with Collision


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This means that in a collision, thedetected energy almost doubles.

However, in a wireless network,

much of the sent energy is lost intransmission.

The received signal has very little

energy. Therefore, a collision mayadd only 5 to 10 percentadditional energy.

This is not useful for effective

Avoidance (CSMA/CA)

Timing in CSMA/CA


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Inter frame Space (IFS)


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-First, collisions are avoided by

deferring transmission even if thechannel is found idle.

Contention Window

-The contention window is anamount of time divided into slots.

-A station that is ready to sendchooses a random number of slotsas its wait time.

The number of slots in the window

CONTROLLED ACCESS


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In controlled access, the stationsconsult one another to find whichstation has the right to send.

A station cannot send unless ithas been authorized by otherstations.

We discuss three popularcontrolled-access methods.

ReservationIn the reservation method, a station


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needs to make a reservation before

sending data.

Time is divided into intervals.

In each interval, a reservation frameprecedes the data frames sent inthat interval.

If there are N stations in the system,there are exactly N reservation minislots in the reservation frame.

Each mini slot belongs to a station

Reservation accessmethod


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PollingPolling works with topologies in


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which one device is designatedas a primary station and theother devices are secondarystations.

All data exchanges must bemade through the primary

device even when the ultimatedestination is a secondarydevice.

Select and poll functionsin polling access method


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p g

Token PassingIn the token-passing method


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In the token passing method,

the stations in a network areorganized in a logical ring. Inother words, for each station,

there is a predecessor and asuccessor.

The predecessor is the station

which is logically before thestation in the ring; thesuccessor is the station which is

The current station is the one

Token Passing


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The current station is the one

that is accessing the channelnow.

The right to this access has

been passed from thepredecessor to the currentstation.

The right will be passed to thesuccessor when the currentstation has no more data to

Logical RingIn a token-passing network, stations


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do not have to be physically

connected in a ring; the ring can bea logical one.

CHANNELIZATION


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Channelization relates tobandwidth of a link.

Three channelization protocols are:

FDMA, TDMA, and CDMA.

Frequency-DivisionMultiple Access (FDMA)


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p

In frequency-division multipleaccess (FDMA), the availablebandwidth is divided intofrequency bands.


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T me-D v s on Mu t p eAccess (TDMA)

I i di i i l i l


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In time-division multiple access(TDMA), the stations share thebandwidth of the channel intime.

Each station is allocated a timeslot during which it can send

data.Each station transmits its datain is assigned time slot.


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Code-Division MultipleAccess (CDMA)


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Code-division multiple access(CDMA) was conceived severaldecades ago.

Recent advances in electronictechnology have finally made itsimplementation possible.

CDMA differs from FDMA becauseonly one channel occupies theentire bandwidth of the link.

It differs from TDMA because all

Wired LANs: EthernetLAN can be used as an isolated


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network to connect computers inan organization for the solepurpose of sharing resources,most LANs today are also linkedto a wide area network (WAN) orthe Internet.

The LAN market has seenseveral technologies such asEthernet, Token Ring, Token Bus,

FDDI and ATM LAN

IEEE STANDARDSIn 1985, the Computer Society of


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p y

the IEEE started a project, calledProject 802, to set standards toenable intercommunication amongequipment from a variety of

manufacturers.

Project 802 does not seek toreplace any part of the OSI or the

Internet model.

Instead, it is a way of specifyingfunctions of the physical layer and

IEEE standard for LAN


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IEEE standard for LAN


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The IEEE has subdivided the datalink layer into two sublayers:logical link control (LLC) andmedia access control (MAC).

IEEE has also created severalphysical layer standards fordifferent LAN protocols.

STANDARD ETHERNET


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ETHERNET


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The original Ethernet was createdin 1976 at Xerox's Palo AltoResearch Center (PARC).

Since then, it has gone throughfour generations: StandardEthernet (lot Mbps), Fast Ethernet(100 Mbps), Gigabit Ethernet (l

Gbps), and Ten-Gigabit Ethernet(l0 Gbps).

MAC Sublayer


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In Standard Ethernet, the MACsublayer governs the operation ofthe access method.

It also frames data received fromthe upper layer and passes themto the physical layer.

Frame FormatThe Ethernet frame contains seven

fields: preamble SFD DA SA


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fields: preamble, SFD, DA, SA,

length or type of protocol data unit(PDU), upper-layer data, and theCRC.

Ethernet does not provide anymechanism for acknowledgingreceived frames, making it what is

known as an unreliable medium.

802.3 MACframe


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Preamble. The first field of the802.3 frame contains 7 bytes (56bits) of alternating 0s and 1s thatalerts the receiving system to the

coming frame and enables it tosynchronize its input timing.

The preamble is actually added

at the physical layer and is not(formally) part of the frame.


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Start frame delimiter (SFD). Thesecond field (l byte: 10101011)signals the beginning of theframe.

The SFD warns the station orstations that this is the lastchance for synchronization.

i i dd ( ) h


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Destination address (DA). The DAfield is 6 bytes and contains thephysical address of thedestination station or stations to

receive the packet.

Source address (SA). The SA fieldis also 6 bytes and contains the

physical address of the sender ofthe packet.

h hi fi ld i


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Length or type. This field isdefined as a type field or lengthfield.

The IEEE standard used it as thelength field to define the numberof bytes in the data field.

D Thi fi ld i d


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Data. This field carries dataencapsulated from the upper-layer protocols.

It is a minimum of 46 and amaximum of 1500 bytes, as wewill see later.

CRC. The last field contains errordetection information, in thiscase a CRC-32.

Frame LengthEthernet has imposed restrictions on

both the minimum and maximum


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lengths of a frame, as shown inFigure.

Addressing

E h t ti Eth t


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Each station on an Ethernetnetwork (such as a PC,workstation, or printer) has itsown network interface card (NIC).

The NIC fits inside the station andprovides the station with a 6-bytephysical address.

Unicast and multicastaddressesThe first byte defines the type of

dd


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address.

If the bit is 0, the address is unicast;otherwise, it is multicast.

A unicast destination addressdefines only one recipient; therelationship between the sender and


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the receiver is one-to-one.A multicast destination address

defines a group of addresses; the

relationship between the sender andthe receivers is one-to-many.

The broadcast address is a special

case of the multicast address; therecipients are all the stations on theLAN.

A b d t d ti ti dd i

Access Method: CSMA/CDStandard Ethernet uses I-persistent CSMA/CD


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persistent CSMA/CD.

Slot Time In an Ethernetnetwork, the round-trip time

required for a frame to travelfrom one end of a maximum-length network to the other plus

the time needed to send the jamsequence is called the slot time.

Slot time =round-trip time +

Physical LayerThe Standard Ethernet defines

several physical layer


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implementations; four of the mostcommon, are shown in Figure.

Categories of Standard Ethernet

10Base5: Thick Ethernet

Th fi t i l t ti i ll d


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The first implementation is called10BaseS, thick Ethernet, or

Thicknet.

The nickname derives from thesize of the cable, which is roughlythe size of a garden hose and toostiff to bend with your hands.

10Base5 was the first Ethernetspecification to use a bustopology with an external

10Base5 implementation


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10Base2: Thin EthernetThe second implementation is called

10Base2 thin Ethernet or


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10Base2, thin Ethernet, or

Cheapernet.

10Base2 also uses a bus topology,but the cable is much thinner and

more flexible.

The cable can be bent to pass veryclose to the stations.

In this case, the transceiver isnormally part of the networkinterface card (NIC), which is

10Base2 implementation


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10Base-T: Twisted-PairEthernet


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The third implementation iscalled 10Base-T or twisted-pair Ethernet.

10Base-T uses a physical startopology.

The stations are connected to

a hub via two pairs of twistedcable.

10Base-T implementation


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10Base-F: Fiber Ethernet

Although there are several types


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Although there are several typesof optical fiber 10-Mbps Ethernet,the most common is called10Base-F.

l0Base-F uses a star topology toconnect stations to a hub.

The stations are connected to thehub using two fiber-optic cables.

10Base-F implementation


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CHANGES IN THESTANDARD ETHERNET


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The 10-Mbps StandardEthernet has gone throughseveral changes before

moving to the higher datarates.

These changes actuallyopened the road to theevolution of the Ethernet to

Raising the BandwidthIn an un bridged Ethernet network,

the total capacity (10 Mbps) is


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p y p

shared among all stations with aframe to send; the stations sharethe bandwidth of the network.

Sharing bandwidth

A network with andwithout a bridge


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Separating CollisionDomainsThe advantage of a bridge is


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The advantage of a bridge isseparation of the collisiondomain.

Figure shows the collisiondomains for an unbridged and abridged network.


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Switched Ethernet

The idea of a bridged LAN can be


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The idea of a bridged LAN can beextended to a switched LAN.

Instead of having two to fournetworks,

Why not have N networks, whereN is the number of stations onthe LAN?

Switched Ethernet


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Full-Duplex Ethernet

One of the limitations of 10Base5


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One of the limitations of 10Base5and lOBase2 is thatcommunication is half-duplex(l0Base-T is always full-duplex).

The next step in the evolutionwas to move from switchedEthernet to full-duplex switched

Ethernet.The full-duplex mode increases

the capacity of each domain from

Full-duplex switchedEthernet


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No Need for CSMA/CD

In full-duplex switched Ethernet


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In full duplex switched Ethernet,there is no need for the CSMA/CDmethod.

In a full duplex switchedEthernet, each station isconnected to the switch via twoseparate links.

FAST ETHERNET

Fast Ethernet was designed to


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Fast Ethernet was designed tocompete with LAN protocols suchas FDDI or Fiber Channel.

IEEE created Fast Ethernet underthe name 802.3u.

Fast Ethernet is backward-compatible with StandardEthernet, but it can transmit data10 times faster at a rate of 100Mbps.

Goals of Fast Ethernet

1 Upgrade the data rate to 100


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1. Upgrade the data rate to 100Mbps.

2. Make it compatible withStandard Ethernet.

3. Keep the same 48-bit address.

4. Keep the same frame format.

5. Keep the same minimum andmaximum frame lengths.

MAC SublayerA main consideration in theevolution of Ethernet from 10 to


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evolution of Ethernet from 10 to100 Mbps was to keep the MACsublayer untouched.

But the bus topologies wasdropped and only the startopology was used.

For the star topology, there aretwo choices, as we saw before:half duplex and full duplex.

In the half-duplex approach, the


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In the half duplex approach, thestations are connected via a hub;in the full-duplex approach, theconnection is made via a switch

with buffers at each port.The access method is the same

(CSMA/CD) for the half-duplex

approach; for full duplex FastEthernet, there is no need forCSMA/CD.

AutonegotiationA new feature added to Fast

Ethernet is called autonegotiation.


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It allows a station or a hub a rangeof capabilities.

To allow incompatible devices toconnect to one another.

To allow one device to have multiplecapabilities.

To allow a station to check a hub'scapabilities.

Physical Layer

The physical layer in Fast


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The physical layer in FastEthernet is more complicatedthan the one in StandardEthernet.

Topology

Fast Ethernet is designed to


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Fast Ethernet is designed toconnect two or more stationstogether.

If there are only two stations,they can be connected point-to-point.

Three or more stations need tobe connected in a star topologywith a hub or a switch at thecenter.


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Implementation


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Encoding

Manchester encoding needs a


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g200-Mbaud bandwidth for a datarate of 100 Mbps.

So an alternativeencoding/decoding scheme hasintroduced.

Encoding for Fast Ethernetimplementation


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GIGABIT ETHERNET

The need for an even higher data


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grate resulted in the design of theGigabit Ethernet protocol (1000Mbps).

The IEEE committee calls theStandard 802.3z.

oa s o e gaEthernet1. Upgrade the data rate to 1 Gbps.

2. Make it compatible with Standard


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por Fast Ethernet.

3. Use the same 48-bit address.

4. Use the same frame format.5. Keep the same minimum andmaximum frame lengths.

Physical and Data Link Layer

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