Networks and Communication
Prepared By: Abdul Wajid Yousafzai
• Is the series of computer systems that are linked
together so that they are able to share computing
power or storage facilities.
• A network is connected with one another through
cables or telephone wires.
• The link way be between computers in the same
building or between computers in different parts of the
• Network may be in form of
– Terminal computer
– Server Computer
• Computer network can be used in offices or
• Internet is an example of network in which
million of people are connected with phone
Advantages of networking
• Information and resource sharing by using printer or
• Money saving by using the same software
• Easy communication with others using large network
• Internet Access Sharing by using one high speed
connection instead of many slower one.
• Avoid duplicate information by using to share same file
• Data Security and Management
• Entertainment through playing multi-player game
• Maintain and backup data easily because these are
stored in center
Disadvantages of networking
• Hardware, software & Setup Costs by designing and
implementing the network
• Hardware & Software Management Costs by hiring a network
• Undesirable sharing by spreading viruses over networks
• Illegal or Undesirable Behavior by downloading
• Data Security Concerns by unauthorized access through
• Whole network becomes unusable if server breaks down
Types of computer networks
• LAN ( LOCAL AREA NETWORK )
• WAN (WIDE AREA NETWROK )
• MAN ( MANIPULATION AREA NETWROK)
• It covers small area
• Most LANs are used to connect computer in a single building or group of
• LANs can be found in industrial plants , office buildings ,colleges or
• Capable of transmitting data at very fast rate
• It is much faster than data transmission over a telephone line
Size Transmission Technology Topology
10 to 100 Mbps
Low delay (ms)
Very few Errors
Ring (Token ring)
Uses of LAN
• Sharing of one copy by all users
• System resources can be share
• Easy to manage data store
• Data is more secure from being
copied or destroyed
• It may be in form of
• Peer-to-peer Network
• Client Server Network
• It covers the large area
• WAN connects computer in different cities and countries
• Normally a WAN consists of two or more interconnected LANs
• Connected through telephone lines or satellites
• WAN is expensive than LAN
• Speed depends on the speed provided by the company
• Use in banks or in air ticketing system
• Is the communication network that covers a
geographical area of the size of city
• A MAN include one or more LANs but covers a small
geographical area than WAN
• It act as a high speed network
• Mobile phone system often use MAN
• The physical layout or the way in which network connections
connection are made is called topology
• It refers especially to the locations of the computer and how
the cable runs between them
• It may be
• Bus or Line Topology
• Star Topology
• Ring Topology
• Tree Topology
Bus or Line Topology
• All computers or network nodes are connected to a common communication
• In a local network this medium is often a central wire called bus
• If a computer wants to send data to the other computer in the network ,it
sends the data via the bus
• The data and address move from one computer to other and checks the
address if it matches then it goes to other computer
Workstation Laser printer
• Simple easy and suitable for small network
• Least amount of cable is required
• Easy to extend a bus
• Heavy network traffic can slow a network
• A cable breaks or loose connector will disconnect the entire
• No of computer increase then speed of network will be slow
• Is one in which all the cables run
from the computers to a central
• Connected with the device called
• Used in client server network
• If two computers want to share
data the sender computer sends
data to the hub and hub sends it
to the receiving computer
• Hub can accommodate multiple cable types
• Easy to modify
• Finding faults becomes easy
• Single computer failure cannot break the network
• If central hubs fails the entire network fails
• It is more expensive because all network cables must
be pulled to a central point
• Each computer is connected to the
next computer with the last one
connected to the first
• Data flow around the circle from
device to device in one direction
• Each computer retransmits what it
receives from the previous computer
• The message flow in one direction
• There is no signal loss problem
• Every computer has a equal access to the token
• Failure of one computer on ring effect the whole
• Difficult to troubleshoot
• Adding or removing computers disrupts the network
• In some networks one or more computers are
more powerful than the others.
• These networks are called hierarchical
• These more powerful computers usually look
after specific functions on the network and are
called servers (e.g. file servers, printer servers).
• File servers manage all the files stored on the
network, including program and data files.
• File servers are also used to store program and
• Printer servers manage all the printing requests
on the network.
Network software and hardware
• Networks require specific software and hardware in
order to work including:
– A network operating system.
– Network cabling (e.g. metal or fibre optic cables) and connectors
or wireless media (e.g. radio, microwave, or infra-red).
– Network cards.
• The quality of the software and hardware used will
determine the speed at which data is transferred (i.e. the
data transfer speed).
• The data transfer speed is the speed at which data is
transferred from the server to a workstation/terminal.
• Download time is the time this data transfer takes.
• Twisted pair (copper cable):
• Two wires twisted together.
• Makes them less susceptible to acting like an antenna and picking up
radio frequency information
– Telephone company uses twisted-pair copper wires to link
• Coaxial cable
– Also two wires:
• One of the wires is woven of fine strands of copper forming a tube.
• The wire mesh surrounds a solid copper wire that runs down the center.
• Space between has a non-conducting material.
• Makes them more impervious to outside noise.
– Light is electromagnetic.
– Can transmit more
information down a
• It can send a wider set of
– Each cable can send
several thousand phone
– Commonly used in TV and
VCR remote controls.
– Use infrared frequencies
radiation that behave
much like visible light.
– Must be in the line of
– Often used to connect
– Often used to communicate
with distant locations.
– Must be line of sight.
– Satellite communications
•Uses radio frequencies.
•Function even though line of sight is
•Not commonly used because of the possible
interference from other sources of
electromagnetic radiation such as old electric
drills and furnace motors.
• Keeping networks secure is very
• This is usually done by giving all users a:
– User ID – a unique name or number that
identifies each individual network user and
allocates each user’s file access.
– Password – a string of letters and/or numbers
that is only known by each individual user.
To do this the computer must be equipped with a
(Modulator – Demodulator).
• The modem converts digital signals into analogue
signals that can be sent along telephone lines.
• It can also convert analogue signals it receives back
into digital signals the computer can use.
Dial up Connection
A different Tone
•Dial up ties up your telephone line and so it cannot be used for anything else.
•It takes a while for the computer to set up a connection i.e many seconds.
•Internet Service Providers may charge you by the minute and so a slow link
means you pay more.
Integrated Services Digital Network
• You do not need a modem but instead need a device called an ISDN terminal adapter.
• In this network sending voice, video and computer data and user can receive telephone
calls using the one line.
• Conduct videoconferencing easily.
• Designed to replace public telecom system
• Wide variety of services
• Entirely digital domain
• First generation ( narrowband ISDN )
– 64 kbps channel is the basic unit
– Circuit-switching orientation
– Contributed to frame relay
• Second generation ( broadband ISDN )
– 100s of mbps
– Packet-switching orientation
– Contributed to ATM ( cell relay )
• Your main telephone socket has to be changed to an ISDN box
by an engineer.
• Your computer has to be fitted with an ISDN modem which is
more expensive than a standard modem.
• It is more expensive than Dial-up on a monthly basis.
• Should you move house, you have to pay for your telephone
line to be converted back again to a normal line.
• Sending data down a communication line is
not just a question of sending the streams of
bits that represent each character.
• The problem is that each computer
manufacturer has its own standards for
• Used for communications between entities in a system
• An agreed-upon format or set of rules for transmitting
data between two devices.
• Must speak the same language
– User applications
– E-mail facilities
– Remote sensor
Key Elements of a Protocol
– Data formats
– Signal levels
– Control information
– Error handling
– Speed matching
• Handshaking is the exchange of signals between
devices to establish their readiness to send or receive
• E.g. Between Computer and Printer
• It is one method of ensuring that both the sender and
receiver are ready before transmission begins.
• The conversation between the two devices is along
the lines of following
• Device 1 Device 2
• Are you ready to receive data Yes go ahead
• Sends data Message received thanks
• File compression is the process of condensing
• It is used when sending a data through
• Since the smaller the file, the cheaper the file
transfer will be.
• File compression may be performed by
software before the data is sent or by the
modem itself as the data is being sent.
• The speed with which you can surf the web or view
data over a wide area network depends on a couple
• The speed of your modem will determine how fast
the pulses are sent and the speed of the receiver’s
modem will determine the speed with which they
• The speed of the slowest modem will determine the
speed of the over link.
• Different types of cables also influence the speed.
• Bandwidth is a measure of how much data can be
carried at one time, measured in Megabits per
• The bandwidth is determined by the transmission
media (cabling) and the protection around it to
– Bandwidth: Digital
• Number of bits per second (bps) that can be
sent over a link.
• The wider the bandwidth, the more diverse
kinds of information can be sent.
• Simplest is voice, most sophisticated is moving
– Bandwidth: Analog
• The difference between the highest and lowest
frequencies that can be sent over an analog link
(like phone lines).
• Measurement is given in hertz (Hz).
– For both: The wider the bandwidth, the more
information can flow over the channel.
Cost of installation
• Terminal costs
• Servers costs
• Cabling costs
• Network cards
• Back up devices
• The cost of modem or ISDN
• Cost of ISP
• Telephone or other communication charges.
Electronic data interchange (EDI)
• EDI is the electronic transmission of business data,
such as purchase orders and invoices from one firm
• Speed and reliability are major advantages of EDI.
• Data such as O level result are now commonly
transmitted direct to schools and colleges rather
then being sent by mail.