cn unit -i.pptx

7/25/2019 CN Unit -I.pptx

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Computer NetworksTopics to be covered

Unit I Basics of Networks

Network Components and categories typesof Connections Topologies Layers Physical

Links Hybrid !ltiple "ccess Techni#!es edi!m "ccess C$" %thernet Token &ing '((I )ireless L"N

1


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)hat is a comp!ter Network*

A network is any collection of independent computers thatcommunicate with one another over a shared network medium.

A computer network is a collection of two or more connectedcomputers. When these computers are joined in a network,people can share les and peripherals such as modems,

printers, tape backup drives, or C!"#$% drives.

When networks at multiple locations are connected usin&services available from phone companies, people can send e"mail, share links to the &lobal 'nternet, or conduct videoconferences in real time with other remote users.

When a network becomes open sourced it can be mana&edproperly with online collaboration software.

As companies rely on applications like electronic mail anddatabase mana&ement for core business operations, computer

networkin& becomes increasin&ly more important. (


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)

%+ery network incl!des,

At least two computers *erver or Client workstation.

Networkin& 'nterface Card+s N'C-

A connection medium, usually a wire or cable,althou&h wireless communication between networkedcomputers and peripherals is also possible.

Network $peratin& system software, such as %icrosoft

Windows NT or (, Novell NetWare, /ni0 and inu0.

Types of Networks,

ANs ocal Area Networks-

networks usually conned to a &eo&raphic area, such as asin&le buildin& or a colle&e campus.

ANs can be small, linkin& as few as three computers, butoften link hundreds of computers used by thousands ofpeople.


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2

WANs Wide Area Networks- combines multiple ANs that are &eo&raphically

separate.

This is accomplished by connectin& the di3erent ANsusin& services such as dedicated leased phone lines,dial"up phone lines both synchronous andasynchronous-, satellite links, and data packet carrierservices.

%ANs %etropolitan area Networks- The refers to a network of computers with in a City.

45N 4irtual 5rivate Network- Tunnelin& to transfer data securely on the 'nternet to a

remote access server on your workplace network. 45N helps you save money by usin& the public 'nternet

instead of makin& lon&6distance phone calls to connectsecurely with your private network

There are two ways to create a 45N connection, by

dialin& an 'nternet service provider '*5-, or connectin&directly to 'nternet.


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7

Categories of Network

Network can be di+ided in to two main categories,

5eer"to"peer. there are no dedicated servers or hierarchy amon& the computers.

All of the computers are e8ual and therefore known as peers.

Normally each computer serves as Client9*erver and there is noone assi&ned to be an administrator responsible for the entire

network.

*erver 6 based. The client9server network is the most e:cient way to provide;

!atabases and mana&ement of applications such as *preadsheets,Accountin&, Communications and !ocument mana&ement.

Network mana&ement. Centrali


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=

At the heart of the model is the concept of splittin& applicationfunctions between a client and a server processor.

The division of labor between the di3erent processors enables

the application desi&ner to place an application function on theprocessor that is most appropriate for that function.

This lets the software desi&ner optimi


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>

Network Hardware Components

What are the ?ardware Components commonly

found in a typical Network . . .@

Cables

Client Adapters

?ubs *witches

Access 5oints

#outers

%odems

%odem #outers


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%thernet Cables

When we reference Cables in Networkin& we are &enerally referrin& to

#B27 thernet Networkin& CablesD /nshielded Twisted 5air Cables /T5-. *hielded Twisted 5air *T5-

These Cables are often referred to by the *tandard that they adhere to i.e.

Cate&ory 7 Cat 7- or Cate&ory = Cat =-.

The Cables are further distin&uished with re&ard to they way in which they

are wiredD *trai&ht"Throu&h 5atch-

Crossover

!ue to the widespread use of Auto"*ensin& AN 5orts we see very little

occasions where a Crossover Cable is actually re8uired now.

Eenerally speakin& Crossover Cables are just used when networkin& two5Cs directly to&ether, or when daisy"chainin& Network ?ubs.

For all other instances 5atch Cables are &enerally used.


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G

%thernet Cables - UTP Cables $olid Core " *olid each Conductor is a sin&le

relatively thick Copper Wire " $tranded" each Conductor consists of a number

of thin Copper Wires

*olid Cable is used for infrastructure Cablin&. Thecable is inHe0ible because of its structure andthe ma0imum len&th would be 1 meters.

*tranded Cables have no 0ed ma0imum len&th,but are &enerally used for short distances. TheCables are He0ible, and the ma0imum len&th

lar&ely depends on Cable Iuality and *hieldin&. *tandard thernet 191- Cables only use 2 out

of the Connectors, whereas Ei&abit uses all .


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1

Client "dapters

There are numerous types of Network 'nterface Cards or ClientAdapters and they are usually identied by their Type and Connection

Technolo&yD very Network 'nterface Card N'C- has a uni8ue %AC address

pointin& to its %anufacturer.


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11

H!b

?ubs tie the Network to&ether.

A Network can be e0tended by daisy"chainin& one ?ub toanother.

?ubs by default echo all tra:c to all 5orts, so when a 5acket ofdata arrives at one 5ort, it is copied to the other 5orts so that allse&ments of the Network can see all 5ackets.

$witch

*witches di3er from ?ubs in that they track %AC Addresses onboth sides of the Network and do not echo any tra:c that doesnot need to be echoed.

Jroadcast messa&es are not ltered via *witches, and these arecontinued to be echoed to all 5orts, but otherwise the *witch isintelli&ently lterin& to ensure that only tra:c destined for aparticular %AC Address on a 5articular 5ort,&ets to that device.


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"ccess Point

An Access 5oint is a device for addin& Wireless Connectivity to ane0istin& Wired Network.

't is clear that an Access 5oint is a Wireless !evice . . . Jut is it a ?ubor a *witch@

Well it is in fact both a ?ub and a *witch.

As Access 5oints are Wireless, and Wireless Jandwidth is by nature ashared resource, the Access 5oint A5- is actin& in this role as a ?ubas it does not lter any of the Wireless Communication.

'f however an Access 5oint 's connected to an e0istin& Wired Network,then the Access 5oint A5- will act like a *witch and lter Tra:caccordin&ly. *hould Client A send !ata to Client J, the Access 5ointwill lter the !ata and ensure that it is not broadcasted to all 5orts.


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1)

&o!ter - A #outer is in fact a . . .#outer K Access 5oint K Network *witch

Whereas a Network *witch works at a %AC Address level a #outer works atlevel, an '5 Address level, and it has an '5 Address for each Network that it

is connected to. Jased on *ource and !estination '5 Address and the Networks to which

they belon&, the #outer decides to which Network to forward the 5acket.

%ost nd"/ser #outers, are Eateway #outers that simply connect a ocalAN- Network with the rest of the World WAN-.

Eateway #outers are usually based around Network Address TranslationNAT-.

The Network Address Translation NAT- element handles the tra:c from#outablee0ternal- '5 Addresses to the non"#outable internal- '5 Addressesof a typical internal Network or AN.

Three special Address #an&es are to be used in ocal NetworksANs- only, and they should not be forwarded to any 5ublic Network.

Class ", /0.1.1.1

Class B, /23./4.1.1 /23.5/.1.1

Class C, /63./47.1.1


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odem

The name L%odemM derives from the functionality of the !evice

in that it is %odem in essence a %odulator9!emodulator.

't is essentially a Communications !evice that converts oneform of a si&nal to another that is suitable for transmissionover a 5hone ine.

Typically !i&ital !ata from a 5C is converted to Analo&ue !atawhich is sent via the 5hone ine and then from converted backfrom this Analo&ue !ata to !i&ital !ata a&ain on the ne0t 5C.

odem &o!ters

A %odem #outer is in fact a . . . #outer K Access 5oint K Network*witch K %odem K NAT device

The WAN 5ort found on a typical #outer is essentially replacedwith a 5hone ine connection 5oint.


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Topolo&ies The &eometrical arran&ement of computer resources, remote

devices and communication facilities is known as Network

structure or Network topolo&y. A computer network is comprised of nodes and links, a node

is the end point of any branch in a computer, a terminaldevice, workstation or interconnectin& e8uipment facility.

A link is a communication path between two nodes. The

terms circuitO and ChannelO are fre8uently used assynonyms for the link.

Topolo&ies can be physical or lo&ical. 5hysical Topolo&ymeans the physical desi&n of a network includin& thedevices, location and cable installation.

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I. B!s Topology This structure is very popular for local

area networks. 'n this structure ortopolo&y, a sin&le network cable runs in

the buildin& or campusand all nodes arelinked alon& with this communication

line with two endpoints called the bus or backbone asshow &ure.

Jy this type of topolo&y, if one node &oes faulty all nodes may be

a3ected as all nodes share the same cable for the sendin& andreceivin& of information.

The cablin& cost of bus systems is the least of all the di3erenttopolo&ies. ach end of the cable is terminated usin& a specialterminator.

"d+antages,

#eliable in very small networks as well as easy to use and understand.

#e8uires least amount of cable to connect the computers nodes-to&ether and therefore is less e0pensive than other cablin&arran&ements.

't+s easy to e0tend, Two cables can be easily joined with a connector,makin& a lon&er cable for more computers to join the network.

A repeater can also be used to e0tend a bus con&uration 1=


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(isad+antages,

?eavy network tra:c can slow a bus considerably because anycomputer can transmit at any time. Jut networks do not coordinatewhen information is sent. Computer interruptin& each other can use

a lot of bandwidth. ach connection between two cables weakens the electrical si&nal.

The bus con&uration can be di:cult to nd and can cause thewhole networks to stop functionin&.

II. &ing Topology

This is yet another structure for local area networks. 'n this topolo&y, the network cable passes from one node to

another until all nodes are connected in the form of a loop orrin&.

There is a direct point"to"point link between two nei&hborin&

nodes the Ne0t and the 5revious-. These links are unidirectional which ensures

that transmission by a node traverses the

whole rin& and comes back to the node,

which made the transmission as shown in

&ure. 1>


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'nformation travels around the rin& from one node to the ne0t.

ach packet of data sent to the rin& is pre0ed by the address ofthe station to which it is bein& sent.

When a packet of data arrives, the node checks to see if the

packet address is the same as its own, if it is, it &rabs the data inthe packet. 'f the packet does not belon& to it, it sends thepacket to the ne0t node in the rin&.

Faulty nodes can be isolated from the rin&. When the workstationis powered on, it connects itself to the rin&. When power is o3, it

disconnects itself from the rin& and allows the information tobypass the node.

The most common implementation of this topolo&y is token rin&.A break in the rin& causes the entire network to fail. 'ndividualnodes can be isolated from the rin&.

"d+antages, #in& networks o3er hi&h performance for a small number of workstations or

for lar&er networks where each station has a similar workload.

#in& networks can span lon&er distances than other types of networks.

#in& networks are easily e0tendable.

/nlike Jus topolo&y, there is no si&nal loss in #in& topolo&y because the

tokens are data packets that are re"&enerated at each node. 1


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(isad+antages,

#elatively e0pensive and di:cult to install

Failure of one computer on the network can a3ect the whole network.

't is di:cult to nd fault in a rin& network.

Addin& or removin& computers can disrupt the network. 't is much slower than an thernet network under normal load.

III. $tar Topology

*tar topolo&y uses a central hub throu&h which, all components areconnected.

'n a *tar topolo&y, the central hub is the host computer, and at the endof each connection is a terminal as shown in Fi&ure.

Nodes communicate across the network by passin& data

throu&h the hub. A star network uses a si&nicant

amount of cable as each terminal is wired back to the

central hub, even if two terminals are side by side but several hundredmeters away from the host. The central hub makes all routin&decisions, and all other workstations can be simple.

An advanta&e of the star topolo&y is that failure, in one of the terminalsdoes not a3ect any other terminalD however, failure of the central huba3ects all terminals. This type of topolo&y is fre8uently used to connect

terminals to a lar&e time"sharin& host computer. 1G


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"d+antages,

't is more reliable if one connection fails, it does not a3ect others-

The center of a star network is a &ood place to dia&nose networkfaults and if one computer fails whole network is not disturbed.

?ub detects the fault and isolates the faulty computer. 't is easy to replace, install or remove hosts or other devices, the

problem can be easily detected"'t is easier to modify or add a newcomputer without disturbin& the rest of the network by simplyrunnin& a new line from the computer to the central location andplu&&in& it to the hub.

/se of multiple cable types in a same network with a hub. 't has &ood performance

(isad+antages

't is e0pensive to install as it re8uires more cable, it costs more tocable a star network because all network cables must be pulled to

one central point, re8uirin& more cable len&th than othernetworkin& topolo&ies.

Central node dependency, if central hub fails, the whole networkfails to operate.

%any star networks re8uire a device at the central point to

rebroadcast or switch the network tra:c. (


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I8. esh Topology

!evices are connected with many redundant interconnections betweennetwork nodes. 'n a well"connected topolo&y, every node has aconnection to every other node in the network. The cable re8uirementsare hi&h, but there are redundant paths built in.

Failure in one of the computers does not cause the network to breakdown, as they have alternative paths to other computers

%esh topolo&ies are used in critical connection of host computerstypically telephone e0chan&es-. Alternate paths allow each computerto balance the load to other computer systems in the network by usin&

more than one of the connection paths available. A fully connected mesh network therefore has no n"1- 9( physical

channels to link n devices. To accommodate these, every device on thenetwork must have n"1- input9output ports.

"d+antages

Pield the &reatest amount of redundancy in the event

that one of the nodes fails where network tra:c can

be redirected to another node.

5oint"to"point link makes fault isolation easy.

5rivacy between computers is maintained as messa&es

travel alon& dedicated path.

Network problems are easier to dia&nose. (1


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(isad+antages

The amount of cablin& re8uired is hi&h.

A lar&e number of '9$ input9output- ports are re8uired.

8. Tree Topology The most common structure or topolo&y known as Tree

topolo&y, Tree topolo&y is a AN topolo&y in which onlyone route e0ists between any two nodes on the network.The pattern of connection resembles a tree in which all

branches sprin& from one root."d+antages

'nstallation and con&uration of network are easy.

The addition of the secondary hub allows more devices

to be attached to the central hub.

ess e0pensive when compared to mesh topolo&y.

Faults in the network can be detected traces

(isad+antages

Failure in the central hub brin&s the entire network to a halt.

%ore cablin& is re8uired when compared to the bus topolo&y

because each node is connected to the central hub. ((


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Network Components

A data communications system has ve components

/. essage, The messa&e is the information data- to be communicated.5opular forms of information include te0t, numbers, pictures, audio, and video.

3. $ender,The sender is the device that sends the data messa&e. 't can be acomputer, workstation, telephone handset, video camera, and so on.

5. &ecei+er,The receiver is the device that receives the messa&e. 't can be acomputer, workstation, telephone handset, television, and so on.

9. Transmission medi!m, The transmission medium is the physical path bywhich a messa&e travels from sender to receiver. *ome e0amples of

transmission media include twisted"pair wire, coa0ial cable, ber"optic cable,and radio waves.

:. Protocol, A protocol is a set of rules that &overn data communications. 'trepresents an a&reement between the communicatin& devices. Without aprotocol, two devices may be connected but not communicatin&, just as aperson speakin& French cannot be understood by a person who speaks only

Bapanese. ()


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L";%&%( T"$


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Physical Layer - responsible for mo+ements of indi+id!al bitsfrom one hop =node> to the ne1t

The physical layer coordinates the functions re8uired to carrya bit stream over a physical medium.

't deals with the mechanical and electrical specications of theinterface and transmission medium.

't also denes the procedures and functions that physical

devices and interfaces have to perform for transmission to$ccur.

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The physical layer is also concerned with the followin&; Physical characteristics of interfaces and medi!m. The physical

layer denes the characteristics of the interface between the devicesand the transmission medium. 't also denes the type of transmissionmedium.

&epresentation of bits.The physical layer data consists of a streamof bits se8uence of $s or 1s- with no interpretation. To be transmitted,bits must be encoded into si&nals""electrical or optical. The physicallayer denes the type of encodin& how $s and ' s are chan&ed tosi&nals-.

(ata rate.The transmission rate"the number of bits sent each second"is also dened by the physical layer. 'n other words, the physical layerdenes the duration of a bit, which is how lon& it lasts.

$ynchroni?ation of bits.The sender and receiver not only must usethe same bit rate but also must be synchroni


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(ata Link Layer responsible for mo+ing frames from one hop =node> to the ne1t.

The data link layer transforms the physical layer, a raw transmission facility, to a reliablelink. 't makes the physical layer appear error"free to the upper layer

$ther responsibilities of the data link layer include the followin&;

'raming. The data link layer divides the stream of bits received from the network layerinto mana&eable data units called frames.

Physical addressing. 'f frames are to be distributed to di3erent systems on thenetwork, the data link layer adds a header to the frame to dene the sender and9orreceiver of the frame. 'f the frame is intended for a system outside the sender+snetwork, the receiver address is the address of the device that connects the network tothe ne0t one.

'low control. 'f the rate at which the data are absorbed by the receiver is less thanthe rate at which data are produced in the sender, the data link layer imposes a How

control mechanism to avoid overwhelmin& the receiver. %rror control. The data link layer adds reliability to the physical layer by addin&

mechanisms to detect and retransmit dama&ed or lost frames. 't also uses amechanism to reco&ni


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Network Layer - responsible for the delivery of individual pkts from the sourceto the destination.

The network layer is responsible for the source"to"destination delivery of apacket, possibly across multiple networks links-.

Whereas the data link layer oversees the delivery of the packet between

two systems on the same network links-, the network layer ensures thateach packet &ets from its point of ori&in to its nal destination.

$ther responsibilities of the network layer include the followin&;

Logical addressing. The physical addressin& implemented by the datalink layer handles the addressin& problem locally. 'f a packet passes thenetwork boundary, we need another addressin& system to helpdistin&uish the source and destination systems. The network layer adds

a header to the packet comin& from the upper layer that, amon& otherthin&s, includes the lo&ical addresses of the sender and receiver.

&o!ting. When independent networks or links are connected to createinternetworks network of networks- or a lar&e network, the connectin&devices called routers or switches) route or switch the packets to theirfnal destination. One o the unctions of the network layer is to provide

this mechanism. (


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Transport Layer - responsible for the delivery of a messa&e from one processto another.

The transport layer is responsible for process"to"process delivery of the entiremessa&e.

A process is an application pro&ram runnin& on a host.

Whereas the network layer oversees source"to"destination delivery of individualpackets, it does not reco&ni


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$egmentation and reassembly. A messa&e is divided intotransmittable se&ments, with each se&ment containin& a se8uencenumber. These numbers enable the transport layer to reassemblethe messa&e correctly upon arrivin& at the destination and toidentify and replace packets that were lost in transmission.

Connection control. The transport layer can be eitherconnectionless or connection oriented. A connectionless transportlayer treats each se&ment as an independent packet and delivers itto the transport layer at the destination machine. A connectionoriented transport layer makes a connection with the transport

layer at the destination machine rst before deliverin& thepackets. After all the data are transferred, the connection isterminated.

'low control. ike the data link layer, the transport layer isresponsible for How control. ?owever, How control at this layer isperformed end to end rather than across a sin&le link.

%rror control. ike the data link layer, the transport layer isresponsible for error control. ?owever, error control at this layer isperformed process"to 6process rather than across a sin&le link. Thesendin& transport layer makes sure that the entire messa&e arrivesat the receivin& transport layer without error dama&e, loss, orduplication-. rror correction is usually achieved throu&h

retransmission. )

*ession ayer " responsible for dialo& control and


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*ession ayer " responsible for dialo& control andsynchroni


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5resentation ayer responsible for translation, compression, and encryption.

The presentation layer is concerned with the synta0 and semantics of theinformation e0chan&ed between two systems.

*pecic responsibilities of the presentation layer include the followin&; Translation.The processes in two systems are usually e0chan&in& information

in the form of character strin&s, numbers, and so on. The information must be

chan&ed to bit streams before bein& transmitted. Jecause di3erent computersuse di3erent encodin& systems, the presentation layer is responsible forinteroperability between these di3erent encodin& methods. The presentationlayer at the sender chan&es the information from its sender"dependent formatinto a common format. The presentation layer at the receivin& machine chan&esthe common format into its receiver"dependent format.

%ncryption.To carry sensitive information, a system must be able to ensureprivacy. ncryption means that the sender transforms the ori&inal information toanother form and sends the resultin& messa&e out over the network. !ecryptionreverses the ori&inal process to transform the messa&e back to its ori&inal form.

Compression.!ata compression reduces the number of bits contained in theinformation. !ata compression becomes particularly important in thetransmission of multimedia such as te0t, audio, and video.

)(

"pplication Layer - responsible for pro+iding ser+ices to the !ser


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"pplication Layer responsible for pro+iding ser+ices to the !ser

The application layer enables the user, whether human or software, toaccess the network.

't provides user interfaces and support for services such as electronicmail, remote le access and transfer, shared database mana&ement, and

other types of distributed information services. *pecic services provided by the application layer include the followin&;

Network +irt!al terminal. A network virtual terminal is a softwareversion of a physical terminal, and it allows a user to lo& on to aremote host. To do so, the application creates a software emulation ofa terminal at the remote host. The user+s computer talks to the

software terminal which, in turn, talks to the host, and vice versa. Theremote host believes it is communicatin& with one of its ownterminals and allows the user to lo& on.

'ile transferA accessA and management. This application allows auser to access les in a remote host to make chan&es or read data-,to retrieve les from a remote computer for use in the localcomputer, and to mana&e or control les in a remote computer locally.

ail ser+ices.This application provides the basis for e"mailforwardin& and stora&e.

(irectory ser+ices.This application provides distributed databasesources and access for &lobal information about various objects and

services. ))


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PH;$IC"L LIN


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'nterference and cross talk may a3ect both the wires and createunwanted si&nals, if the two wires are parallel.

Jy twistin& the pair, a balance is maintained. *uppose in one twist onewire is closer to noise and the other is farther in the ne0t twist thereverse is true. Twistin& makes it probable that both wires are e8uallya3ected by e0ternal inHuences.

Twisted 5air Cable comes into two forms; Unshielded

$hielded

Unshielded +ers!s shielded Twisted-Pair Cable

*hielded Twisted"5air *T5- Cable has a metal foil or braided"meshcoverin& that encases each pair of insulated conductors.

%etal casin& improves that 8uality of cable by preventin& thepenetration of noise or cross talk.

't is more e0pensive.

"pplications

Twisted 5air cables are used in telephone

lines to provide voice and data channels.

ocal area networks also use twisted pair

cables.

Connectors

The most common /T5 connector is #B27. )7

C i l C bl


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Coa1ial Cable Coa0ial cable coa0- carries si&nals of hi&her fre8uency ran&es

than twisted pair cable.

'nstead of havin& two wires, coa0 has a central core conductor

of solid or stranded wire usually copper- enclosed in aninsulatin& sheath, and with outer conductor of metal foil.

The outer metallic wrappin& serves both as a shield a&ainstnoise and as the second conductor and the whole cable isprotected by a plastic cover.

"pplications

't is used in analo& and di&ital telephone networks

't is also used in Cable T4 networks

't is used in thernet AN

)=


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'iber ptic Cable

A ber"optic cable is made of &lass or plastic and transmitssi&nals in the form of li&ht.

Properties of light

i&ht travels in a strai&ht line as lon& as it moves throu&h asin&le uniform substance. 'f travelin& throu&h one substancesuddenly enters another, ray chan&es its direction.

'f the an&le of incidencethe an&le the ray makes with theline perpendicular to the interface between the twomedium- is less than the critical an&le the ray refracts andmove closer to the surface.

'f the an&le of incidence is e8ual to the critical an&le, theli&ht bends alon& the interface.

'f the an&le of incidence is &reater than the critical an&le,the ray reHects and travels a&ain in the denser substance.Critical an&le di3ers from one medium to another medium.$ptical ber use reHection to &uide li&ht throu&h a channel.

)>


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A Elass or plastic core is surrounded by a claddin& of lessdense &lass or plastic.

R

)

%ode

*tep"'nde0

%ultimode *in&le mode

Eraded "'nde0

!ltimode'n the multiple mode, multiple li&htbeams from a source move throu&hthe core in di3erent paths$ingle ode*in&le mode uses step"inde0 ber

and a hi&hly focused source of li&htthat limits beams to a small ran&e ofan&les, all close to the hori


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Ung!ided media

/n&uided media transport electroma&netic waves withoutusin& a physical conductor. This type of communication is oftenreferred to as wireless communication.

*i&nals are normally broadcast throu&h air and thus availableto anyone who has device capable of receivin& them.

/n&uided si&nals can travel from the source to destination inseveral ways;

ro!nd propagation6 waves travel throu&h lowestportion on atmosphere.

$ky propagation6 ?i&h fre8uency waves radiate upwardinto ionosphere and reHected back to earth.

Line-of-sight propagation6 4ery hi&h fre8uency si&nalstravel in a strai&ht line

)G

& di )


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&adio )a+es

lectroma&netic waves ran&in& in fre8uencies between ) k?< and 1E?< are normally called radio waves.

Properties

#adio waves are omni directional. When an antenna transmits radiowaves, they are propa&ated in all directions. This means that thesendin& and receivin& antennas do not have to be ali&ned.

A sendin& antenna sends waves that can be received by any receivin&antenna.

#adio waves, particularly those of low and medium fre8uencies, can

penetrate walls.icrowa+es

lectroma&netic waves havin& fre8uencies between 1 and ) E?


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Carrier $ense !ltiple "ccess with Collision (etection =C$"DC(>

The basic idea;

When a station has a frame to transmit;

1. isten for !ata Transmission on Cable Carrier *ense-

(. When %edium is Iuiet no other station transmittin&-;

a- Transmit Frame, istenin& for Collision

b- 'f collision is heard, stop transmittin&, wait random time, and transmit a&ain.

Carrier $ense !ltiple "ccess with Collision "+oidance=C$"DC">. C*%A9CA Carrier *ense %ultiple Access9Collision Avoidance- is a protocol for

carrier transmission in (.11 networks.

. /nlike C*%A9C! which deals with transmissions after a collision has occurred,C*%A9CA acts to prevent collisions before they happen.

. 'n C*%A9CA, as soon as a node receives a packet that is to be sent, it checks to besure the channel is clear no other node is transmittin& at the time-.

. 'f the channel is clear, then the packet is sent. 'f the channel is not clear, the node

waits for a randomly chosen period of time, and then checks a&ain to see if thechannel is clear.

.This period of time is called the backp"o3 factor, and is counted down by a back"o3 counter.

. 'f the channel is clear when the backo3 counter reaches


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The ori&inal thernet was developed as an e0perimental coa0ial cablenetwork in the 1G>s by Sero0 Corporation to operate with a data rate of) %bps usin& a carrier sense multiple access collision detectionC*%A9C!- protocol for ANs with sporadic tra:c re8uirements.

*uccess with that project attracted early attention and led to the 1Gjoint development of the 1"%bps thernet 4ersion 1. specication bythe three"company consortium; !i&ital 8uipment Corporation, 'ntelCorporation, and Sero0 Corporation.

The ori&inal ' (.) standard was based on, and was very similar to,the thernet 4ersion 1. specication.

*ince then, a number of supplements to the standard have been denedto take advanta&e of improvements in the technolo&ies and to supportadditional network media and hi&her data rate capabilities, plus severalnew optional network access control features.

From then onwards, the term Ethernet reers to the amily o local-area

network (LAN) products coered !y the "EEE #$%.& standard that defneswhat is commonly known as the 'A*'+ protocol. ,hree data rates arecurrently defned or operation oer optical f!er and twisted-pair ca!les

1 %bps1Jase"T thernet

1 %bpsFast thernet

1 %bpsEi&abit thernet2=

thernet Architecture


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thernet Architecture

thernet architecture can be divided into two layers;

Physical layer, this layer takes care of followingf!nctions.

ncodin& and decodin& Collision detection

Carrier sensin&

Transmission and receipt

(ata link layer,

'ollowing are the maEor f!nctions

*tation interface

!ata ncapsulation 9!ecapsulation

ink mana&ement

Collision %ana&ement

The Jasic thernet Frame Format

2>

The ' ( ) standard denes a basic data frame format that is


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The ' (.) standard denes a basic data frame format that isre8uired for all %AC implementations, plus additional optionalformats that are used to e0tend the protocols basic capability.

The basic data frame format contains the followin& seven elds

5reamble 5A-; 't consists of > bytes. The 5A is an alternatin& pattern of ones and


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en&th9Type; 't consists of 2 bytes. This eld indicates either the number of %AC"client data

bytes that are contained in the data eld of the frame, or the frame type '! if theframe is assembled usin& an optional format.

!ata;

't is a se8uence of n bytes of any value, where n is less than or e8ual to 17. 'fthe len&th of the !ata eld is less than 2=, the !ata eld must be e0tended byaddin& a ller a pad- su:cient to brin& the !ata eld len&th to 2= bytes.

Frame check se8uence FC*-; 't consists of 2 bytes. This se8uence contains a )("bit cyclic redundancy check

C#C- value, which is created by the sendin& %AC and is recalculated by thereceivin& %AC to check for dama&ed frames. The FC* is &enerated over the !A,*A, en&th9Type, and !ata elds.

Token #in&; (.7- A Jrief ?istory

$ri&inally, 'J% developed Token #in& network in the 1G>s.

't is still 'J%+s primary local"area network AN- technolo&y.

The related ' (.7 specication is almost identical to andcompletely compatible with 'J%+s Token #in& network.

'n fact, the ' (.7 specication was modeled after 'J% Token#in&, and on the same lines.

The term ,oken ing is generally used to reer to !oth "/0s ,oken

ing network and "EEE #$%.1 networks. 2G

'ntroduction


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'ntroduction

Token #in& and '(.7 are based on token passin& %ACprotocol with rin& topolo&y.

They resolve the uncertainty by &ivin& each station a turn on by

one. ach node takes turns sendin& the dataD each station may

transmit data durin& its turn.

The techni8ue that coordinates this turn mechanism is calledToken passin&D as a Token is passed in the network and the

station that &ets the token can only transmit. As one node transmits at a time, there is no chance of collision.

*tations are connected by point"to"point links usin& repeaters.

%ainly these links are of shielded twisted"pair cables.

The repeaters function in two basic modes; isten mode,

Transmit mode.

A disadvanta&e of this topolo&y is that it is vulnerable to link orstation failure. Jut a few measures can be taken to take care of

it. 7

Token &ing peration


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Token &ing peration

Token"passin& networks move a small frame, called a token2 aroundthe network. 3ossession o the token grants the right to transmit.

" a node receiing the token has no inormation to send2 it passes thetoken to the ne4t end station. Each station can hold the token or ama4imum period o time.

'f a station possessin& the token does have information to transmit, itsei


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Priority $ystem

Token #in& networks use a sophisticated priority system that permitscertain user"desi&nated, hi&h"priority stations to use the network morefre8uently.

Token #in& frames have two elds that control priority; the priority feld and

the reseration feld.

$nly stations with a priority e8ual to or hi&her than the priority valuecontained in a token can sei


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The monitor detects the lost condition usin& a timer by time-outmechanism and recoers !y using a new ree token.

,o detect a circulating !usy token2 the monitor sets a 5monitor !it6 to oneon any passing !usy token.

" it detects a !usy token with the monitor !it already set2 it implies thatthe sending station has ailed to remoe its packet and recoers !ychanging the !usy token to a ree token. Other stations on the ring haethe role o passie monitor.

,he primaryjob of these stations is to detect failure of the active monitorand assume the role of active monitor. A contention"resolution is used to

determine which station to take over.'rame 'ormat

Token #in& and ' (.7 support two basic frame types; tokens and

data9command frames.

Tokens are ) bytes in len&th and consist of a start delimiter, an access

control byte, and an end delimiter. !ata9command frames vary in si


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Token Frame contains three elds, each of which is 1 byte inlen&th;

*tart delimiter 1 byte-; Alerts each station of the arrival of a token or data9command

frame-.

Access"control 1 byte-; Contains the 5riority eld the most si&nicant ) bits- and

the #eservation eld the least si&nicant ) bits-,

as well as a token bit used to di3erentiate a token from adata9command frame- and

a monitor bit used by the active monitor to determine whether a

frame is circlin& the rin& endlessly-.

nd delimiter 1 byte-; *i&nals the end of the token or data9command frame.

(ataDCommand 'rame 'ields

!ata9command frames have the same three elds as TokenFrames, plus several others. The !ata9command frame eldsare described below;

72

Frame"control byte 1 byte-;


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Frame control byte 1 byte-; 'ndicates whether the frame contains data or control information.

'n control frames, this byte species the type of controlinformation.

!estination and source addresses ("= bytes-; Consists of two ="byte address elds that identify the destination

and source station addresses.

!ata up to 27 bytes-; 'ndicates that the len&th of eld is limited by the rin& token

holdin& time, which denes the ma0imum time a station can holdthe token.

Frame"check se8uence FC*" 2 byte- ; 's led by the source station with a calculated value dependent on

the frame contents. The destination station recalculates the valueto determine whether the frame was dama&ed in transit. 'f so, the

frame is discarded. Frame *tatus 1 byte-;

This is the terminatin& eld of a command9data frame. The Frame*tatus eld includes the address"reco&ni


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I%%% 703.// )ireless L"N

Fundamental to the ' (.11 architecture is the concept of /asicerice et (/) or wireless LAN cell.

A / is defned as a group o stations that coordinate their access to

the medium under a gien instance o medium access control. ,he geographic area coered !y a / is known as the /asic erice

Area (/A)2 which is ery similar to a cell in a cellular communicationnetwork.

All stations with in a /A with tens o meters in diameter maycommunicate with each other directly.

,he #$%.77 standard support the ormation o two distinct types o/s

ad hoc network and

"nrastructure /.

Two or more J**+s are interconnected usin& a +istri!ution ystem or

+. ,his concept o + increases network coerage. Each / !ecomes a

component o an e4tended2 larger network.

Entry to the + is accomplished with the use o Access 3oints (A3).

An access point is a station2 thus addressa!le. o data moes

!etween the / and the + with the help o these access points. 7=

Creatin& lar&e and comple0 networks usin& J**+s and !*+s


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Creatin& lar&e and comple0 networks usin& J** s and !* sleads us to the ne0t level of hierarchy, the E4tended ericeet or E.

,he !eauty o the E is the entire network looks like an

independent !asic serice set to the Logical Link 'ontrol layer(LL'). ,his means that stations within the E cancommunicate or een moe !etween /0s transparently tothe LL'.

The rst type of J** is known as ad hoc network2 which

consists o a group o stations within the range o each other. As its name implies2 ad hoc networks are temporary in nature2

which are typically created and maintained as needed withoutprior administratie arrangement. Ad hoc networks can !eormed anywhere spontaneously and can !e dis!anded ater a

limited period o time.

7>

%edium Access Control


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the wireless medium presents some uni8ue challen&es not present inwired ANs that must be dealt with by the %AC used for ' (.11.*ome of the challen&es are;

The wireless AN is prone to more interference and is less reliable.

The wireless AN is prone to unwanted interception leadin& to security problems. There are so called hidden station and e4posed station pro!lems.

The Hidden $tation Problem Consider a situation when A is transmittin& to J, 'f C senses the media, it will not hear anythin&

because it is out of ran&e, and thus will falsely

conclude that no transmission is &oin& on andwill start transmit to J. the transmission willinterfere at J, wipin& out the frame from A. The problem of a station not been

able to detect a potential competitor for the medium because the competitor istoo far away is referred as Hidden Station Problem.

%1posed $tation problem

Now consider a di3erent situation where J is transmittin& to A, and C sense themedium and detects the on&oin& transmission between J and A. C falselyconclude that it can not transmit to !, when the fact is that such transmissionwould cause on problem. A transmission could cause a problem only when thedestination is in


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p=C$"-C">

The solution to these problems is Carrier *ense %ultiple Accesswith Collision Avoidance or C*%A9CA

ain steps can be s!mmari?ed as, *ender sends a short frame called e8uest to send , (%$!ytes)

to the destination. , also contains the length o the data rame.

!estination station responds with a short 12 bytes-

clear to send (',) rame.

After receivin& the CT*, the sender starts sendin& the data frame. 'f collision occurs, CT* frame is not received within a certain period

of time.

'raming - format of the ' (.11

The frames can be cate&ori


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yframe check se8uence FC*-.

Frame Jody varies from "()1( bytes.

At last is the FC* eld. The rame check se8uence is a &%-!itcyclic redundancy check which ensures there are no errors inthe rame.

The %AC header consists of seven elds and is ) bytes lon&.

The elds are frame control, duration, address 1, address (,

address ), se8uence control, and address 2. The frame control eld is ( bytes lon& and is comprised of 11

subelds.

=

'rame Control 'ield =in "C header>


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The protocol version eld is ( bits in len&th and will carry the version ofthe (.11 standard. The initial value of (.11 is D all other bit valuesare reserved.

Type and subtype elds are ( and 2 bits, respectively. They work to&ether

hierarchically to determine the function of the frame. The remainin& elds are all 1 bit in len&th.

The To !* eld is set to 1 if the frame is destined for the distributionsystem.

From !* eld is set to 1 when frames e0it the distribution system. Note

that frames which stay within their basic service set have both of theseelds set to .

The %ore Fra& eld is set to 1 if there is a followin& fra&ment of thecurrent %*!/.

#etry is set to 1 if this frame is a retransmission.

5ower %ana&ement eld indicates if a station is in power save mode set

to 1- or active set to -.

%ore data eld is set to 1 if there is any %*!/s are bu3ered for thatstation.

The W5 eld is set to 1 if the information in the frame body wasprocessed with the W5 al&orithm.

The $rder eld is set to 1 if the frames must be strictly ordered. =1

The !uration9'! eld is ( bytes lon&. 't contains the data on the


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duration value for each eld and for control frames it carriesthe associated identity of the transmittin& station.

The address elds identify the basic service set, the destination

address, the source address, and the receiver and transmitteraddresses. ach address eld is = bytes lon&.

The se8uence control eld is ( bytes and is split into (subelds, fra&ment number and se8uence number.

Fra&ment number is 2 bits and tells how many fra&ments the

%*!/ is broken into. The se8uence number eld is 1( bits that indicates the

se8uence number of the %*!/. The frame body is a variablelen&th eld from " ()1(. This is the payload.

=(

'((I = 'iber (istrib!ted (ata Interface>


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F!!' is similar to (.7 and 'J% Token #in&s but it uses opticalber cable instead of copper cables.

Physical Properties

/nlike (.7 networks, an F!!' network consists of a dual rin&twoindependent rin&s that transmit data in opposite directions.

The second rin& is not used durin& normal operation but instead comesinto play only if the primary rin& fails. That is, the rin& loops back onthe secondary ber to form a complete rin&, and as a conse8uence, anF!!' network is able to tolerate a sin&le break in the cable or the failureof one station.

Jecause of the e0pense of the dual"rin& con&uration, F!!' allows nodesto attach to the network by means of a sin&le cable. *uch nodes arecalled single attachment stations (A)9 their dual-connectedcounterparts are called2 not surprisingly2 dual attachment stations

(+A).*hould this *A* fail, the concentrator detects this situation anduses an optical !ypass to isolate the ailed A2 there!y keeping the ringconnected.

=)

'((I has other physical characteristics


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at most 7 stations.

ma0imum distance of ( km between any pair of stations .

$verall, the network is limited to a total of ( km of ber, which

means that, because of the dual nature of the rin&, the total amount of cable

connectin& all

stations is limited to 1 km.

F!!' uses 2J97J encodin&.

Token aintenance First, all nodes on an F!!' rin& monitor the rin& to be sure that the

token has not been lost.

$bserve that in a correctly functionin& rin&, each node should see avalid transmissioneither a data frame or the tokenevery so often.

The &reatest idle time between valid transmissions that a &iven nodeshould e0perience is e8ual to the rin& latency plus the time it takes totransmit a full frame, which on a ma0imally si


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contain the nodeMs !id or the ,,,2 that is2 the token rotationtime that the node needs so that the applications running on thenode can meet their timing constraints.

'f this claim frame makes it all the way around the rin&, then thesender removes it, knowin& that its TT#T bid was the lowest.

That node now holds the tokenthat is, it is responsible forinsertin& a valid token on the rin&and may proceed with thenormal token al&orithm.

When a node receives a claim frame, it checks to see if the TT#Tbid in the frame is less than its own. 'f it is, then the node resetsits local denition of the TT#T to that contained in the claimframe and forwards the frame to the ne0t node.

'f the bid TT#T is &reater than that nodeUs minimum re8uiredTT#T, then the claim frame is removed from the rin& and thenode enters the biddin& process by puttin& its own claim frameon the rin&.

*hould the bid TT#T be e8ual to the nodeUs re8uired TT#T, thenode compares the address of the claim frameUs sender with itsown and the hi&her address wins

cn unit -i.pptx

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