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Networking Devices

Computer network devices also known as communicationdevices and they constitute a data communication network.

These devices are routers, switches, hubs, LAN cards, gateway,modems, In network, the data communication cannot beperformed without these devices.

The top vendors for network devices are Cisco, D-Link, LinkSys,Baynet, RealTek, 3Com, NetGear, Intel, Nortel, and Lucent etc.

Routers

A router is a communication device that is used to connect twologically and physically different networks, two LANs, two WANsand a LAN with WAN. The main function of the router is tosorting and the distribution of the data packets to theirdestinations based on their IP addresses. Cisco routers arewidely used in the world. Every router has routing software.Router does not broadcast the data packets.

Hubs

The central connecting device in a computer network is knownas a hub. Every computer is directly connected with the hub.When data packets arrives at hub, it broadcast them to all theLAN cards in a network and the destined recipient picks themand all other computers discard the data packets. Hub has five,eight, sixteen and more ports and one port is known as uplinkport, which is used to connect with the next hub.

Switches

Unlike the hubs, a switch does not broadcast the data to all thecomputers; it sends the data packets only to the destinedcomputer. Switches are used in the LAN, MAN and WAN. In anEthernet network, computers are directly connected with theswitch via twisted pair cables

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LAN Cards

LAN cards or network adapters are the building blocks of acomputer network. No computer can communicate without aproperly installed and configured LAN card. An UTP/STP cableconnects a computer with the hub or switch. Both ends of thecable have the RJ-45 connectors one is inserted into the LANcard and one in the hub/switch. LAN cards are inserted into theexpansion slots inside the computer.

Repeaters

A repeater is a communication device that retransmits the weaksignals with greater power. It receives the signals over thewireless, optical transmission and network cable mediums suchas copper wire, UTP(Unshielded twisted-pair)/STP(shieldedtwisted-pair) and fiber optic cables and regenerates the analogor digital signals. It can relay the data signals between thedifferent subnetworks.

Gateway

A gateway can translate information between different networkdata formats or network architectures. It can translate TCP/IP toAppleTalk so computers supporting TCP/IP can communicatewith Apple brand computers.

Bridge

A bridge is a network communication device that is used toconnect one segment of the network with another that uses thesame protocol. Bridges are fast devices for forwarding the databut not as fast as the routers and switches. In the bridgednetwork, the messages are sent to all the nodes and are pickedup only by the destined computers. Bridges has now replacedthe switches and routers.



Network Categories

There are two main types of network categories which are:

• Server based• Peer-to-peer



Transmission Mode

The term transmission mode is used to define the direction of

signal flow between two linked devices.

Simplex: Information flows in only one direction

Half-duplex: Information flows in two directions, but only inone direction at a time.

Full-duplex: Information flows in two directions at the same

time.

Transmission Media

Guided Media

Unguided Media



Guided Media

Guided media are those in which signal travel along physical

medium.E.g.

Twisted –Pair Cable

Coaxial cable

Fiber optics cable

Un-guided Media

Unguided-Media or wireless communication transportelectromagnetic wave without using a physical medium.

E.g

Radio Frequency Allocation

Satellite communication

Cellular phones

Twisted-Pair Cable

Unshielded twisted Pair Cable

Shielded Twisted Pair cable

Twisted Pair



Fig 2-3. (a) Category 3 UTP. (b) Category 5

UTP.

Unshielded twisted Pair Cable

It is most common type of telecommunication medium use

today.e.g Telephone system.

Its frequency range allow to transmit both data and voice.

It consists of two conductor (copper) each with its own

colored plastic insulation.

Advantage of Unshielded Twisted Pair cable

It is cheap, flexible and easy to install.

Higher grade UTP used in LAN technologies.

Standard Grade Of UTP

Category1: Used in telephone system level of quality is fine

for voice but low speed data communication.

Category2: Higher grade suitable for data transmission up to

4Mbps.

Category3: It transmit data up to 10Mbps.It is now the

standard cable for telephone system.

Category4: Transmission rate is 16Mbps.

Category 5: Used for data transmission up to 100Mbps.



Shielded Twisted Pair Cable

This cable has metal foil covering that encase each pair of

insulated conductor.

Coaxial CableCoaxial cable carries signals of higher frequency range than

twisted-pair cable.

Coaxial has a central core conductor of solid wire copper

enclosed in a insulating sheath, which in turn encased in outer

conductor of metal foil. This outer conductor also enclosed in

insulating sheath, and cable is protected by plastic cover.



Optical Fiber

Optical fiber : Optical fiber consists of a very fine cylinder of

glass (core) surrounded by concentric layers of glass

(cladding). It transmit signal in form of light.



A signal-encoded beam of light is transmitted by total

internal reflection.

Protocols of Computer Communications and

Networks

Protocol are used for communication between computers in

different computer networks. PROTOCOLS give the rules that

specify how the different parties may communicate.

THE ISO/OSI MODEL

ISO (International Standards Organization) has developed a

standard model for communications, called the OSI Model (Open

Systems Interconnection model)

ISO is the organization.

OSI is the model.

An Open System is a model that allows any two different

systems to communicate regardless of their underlying

architecture.



The purpose of OSI model is to open communication

between different system without requiring change to the

logic of the underlying hardware and software.

It consists of seven separate but related layers each of

which define a segment of the process of movinginformation across a network.

Seven Layers

1. Physical Layer 2. Data link layer

3. Network layer 4. Transport layer

5. Session layer

6. Presentation layer

7. Application layer



Layers

In the sense of purpose and responsibility, each layer

is separate and independent

Each has its own function, but also provides a service

to those layers above and below itself

When communicating, each OSI layer talks with the

same layer in the other device

The Layers at Work



E.g. the Application Layer of Device A communicates with

the Application Layer of Device B, by passing the data

through the other layers

1 Application layer:

The Application layer represents the interface to theapplication process. Basic functions such as file transfer,virtual terminal, and job transfer (remote execution) arerealized. Examples of the Application layer are

SMTP (Simple Mail Transfer Protocol)

FTP (File Transfer Protocol)

TELNET (Remote Terminal Protocol)

2 Presentation layer:

Presentation layer converts local host computer datarepresentations into a standard network format fortransmission on the network. On the receiving side, itchanges the network format into the appropriate hostcomputer's format so that data can be utilizedindependent of the host computer. ASCII and EBCDIC

conversions, cryptography are handled here. Examples of Presentation layer coding and conversion schemesinclude common data representation formats, conversionof character representation formats, common datacompression schemes, and common data encryptionschemes.



Some well-known standards for video include QuickTimeand Motion Picture Experts Group (MPEG). QuickTime isan Apple Computer specification for video and audio, andMPEG is a standard for video compression and coding.

ASCII-( Acronym for the A merican S tandard C ode for Information Interchange. Pronounced ask-ee, ASCII is a code for representing English characters as numbers, with each letter assigned a number from 0 to 127. Forexample, the ASCII code for uppercase M is 77. Mostcomputers use ASCII codes to represent text, which makes it possible to transfer data from one computer to

another. )EBCDIC-( Extended Binary Coded Decimal InterchangeCode ( EBCDIC ) is an 8-bit character encoding used mainlyon IBM mainframe and IBM midrange computer operatingsystems. )

3. Session layer:

The session layer establishes, manages, and terminatescommunication sessions. Communication sessions consistof service requests and service responses that occurbetween applications located in different networkdevices. These requests and responses are coordinatedby protocols implemented at the session layer. Someexamples of session-layer implementations include

AppleTalk's Zone Information Protocol (ZIP), and DecentPhase Session Control Protocol (SCP).

4.Transport layer:

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Transport layer is responsible for providing reliableservice between the hosts. Upper layer datagrams arebroken down into manageable datagram and thenappropriate header information (such as sequencenumber, port number, etc.) is added to the datagram

before passing it on to the Network layer. Two frequentlyused transport protocols are the TCP (TransmissionControl Protocol) and the UDP (User Datagram Protocol).

Important features of Transport layer:

• Transport layer ensures reliable service.• Breaks the message (from sessions layer) into

smaller datagrams, and appends appropriate unitheader information.

• Responsible for communicating with the Session layer

Important features of TCP/UDP:

• TCP/IP widely used protocol for Transport/Networklayers

• TCP: (Transport Control Protocol) TCP ensures that apacket has reached its intended destination by usingan acknowledgement. If not, it retransmits the lostmessages. Hence, TCP is called a connection oriented

protocol.• UDP (Universal Data gram Protocol): UDP simply

transmits packets over the internet. It does not waitfor an acknowledgement. It is the responsibility of upper layer protocols to ensure that the informationhad reached the intended partner(s). Hence, UDP isoften called connectionless protocol.



• Application programs that do not need connection-oriented protocol generally use UDP.

5. Network layer:

The Network layer protocol ensures that messages reachtheir destination system via an optimal route. To do this,a system uses a routing table to determine the next,directly accessible computer on the route to the packet’sdestination and then transmits to it with the aid of aservice which is made available by the Data Link layer. This next computer is either the destination itself or the

next gateway to the destination. The layer uses logical addressing. This layer also providesfor congestion control, and accounting information for thenetwork. IP (Internet Protocol) is an example of a networklayer protocol.

6. Data link layer:

Data link layer provides delivery of information framesbetween communicating partners. This layer isresponsible for flow regulation, error detection andcorrection, and framing of bits for transmission. Thenetwork data frame is made up of checksum, sourceaddress, destination address, and the data itself. Thelargest frame size that can be sent is known as the

maximum transmission Unit (MTU).

Important features of Data link layer:

• Assembles bits into frames, making them ready fortransmission over the network.



• Provides error detection, and correction totransmitted frames. If the checksum is not correct, itasks for retransmission. (Send a control message).

7. Physical layer:

This is the bottom-most layer of the OSI model. ThePhysical layer handles the bit-level communicationsacross the physical medium. The physical medium could

be made up of wired electrical signals, or light, or radio(wireless) signals. Physical layer specifications definecharacteristics such as media, data rates, maximumtransmission distances, and physical connectors.

TCP/IP Network Model

Layered Model

The TCP/IP protocol suite is structured as a hierarchy of five layers, sometimes referred to collectively as aprotocol stack. This architectural scheme provides thefollowing benefits: _ Each layer is designed for a specific purpose and existson both the sending and receiving hosts. _ Each layer is designed so that a specific layer on onemachine sends or receives exactly the same object sentor received by its peer process on another machine.



_ Each layer on a host acts independently of other layerson the same machine, and in concert with the same layeron other hosts.

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