Download Computer Networks & The Internet

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.