Computer Networks

Computer Networks

1.Computer Networks  A number of computers connected to one

another is called a computer network  A network is a set of technologies including H/W, S/W and media- that can be used to connect computer systems together for Communicating with each other (email, instant messaging etc.)  Exchanging information  Share resources in real time 

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Uses of Networks 

Simultaneous Access    

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Shared Peripheral devices- colour laser printers (either directly to network or through Print Server), scanners, fax etc. Personal Communications- email, instant messaging, teleconferencing   

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Commonly used data and resources can be kept on a central location, called Network Server, from where users can simultaneously access. If data is stored centrally, it is called a file server Programs also can be centrally located in the server, which can be accessed by users on the network- Client server model Applications also can be centrally located on server ( Application Server)or in clients in distributed mode of processing.

Video conferencing- real time communication over adistance by people at two or more sites using video picture- cameras, mic, speakers, monitors etc. Audio conferencing- digital audio between groups of participants Data- conferencing- to have shared workspace on their computer desktopsshared “whiteboards”, where they can draw, write, import or manipulate images con real time. Used in conjunction with video and audio conferencing VOIP- Voice over internet protocol- cheap audio communication on long distance calls

Easier Backup- Keep backup on shared storage devices 3

The Telecommunications System

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2. The Telecommunications system  Telecommunications system: Combination of hardware and

software that transmits information (text, data, graphics, and voice) from one location to another. Its components are:  

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Hardware – computers, communications processors like modems Communications media – physical media through which electronic signals are transmitted incl wireless media Communications Networks – links amongst computers and communication devices Communications software – s/w that controls the entire telecommunications system and the entire transmission process Data Communications providers – firms providing data communication services Communications protocols – rules for transmitting information across the system Communications applications – EDI, teleconferencing etc. 5

Communications processors  Hardware devices that support data transmission and

reception across a telecommunications system.  Modem: Device that converts signals from analog to digital and vice versa. 

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Analog Signal – continuous waves that transmit information by altering the amplitude and frequency of the waves. Digital signal: A discrete pulse, either on or off, that conveys information in a binary form.

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 Multiplexer: Electronic device that allows a

single communications channel to carry data transmission simultaneously from many sources.  Front-end processor: A small secondary computer, dedicated solely to communication, that manages all routing communications with peripheral devices.

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Communication media and channels  Communication channel. Pathway for

communicating data form one location to another. Cable Media Broadcast Media

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 Cable media: Communications channels that use physical wires or cables to transmit data and information.  Twisted pair wire – strands of copper wire twisted in pairs –inexpensive, widely available, easy to work with, unobtrusive – slow(low bandwidth- 10Mbps), subject to interference, easily tapped (low security)  Coaxial Cable – insulated and shielded copper wire – higher bandwidth(100Mbps), less susceptible to interference- expensive, easily tapped 9low to medium security), difficult to work with  Fibre optic cable- Thousand of very thin filaments of glass fibers, surround by cladding, that transmit information via light pulses generated by lasers – high bandwidth (6 Tbps) , theoretically upto 25Tbps– difficult to work with.

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 Broadcast ( wireless) media: Communication

channels that use electromagnetic media (the ‘’airwaves’’) to transmit data.  Microwave transmission. Communication channel that uses towers to send wireless signals; used for high-volume, longdistance, point-to-point communication on line of sight (every 30 miles you need towers to receive , amplify and retransmit)

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Satellite transmission. Communications channel that uses orbiting satellites to send digital transmission point-to-point. Geostationary satellites at 22,300miles above. Three satellites sufficient to give global coverage. GEOGeostationary earth orbit Medium earth orbit (MEO) satellites- 6000 miles above, orbits inclined to equator. Low earth orbit (LEO) satellites – 400-700 miles aboverequire less power- cellular telephones- low battery power reqd to reach (Iridium Satellites)

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Propagation delay. Brief pause between the sending and receipt of a satellite transmission (about 0.25 second). 11

 Radio transmission(R.F) Communications channel

that uses radio wave frequencies to send data directly between transmitters and receivers over short distances.  Satellite radio (digital radio). Form of radio transmission that offers uninterrupted, near CDquality music beamed to your radio from space.  Infrared. Red light not commonly visible to human eyes; can be modulated or pulsed for conveying information. 12

3. Network processing strategies  Distributed processing. Network architecture that

divides processing work between two or more computers, linked together in a network.  Client/ server computing. Form of distributed processing in which some machines (servers) perform computing functions for end –user PCs (clients).  Peer- to–peer processing. A type of client/ server distributed processing that allows two or more computers to pool their resources, making each computer both a client and a server 13

4. Types of computer networks  Local area network (LAN). Network that connects

communications devices in a limited geographical region within 2000’ (e.g., a building ), so that every user device on the network can communicate with every other device.  Topology. The physical layout and connectivity of a network Bus Topology Star Topology Ring Topology Mesh Topology

•Hierarchical Topology •Hybrid Topology

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Bus Topology  

One common cable with terminators at the ends is used Advantage- uses least amount of cabling

Server Computer n Laser Printer

Scanner Common Bus

Computer 1

Computer 2

Computer 3

Disadvantages Extra Circuitry and s/w needed for collision prevention- CSMA/CD used- carrier sensing multiple access with collision detection 15 broken connection can bring down all or part of network

Star Topology Most common topology All nodes connected through hub Some hubsintelligent hubs can monitor traffic and prevent collisions

Server Comp4

Comp1

Hub

Comp2

Comp3

Broken connection affects node only Broken hub affects all

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Ring Topology Nodes are connected in circular chain

Server

Each node examines data as it travels through ring A small packet called token is circulated in the ring

comp1

If token is not addressed to the node examining it, it passes to the next node No danger of collisions as only one packet of data travels at a time in the ring

comp2

comp4

comp3

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Mesh Topology Least used topology Most expensive to implement

Comp1

comp2

A cable runs from every computer to other computers Advantagedata can never fail to be delivered as there are alternate paths

comp3

comp4 18

Hierarchical Topology

Main Frame

Mini1

comp1

comp2

Mini2

Mini3

compn

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Hybrid Topology comp1

Star Server Printer

Ring

Printer

Main frame

Hierarchical

Mini

Mini 20

LAN technology  File server. A repository of various software and data files

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for the network, which determines who gets access to what and in what sequence Network interface card. Hardware that specifics the data transmission rate, the size of message units, the addressing information attached to each message and network topology. Gateway. A communications processor that connects dissimilar networks by translating from one set of protocols to another Bridge. A communications processor that connects two networks of the same type. Router. A communications processor that routes message through several connected LANs or to a wide area network. 21

Lan Technology- contd.  LANs employ

Base Band technology or  Broad Band Technology 

 In base band entire cable capacity used to

transmit a single digitally coded signal  In broad band, several signals can be carried by the cable at the same time at different frequencies

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Wireless local area networks (WLANs)  WLANs: Technologies (e.g., Bluetooth and

wifi) that provide LAN connectivity over short distance, typically limited to less than 150 meters. 

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Bluetooth. A wireless technology that enables temporary, short-range connection between wireless devices and enables these devices to communicate with each other via low-power radio frequencies. Max transmission speed 720Kbps Wi-Fi (Wireless Fidelity). A wireless technology that can transmit information at a range up to 300 feet; another name for the 802.11b standard on which most WLANs run. 23

Wide area networks (WAN)  A long- haul broad band (analog)networks,

generally provided by a common telecommunications carrier, that covers a wide geographic area.They can use any of the five topologies and can include switched and dedicated lines, microwave and satellite communications. They are generally star type  Value-added network (VAN). A private, dataonly network that is managed by an outside third party and used by multiple organizations to obtain economies in the cost of network service and network management.

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Wide area networks (WAN) cont…  Virtual private network (VPN). A WAN

operated by a common carrier; provides a gateway between a corporate LAN and the Internet.

 Tunnelling. The process of sending data

over the Internet in encrypted form.

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Enterprise Networking  Interconnected multiple LANs and

WANs.

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