Computer Networks

Computer Networks

• A network consists of two or more entities or objects sharing resources and information.

• A computer network consists of two or more computing devices connected to each other to share resources and information.

Introduction to Computer Networks

Communication Model

Communication Model

• Source– generates data to be transmitted

• Transmitter– Converts data into transmittable signals

• Transmission System– Carries data

• Receiver– Converts received signal into data

• Destination– Takes incoming data

Communication Model

– To transfer data from one computer to another requires some type of link through which the data can be transmitted. This link is known as the communications channel.

– To send data through the channel requires some type of transmission media, which may be either physical or wireless.

Physical Media• Twisted-pair cable – consists of two independently insulated wires

twisted around each other (least expensive type of cable—the kind that is used in many telephone systems)

• Coaxial cable – consists of an insulated center wire grounded by a shield of braided wire (the primary type of cabling used by the cable television industry; it is more expensive than twisted pair)

• Fiber-optic cable – contains hundreds of clear fiberglass or plastic fibers (threads) (made from thin, flexible glass tubing; bandwidth is greater, so it can carry more data; it is lighter than metal wires and is less susceptible to interference; it is fragile and expensive)

• ISDN line – a special digital telephone line that transmits and receives information at very high speeds

Twisted-Pair Cables

• TPC are the simplest and the oldest cable medium.• It is made of two insulated copper wires typically twisted around

each other in a continuous spiral.• The purpose of twisting the wires is to reduce electrical interference

from similar pairs close by.

Coaxial Cable

• Type of cable that has an inner conductor surrounded by a tubular insulating layer, surrounded by a tubular conducting shield. Many coaxial cables also have an insulating outer sheath or jacket.

A. Outer plastic sheathB. Woven copper shieldC. Inner dielectric insulatorD. Copper coreSource: Wikipedia

Fiber Optics

• A fiber optic cable is a network cable that contains strands of glass fibers inside an insulated casing. These cables are designed for long distance and very high network communications.

• Fiber optic cables carry communication signals using pulses of light. While expensive, these cables are increasingly being used instead of traditional copper cables.

Source: Google Images

Wireless Media

• Microwave Technology • Satellite system• Cellular technology • Infrared technology

Microwave

• Microwave transmission refers to the technology of transmitting information or energy by the use of radio waves whose wavelengths are conveniently measured in small numbers of centimeter; so these are referred as microwaves.

Source: Google Images

Microwave Technology

• A disadvantage is that microwaves are limited to line of sight propagation.

• Microwave radio transmission is commonly used in satellite communications, deep space radio communications, radars, radio navigation systems, sensor systems and radio astronomy.

Source: Google Images

Radio Frequency

• Radio frequency (RF) engineering is a subset of electrical engineering that deals with devices that are designed to operate in the Radio Frequency spectrum. These devices operate within the range of about 3 kHz up to 300 GHz.

• RF engineering is incorporated into almost everything that transmits or receives a radio wave, which includes, but is not limited to,Mobile Phones, Radios, Wi-Fi, and two-way radios.

Source: Google Images

Satellite Communication

• Satellite communication, in telecommunications, the use of artificial satellites to provide communication links between various points on Earth.

• A satellite is basically a self-contained communications system with the ability to receive signals from Earth and to retransmit those signals back with the use of a transponder—an integrated receiver and transmitter of radio signals.

Source: Google Images

• Satellite communication has two main components: the ground segment, which consists of fixed or mobile transmission and the space segment, which primarily is the satellite itself. A typical satellite link involves the transmission of a signal from an Earth station to a satellite. The satellite then receives and amplifies the signal and retransmits it back to Earth, where it is received and reamplified by Earth stations and terminals.

• Satellite receivers on the ground include direct-to-home (DTH) satellite equipment, mobile reception equipment in aircraft, satellite telephones.

Networking Hardware and Software

• Hub – electronic device used in a LAN to link groups of computers• Routers - devices used to ensure messages are sent to their intended

destinations• Repeaters – electronic devices that receive signals and amplify and

send them along the network

Hub

An electronic device used for data transmission, a hub does not examine or manage any of the traffic that comes through it: any packet entering any port is rebroadcast on all other ports .Simply put, the hub's major function is to replicate data it receives from one device attached to it to all others.

Repeaters

• A common problem in the networking world is that of weakening electrical signals.  Electrical signals traveling through wires (such as copper wires used in most networks), weaken due to the wire's electrical resistance. 

• A repeater is connected to two cable segments.  Any electrical signal reaching the repeater from one segment, will be amplified and retransmitted to the other segment.

The electrical signal entering the repeater at one end is weakened. The repeater amplifies the electrical signals and resends the data.

Routers

• Routers connect the networks and forward data packets between them.  When data arrives from one of the segments, the router decides, according to it's routing table, to which segment to forward that data.

The router decides which is the shortest route the packet should travel to reach the recipient.

Modem

A modem is a device that converts digital data originating from a terminal or computer, to analog signals used by voice communication networks such as the telephone system. At one end, modems convert the digital pulses to audible tones and convert audio tones back to digital pulses at the other.

The word "Modem" stands for "MOdulator-DEModulator".

Network Category

• Networks are usually classified using three properties:• Topology• Protocol• Architecture

Network/Communication Protocols

• A protocol is simply an agreed-on set of rules and procedures for transmitting data between two or more devices or the information exchanged between devices—through a network, or other media—is governed by rules and conventions that can be set out in technical specifications called communication protocol standards.

• The best known frameworks, to implement a networking protocol are the TCP/IP model and the OSI model.

Network Topology

• The network topology defines the way in which computers, printers, and other devices are connected. A network topology describes the layout of the wire and devices as well as the paths used by data transmissions.

Introduction to Computer Networks

Bus Topology

• Commonly referred to as a linear bus, all the devices on a bus topology are connected by one single cable.

Introduction to Computer Networks

Star Topology

Introduction o Computer Networks

• In star topology, every node (computer workstation or any other peripheral) is connected to central node called hub or switch.

• This consists of a central node, to which all other nodes are connected; this central node provides a common connection point for all nodes through a hub.

• The star topology is the most commonly used architecture in Ethernet LANs.

Ring TopologyIntroduction to Computer Networks

A frame travels around the ring, stopping at each node.

If a node wants to transmit data, it adds the data as well as the destination address to the frame.

Mesh Topology

• The mesh topology connects all devices (nodes) to each other for redundancy and fault tolerance.

• It is used in WANs to interconnect LANs and for mission critical networks like those used by banks and financial institutions.

Network Architecture

• Network architecture – refers to the way a network is designed and built. The two major types are:– Peer-to-peer architecture – Computers connect with

each other in a workgroup to share files, printers, and Internet access. This is used to connect a small number of computers.

– Client/server architecture – sends information from a client computer to a server, which then relays the information back to the client computer, or to other computers on the network

Hub/Switch/Router

• To connect multiple segments of networks into a larger one• Hub

– A multiport repeater to enhance signal within the same LAN• Switch

– Like hub but with intelligent– Better performance

• Router– Forward packets from one LAN to another

• A network of computers that are in the same physical location, such as home or building

• Usually connected using Ethernet– A standard on how

computers communicate over a shared media (cable)

LAN(Local Area Network)

RJ45 for twisted pair cable

WAN(Wireless Area Network)

• A network that uses long-range telecommunication links to connect 2 or more LANs/computers housed in different places far apart.• Towns, states, countries

• Examples:• Network of our Campus• Internet

WANStudent Computer Centre

Your home

USA

Clients and Servers

• The client–server characteristic describes the relationship of cooperating programs in an application.

• The server component provides a function or service to one or many clients, which initiate requests for such services. A server is a computer system that selectively shares its resources;

• a client is a computer or computer program that initiates contact with a server in order to make use of a resource. 

Peer-to-Peer Networks

• Peer-to-peer network is also called workgroup

• No hierarchy among computers all are equal

• No administrator responsible for the network

Applications of Networks

• Resource Sharing– Hardware (computing resources, disks, printers)– Software (application software)

• Information Sharing– Easy accessibility from anywhere (files, databases)– Search Capability (WWW)

• Communication– Email– Message broadcast

• Remote computing

Enhance Communication

• Computer networks use electronic mail (e-mail) as the choice for most of the communication.

• By using networks, information can be sent to a larger audience in an extremely fast and efficient manner.

• Peripheral devices include faxes, modems, scanners, plotters, and any other device that connects to the computers.

• Equipments having common requirements can be shared in order to reduce maintenance cost.

• Important data can also be stored centrally to make it accessible to users, thereby saving storage space on individual computers.

• Computer applications, which take up a considerable amount of storage space, can be installed centrally on the network, saving storage space.

Share Resources

Transmission Mode

• Simplex– One direction

• e.g. Television• Half duplex

– Either direction, but only one way at a time• e.g. police radio

• Full duplex– Both directions at the same

time• e.g. telephone