Computer Networks&

the Internet

Definition

• A computer network is a system of interconnected computers.

• The computers in a network communicate with one another and share applications, data, voice, video and sometimes hardware components.

Basic elements of a communication system

• Communication is the process of transferring messages from one point to another.

• Three basic elements of communication system are:– Sender (source): creates the message to be

transmitted. (your computer)– Medium: carries the message. (the phone

line)– Receiver (sink): receives the message that

was created by the sender. (your friend’s computer)

Data transmission modes• There are 3 modes of transmitting data

from one point to another:– Simplex: communication is possible only in

one direction. The connected devices in such a system are send-only or receive-only. Example, a terminal (send-only) and printer (receive-only). This type of communication is very troublesome, inefficient and hard to use and is not used that much now-a-days. There is no chance of acknowledgement from the receiver and hence the sender will never know if the receiver actually got the message or not.

• Half duplex: this system can transmit data in both directions but only one direction at a time. Example, walki-talki. It requires 2 wires.

• Full duplex: this system can transmit data in both directions simultaneously. Example, telephone system. It requires 4 wires. It is the most efficient method of data communication. One end does not have to wait for the other end to finish before sending data/message.

Data transmission media• The main transmission media are:

– Metallic cable (twisted pair and coaxial cable)– Optical fiber (single mode and multi mode)– Wireless (microwave and infrared)

• Twisted pair:– Consists of 2 bunches of thin copper wires, each

bunch separately in a plastic insulation, then twisted around each other to reduce interference by adjacent wires. Also called UTP.

– There is a another type called STP where each pair is surrounded by an extra plastic insulation.

– Inexpensive, easy to use and install and is the most commonly used cable for telephone lines and LANs

– Problem: affected by interference

• Coaxial cable:– Consists of one copper conductor within and insulated

from another copper conductor of longer diameter.– The conductors are concentric.– Less affected by interference than twisted pair– Supports higher data rate than twisted pair.– Widely used for TV channel distribution.

• Fiber optics cable– Consists of one or more hair-thin filaments of glass

(or plastic) fiber wrapped in a protective jacket.– Each fiber is again protected by the cladding.– A bunch of these cladded fibers are protected by a

single jacket.– Multi-mode has core diameter of 62.5 microns and

cladding diameter of 125 microns.– Single-mode has core diameter of 8.3 microns and

cladding diameter of 125 microns.– 1 micron = 1 / 1,000,000 meter

• Advantages of fiber optics:– Extremely high data rate.– Not affected by electromagnetic interference.– Small size and lightweight.– Secure

• Wireless– Uses the air medium.– Microwave uses parabolic shaped antenna to

transmit and receive data.– Microwaves are useful for communication

among different places on the earth– Infrared are used in remote controls and

indoor LANs. They need line of sight.

Types of computer network• LAN (local area network)

– Provides communication between computers situated in a limited distance like within a department.

– Usually owned by a single organization.– Data transmission rate 1Mbps – 1Gbps.– Less possibility of errors.– Transmission media used: twisted pair,

coaxial cable and fiber optics.– Cost to transmit data is negligible.

• WAN (wide area network)– Spreads over large geographic area.– May connect computer in different countries.– May be owned by single or group of

organizations.– Implemented using circuit switching or packet

switching technologies.– May connect many LANs.– Example, the Internet.– Data transmission rate 1200bps – 2Mbps.– More possibility of errors than LAN.– Transmission media used: telephone line,

microwave links and satellite links.– Cost to transmit data is may be high due to

the use of leased telephone line, satellite etc.

• MAN (metropolitan area network)– Covers a city.– Technologies used are similar to LAN.– Can be used to connect different branches of

an organization.

• Use of computer networks– Simultaneous access to data.– Sharing peripheral devices.– Personal communication like e-mail.– Making backup of information.– Teleconferencing and video-conferencing.– Protection via password.

Network topology• The way the nodes (computers or other devices

that are communicating) of a network are linked together.

• Popular network topologies are: star, bus, ring, mesh.

• Bus– A single transmission media is shared by all nodes.

All the nodes are connected by a single cable with a terminator at each end.

– Transmission media is usually coaxial cable.– It’s the simplest topology used for LAN.– Data sent from one node to another is sent to all

nodes and only the desired destination will accept the data.

• Star– All nodes are connected to a central switch which

control the data movement.– Data from one node to another must go through the

switch which actually makes the routing decision.– It is the most popular and currently the most widely

used topology for LANs.

• Ring– Nodes are connected in a closed loop.– Data travels around the ring through the nodes which

act as repeaters.– A particular node has connection with only 2 other

nodes.– When a node receives data from a node, if it is meant

for that that node, it uses it, otherwise it forwards the data to the other node.

• Mesh– All the nodes are connected to all other

nodes.– Needs the most amount of links or physical

connections.

• Network protocols– Nodes of a network must obey some rules if they want to

communicate with each other. The set of rules is called network protocol.

– The set of programs or modules that implement the protocol is called protocol architecture.

– Example of protocol architecture, TCP/IP.

• Network interface cards (NIC) & protocols– Also called LAN adapter.– Can be plugged into the PCI slot.– Makes the physical connection between the computer and

the network.– It converts parallel data of the computer’s bus into serial

data for LAN.– It boosts the strength of a signal so that it can flow through

the transmission link.– Only one node is allowed to transmit in a LAN otherwise

signals will interfere.– LAN needs MAC scheme to prevent interference.– NIC determines the access method used by the network.– Example, Ethernet, Token Ring, FDDI, Arcnet etc.

• Client: A client computer uses, does not provide. Runs client OS. Example, Windows 98.

• Server: A server computer provides network resources. Runs network OS. Example, Windows NT server.

• Client/Server network– It’s a hierarchical strategy where all the nodes share

resources provided by one or more central servers.– This arrangement requires specialized software for

both the individual nodes and the server node.– Commonly used model in the Internet.– Example, Yahoo Web Server and Internet Explorer

Web Client.• Peer-to-peer network

– All the nodes have equal role in the network.– All have similar types of software.– Commonly used in small organizations. Example,

your lab.– Primary benefit is sharing of files, printers etc.

The Internet• Largest network in the world.• It is owned by everyone. • It is the key platform for all kinds of services

like entertainment, business etc.• It evolved from ARPANET – a research

network developed by United States Defense Department in 1969. Initially created to help academic and government researchers to use e-mail and share files.

• The Internet is growing very fast and eventually will connect all the computers and networks in the world together.

• The Internet is based on the TCP/IP protocol suit which is used to identify every computer uniquely in the Internet.

• Internet services– Popular Internet applications are: e-mail,

browsing, news.– Some protocols supported by the Internet for

various services are:• HTTP (Hyper Text Transfer Protocol) – Web browsing.• SMTP (Simple Mail Transfer Protocol) – Sending e-mail.• POP (Post Office Protocol) – Receiving e-mail.• FTP (File Transfer Protocol) – Transferring data files.

• Internet address– Every computer on the Internet is identified by a 4

part address called IP address.– Each of the 4 part is a number between 0 to 255.

– Example, 203.160.6.23.– Hard for humans to deal with IP address.– DNS addresses are also used for user

convenience which uses words instead of numbers.

• DNS addresses have 2 parts: machine name + domain name.

• Example, www.yahoo.com

– www = machine/computer name.

– yahoo.com = domain name.

• Domains identify the type of institution that uses the address. Example, .edu for educational institution.

• Large companies may divide their domain into smaller sub-domains for their convenience.

• Outside USA domains usually identify the country in which the system is located called geographic domain. Example, .jp for Japan.

• Example, www.times.ac.uk

Internet Access• There are 2 ways to access the

Internet:– Dial-up telephone access: uses a modem.

Normally used by PC users at home. Data transfer rate is low.

– Leased line access: uses LAN card. Normally used by large companies. They have to pay a monthly rent to use the line. Connection is dedicated and any one in the company can access the Internet any time. More costly and more faster than dial-up scheme.