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Data communications refers to the transmission of this digital data between two ormore computers and a computer network or data network is a telecommunicationsnetwork that allows computers to exchange data. The physical connection betweennetworked computing devices is established using either cable media or wirelessmedia. The best-known computer network is the Internet.

Generally, networks are distinguished based on their geographical span. A network can be as

small as distance between your mobile phone and its Bluetooth headphone and as large as theinternet itself, covering the whole geographical world,

Personal Area Network

A Personal Area Network (PAN is smallest network which is very personal to a user. !his may

include Bluetooth enabled devices or infra"red enabled devices. PAN has connectivity range upto #$ meters. PAN may include wireless computer keyboard and mouse, Bluetooth enabled

headphones, wireless printers and !% remotes.

&or e'ample, Piconet is Bluetooth"enabled Personal Area Network which may contain up to devices connected together in a master"slave fashion.

Local Area NetworkA computer network spanned inside a building and operatedunder single administrative system is generally termed as )ocal Area Network ()AN.

*sually,)AN covers an organi+ation offices, schools, colleges or universities. Number ofsystems connected in )AN may vary from as least as two to as much as #- million. )AN

provides a useful way of sharing the resources between end users.!he resources such as printers,

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file servers, scanners, and internet are easily sharable among computers.

)ANs are composed of ine'pensive networking and routing euipment. /t may contains localservers serving file storage and other locally shared applications. /t mostly operates on private /P

addresses and does not involve heavy routing. )AN works under its own local domain and

controlled centrally.

)AN uses either 0thernet or !oken"ring technology. 0thernet is most widely employed )ANtechnology and uses 1tar topology, while !oken"ring is rarely seen.

)AN can be wired,wireless, or in both forms at once.

Metropolitan Area Network

!he 2etropolitan Area Network (2AN generally e'pands throughout a city such as cable !%network. /t can be in the form of 0thernet,!oken"ring, A!2, or &iber 3istributed 3ata /nterface

(&33/.

2etro 0thernet is a service which is provided by /1Ps. !his service enables its users to e'pand

their )ocal Area Networks. &or e'ample, 2AN can help an organi+ation to connect all of itsoffices in a city.

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Backbone of 2AN is high"capacity and high"speed fiber optics. 2AN works in between )ocalArea Network and 4ide Area Network. 2AN provides uplink for )ANs to 4ANs or internet.

Wide Area Network

As the name suggests,the 4ide Area Network (4AN covers a wide area which may span across

provinces and even a whole country. Generally, telecommunication networks are 4ide AreaNetwork. !hese networks provide connectivity to 2ANs and )ANs. 1ince they are euipped

with very high speed backbone, 4ANs use very e'pensive network euipment.

4AN may use advanced technologies such as Asynchronous !ransfer 2ode (A!2, &rame

5elay, and 1ynchronous 6ptical Network (16N0!. 4AN may be managed by multiple

administration.

Internetwork

A network of networks is called an internetwork, or simply the internet. /t is the largest networkin e'istence on this planet.!he internet hugely connects all 4ANs and it can have connection to

)ANs and 7ome networks. /nternet uses !8P9/P protocol suite and uses /P as its addressing

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protocol. Present day, /nternet is widely implemented using /Pv:. Because of shortage of address

spaces, it is gradually migrating from /Pv: to /Pv-.

/nternet enables its users to share and access enormous amount of information worldwide. /t uses444, &!P, email services, audio and video streaming etc. At huge level, internet works on

8lient"1erver model.

/nternet uses very high speed backbone of fiber optics. !o inter"connect various continents, fibers

are laid under sea known to us as submarine communication cable.

/nternet is widely deployed on 4orld 4ide 4eb services using 7!2) linked pages and isaccessible by client software known as 4eb Browsers. 4hen a user reuests a page using some

web browser located on some 4eb 1erver anywhere in the world, the 4eb 1erver responds with

the proper 7!2) page. !he communication delay is very low.

/nternet is serving many proposes and is involved in many aspects of life. 1ome of them are;

4eb sites

0"mail

/nstant 2essaging

Blogging

1ocial 2edia

2arketing

Networking

5esource 1haring

Audio and %ideo 1treaming

Network LAN technologies

Ethernet

0thernet is a widely deployed )AN technology.!his technology was invented by Bob 2etcalfe

and 3.5. Boggs in the year #.? in #

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random amount of time, and then re"transmit the data . 0thernet connector is,network interface

card euipped with :"bits 2A8 address. !his helps other 0thernet devices to identify and

communicate with remote devices in 0thernet.

!raditional 0thernet uses #$BA10"! specifications.!he number #$ depicts #$2BP1 speed,

BA10 stands for baseband, and ! stands for !hick 0thernet. #$BA10"! 0thernet providestransmission speed up to #$2BP1 and uses coa'ial cable or 8at"@ twisted pair cable with 5"@

connector. 0thernet follows star topology with segment length up to #$$ meters. All devices are

connected to a hub9switch in a star fashion.

Fast-Ethernet

!o encompass need of fast emerging software and hardware technologies, 0thernet e'tends itself

as &ast"0thernet. /t can run on *!P, 6ptical &iber, and wirelessly too. /t can provide speed up to

#$$ 2BP1. !his standard is named as #$$BA10"! in /000 $?.> using 8at"@ twisted pair cable.

/t uses 812A983 techniue for wired media sharing among the 0thernet hosts and 812A98A

(8A stands for 8ollision Avoidance techniue for wireless 0thernet )AN.

&ast 0thernet on fiber is defined under #$$BA10"& standard which provides speed up to #$$

2BP1 on fiber. 0thernet over fiber can be e'tended up to #$$ meters in half"duple' mode and

can reach ma'imum of >$$$ meters in full"duple' over multimode fibers.

iga-Ethernet

After being introduced in #

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/n this diagram, different %)ANs are depicted in different color codes. 7osts in one %)AN, even

if connected on the same 1witch cannot see or speak to other hosts in different %)ANs. %)AN is

)ayer"> technology which works closely on 0thernet. !o route packets between two different

%)ANs a )ayer"? device such as 5outer is reuired.

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A Network !opology is the arrangement with which computer systems or network devices are

connected to each other. !opologies may define both physical and logical aspect of the network.

Both logical and physical topologies could be same or different in a same network.

Network topologies

Point-to-Point

Point"to"point networks contains e'actly two hosts such as computer, switches or routers, serversconnected back to back using a single piece of cable. 6ften, the receiving end of one host is

connected to sending end of the other and vice"versa.

/f the hosts are connected point"to"point logically, then may have multiple intermediate devices.

But the end hosts are unaware of underlying network and see each other as if they are connecteddirectly.

Bus Topology/n case of Bus topology, all devices share single communication line or cable.Bus topology mayhave problem while multiple hosts sending data at the same time. !herefore, Bus topology either

uses 812A983 technology or recogni+es one host as Bus 2aster to solve the issue. /t is one of

the simple forms of networking where a failure of a device does not affect the other devices. But

failure of the shared communication line can make all other devices stop functioning.

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Both ends of the shared channel have line terminator. !he data is sent in only one direction and

as soon as it reaches the e'treme end, the terminator removes the data from the line.

Star Topology

All hosts in 1tar topology are connected to a central device, known as hub device, using a point"to"point connection. !hat is, there e'ists a point to point connection between hosts and hub. !he

hub device can be any of the following;

)ayer"# device such as hub or repeater

)ayer"> device such as switch or bridge

)ayer"? device such as router or gateway

As in Bus topology, hub acts as single point of failure. /f hub fails, connectivity of all hosts to all

other hosts fails. 0very communication between hosts, takes place through only the hub.1tar

topology is not e'pensive as to connect one more host, only one cable is reuired andconfiguration is simple.

Ring Topology

/n ring topology, each host machine connects to e'actly two other machines, creating a circular

network structure. 4hen one host tries to communicate or send message to a host which is notadCacent to it, the data travels through all intermediate hosts. !o connect one more host in the

e'isting structure, the administrator may need only one more e'tra cable.

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&ailure of any host results in failure of the whole ring.!hus, every connection in the ring is a

point of failure. !here are methods which employ one more backup ring.

Mesh Topology/n this type of topology, a host is connected to one or multiple hosts.!his topology has hosts inpoint"to"point connection with every other host or may also have hosts which are in point"to"

point connection to few hosts only.

7osts in 2esh topology also work as relay for other hosts which do not have direct point"to"

point links. 2esh technology comes into two types;

ull Mesh; All hosts have a point"to"point connection to every other host in the network.

!hus for every new host n(n"#9> connections are reuired. /t provides the most reliablenetwork structure among all network topologies.

Partially Mesh; Not all hosts have point"to"point connection to every other host. 7osts

connect to each other in some arbitrarily fashion. !his topology e'ists where we need to

provide reliability to some hosts out of all.

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Tree Topology

Also known as 7ierarchical !opology, this is the most common form of network topology in use

presently.!his topology imitates as e'tended 1tar topology and inherits properties of bus

topology.

!his topology divides the network in to multiple levels9layers of network. 2ainly in )ANs, a

network is bifurcated into three types of network devices. !he lowermost is access"layer where

computers are attached. !he middle layer is known as distribution layer, which works asmediator between upper layer and lower layer. !he highest layer is known as core layer, and is

central point of the network, i.e. root of the tree from which all nodes fork.

All neighboring hosts have point"to"point connection between them.1imilar to the Bus topology,if the root goes down, then the entire network suffers even.though it is not the single point of

failure. 0very connection serves as point of failure, failing of which divides the network into

unreachable segment.

!aisy "hain

!his topology connects all the hosts in a linear fashion. 1imilar to 5ing topology, all hosts are

connected to two hosts only, e'cept the end hosts.2eans, if the end hosts in daisy chain are

connected then it represents 5ing topology.

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0ach link in daisy chain topology represents single point of failure. 0very link failure splits thenetwork into two segments.0very intermediate host works as relay for its immediate hosts.

#y$rid Topology

A network structure whose design contains more than one topology is said to be hybrid topology.

7ybrid topology inherits merits and demerits of all the incorporating topologies.

!he above picture represents an arbitrarily hybrid topology. !he combining topologies may

contain attributes of 1tar, 5ing, Bus, and 3aisy"chain topologies. 2ost 4ANs are connected bymeans of 3ual"5ing topology and networks connected to them are mostly 1tar topology

networks. /nternet is the best e'ample of largest 7ybrid topology.

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ISO/OSI Model in Communication

Networks!here are nnumbers of users who use computer network and are located over the world. 1o to

ensure, national and worldwide data communication, systems must be developed which arecompatible to communicate with each other. /16 has developed this. /16 stands for

International organi%ation o& Standardi%ation. !his is called a model for 'pen Syste(

Interconnection(61/ and is commonly known as 61/ model.

!he /16"61/ model is a seven layer architecture. /t defines seven layers or levels in a complete

communication system.

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Feature of OSI Model :

%. &ig picture of communication over network is understandable through this'(I model.

). *e see how hardware and software work together.

+. *e can understand new technologies as they are developed.

,. Troubleshooting is easier by separate networks.

. an be used to compare basic functional relationships on di/erent networks.

Functions of Dierent Layers :

Layer 1: !e "!ysical Layer :

%. It is the lowest layer of the '(I 0odel.

). It activates1 maintains and deactivates the physical connection.

+. It is responsible for transmission and reception of the unstructured raw dataover network.

,. !oltages and data rates needed for transmission is de2ned in the physicallayer.

. It converts the digital3analog bits into electrical signal or optical signals.

4. Data encoding is also done in this layer.

Layer #: Data Link Layer :

%. Data link layer synchroni5es the information which is to be transmitted overthe physical layer.

). The main function of this layer is to make sure data transfer is error free fromone node to another1 over the physical layer.

+. Transmitting and receiving data frames se6uentially is managed by this layer.

,. This layer sends and expects acknowledgements for frames received andsent respectively. 7esending of non-acknowledgement received frames is alsohandled by this layer.

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. This layer establishes a logical layer between two nodes and also managesthe Frame tra8c control over the network. It signals the transmitting node tostop1 when the frame bu/ers are full.

Layer $: !e Network Layer :

%. It routes the signal through di/erent channels from one node to other.

). It acts as a network controller. It manages the (ubnet tra8c.

+. It decides by which route data should take.

,. It divides the outgoing messages into packets and assembles the incomingpackets into messages for higher levels.

Layer %: rans&ort Layer :

%. It decides if data transmission should be on parallel path or single path.

). Functions such as 0ultiplexing1 (egmenting or (plitting on the data are doneby this layer

+. It receives messages from the (ession layer above it1 convert the messageinto smaller units and passes it on to the $etwork layer.

,. Transport layer can be very complex1 depending upon the network

re6uirements.

. !ransport layer breaks the message (data into small units so that they are handled more

efficiently by the network layer.

Layer ': !e Session Layer :

%. (ession layer manages and synchroni5e the conversation between twodi/erent applications.

). Transfer of data from source to destination session layer streams of data aremarked and are resynchroni5ed properly1 so that the ends of the messagesare not cut prematurely and data loss is avoided.

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Layer (: !e "resentation Layer :

%. 9resentation layer takes care that the data is sent in such a way that thereceiver will understand the information :data; and will be able to use thedata.

). *hile receiving the data1 presentation layer transforms the data to be readyfor the application layer.

+. "anguages:syntax; can be di/erent of the two communicating systems.