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Download 9th Class Computer Science Notes (Unit # 4) for Federal Board (FBISE) & Punjab Boards

9th Class Computer Science Notes (Unit # 4) for FBISE Islamabad / Punjab Boards. Class 9 / IX / SSC / Matric. Download Complete Guide / Key Book as PDF. These notes are as per latest syllabus / course prescribed by Federal Board of Intermediate & Secondary Education (FBISE) Islamabad and all Boards of Punjab Province. Students of Sindh, KPK, Balochistan and Azad Kashmir can also utilize these notes as per their course / books.

9th Class Computer Science Notes (Unit # 4) for FBISE Islamabad / Punjab Boards


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9th Class Computer Science Notes (Unit # 4) for FBISE Islamabad / Punjab Boards


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Differentiate between analog and digital signals.


Analog signalsDigitals signals
It is a continuous wave that changes with time.It is a discrete wave that carries binary information.
It is represented by a sine wave.It is represented by square wave.
It is described by the amplitude, period or frequency, and phase.It is described by bit rate and bit intervals.
It has no fixed range.It has a finite range i.e. between 0 and 1.
It is more prone to distortion.It is less prone to distortion.
It transmit data in the form of a wave.It carries data in the binary form i.e.0 and 1.
For example, human voiceFor example, Signals used for transmission in a computer.


Why digital signals are used in computer systems?

Digital signals are used in computer system because:

  1. Information represented in digital form can be easily transmitted by series or “ON” and “OFF” signals by pulses of electricity.
  2. A pulse “ON” can represent 1 and the absence of pulse “OFF” can represent 0.
  • Multiple bit (0,1) streams are used in a computer network.
  1. Digital data can be compressed relatively easily to increase the efficiency of transmission.


Name the properties of a good communication system.



  • A good communication system must deliver the message to the correct destination.
  • For example, when e-mail is sent to a person, it is received only by the person to whom it is addressed.
  • This is managed by the protocol.


  • System must deliver the message accurately without any change.
  • For example, when data is transmitted over a long distance, it may get corrupted due to transmission errors.
  • The data that is not correctly received at the destination is retransmitted from the source.
  • This is also ensured by the protocol.


  • The system must deliver the data without significant delay.
  • It is very important in real time transmission such as video conferencing.
  • For example, a computerized real-time system is used to monitor the temperature in an oil refinery. If the temperature is getting too high, it must be transmitted immediately otherwise there can be an explosion.


Give any three reason why guided communication medium is more reliable than unguided medium.

  • Guided media reduce cross talk and electromagnetic interference.
  • It provides high quality transmission at extremely fast speed.
  • It can transmit trillions of bits per seconds.
  • It is not affected by electromagnetic fields and can transmit both analog and digital signals.
  • It is used for data transmission over long distance.
  • It has multipath interference, due to reflections from land, water, natural and human-made objects.


What is meant by transmission impairment?

  • The errors that occur during data communication from one point to another are called transmission impairments.
  • Impairments occur due to imperfect characteristics of communication medium.
  • Due to this, the received and the transmitted signals are not always the same.
  • The types of impairments are:
    • Attenuation
    • Amplification
    • Distortion
    • Cross talk

Differentiate between attenuation and distortion.


  • Attenuation is the fall of signal strength with the distance.
  • If the attenuation is too much, the receiver may not be able to detect the signal at all.


  • Distortion refers to change in shape or frequency of digital signal.
  • Communication line delays the signal frequency by different amounts because different frequency components travel at different speed.
  • This causes distortion in digital signals.

What is cross talk?

  • Cross talk occurs is guided media.
  • As signal is transmitted through a wire, undesired signals enter the path of the transmitted signal due to electromagnetic radiation.
  • It is caused because of putting several wires together in a single cable.
  • Sometimes, user can hear another conversion in the background when talking on the phone.
  • This happens by the coupling between two wires that are close to each other.

What is Dial-up modem? Why is it used?

  • Dial-up modem provides internet connection through telephone line.
  • Maximum speed of Dial-up modem is 56 Kilobits per second which is very slow.
  • It is being replaced by faster DSL connection for Internet.

Uses of Dial-up Modem:

  • A telephone line is used for voice transmission which is analog signal.
  • A modem converts digital computer signal to analog form for transmission over telephone line. This process is called modulation.
  • Another modem at the receiving end, converts the analog signals back to digital form which is called demodulation.

Define data rate and baud rate.

Data Rate:

Data rate is the speed with which data can be transmitted from one device to another. It is generally measured in Kilobits (thousand bits) or Megabits (million bits) per second.

Baud Rate:

Baud is the rate of change of electrical signals per second during data communications. An electrical signal can have two or more than two states to represent binary digits 0 and 1.

Define bandwidth.

  • Bandwidth describes the overall data transmission capacity of a medium.
  • It represents the amount of data that passes through a network connection per unit of time.
  • Bandwidth is also measured in bits per second like data rate.




Describe the components of communication system with the help of diagram.

Communication system consists of the following five basic components:


It is the device, which sends the message. For example telephone handset etc.


It is the device, which receives the message in other words it is the destination of message that can be a computer, radio, telephone handset, etc.


It is the data to be transmitted. It can be text, graphics, image, sound or video.

Transmission Medium:

It is the physical pathway over which the message is sent from sender to receiver. Some examples of transmission media are coaxial cable, Fibre optic cable, microwaves etc.


It is the set of rules between the two communicating devices that governs the process of data communication. Without a protocol, two devices may be connected but they cannot communicate with each other.


Explain asynchronous and synchronous transmission modes with examples.

Asynchronous Transmission:

  • The transmission mode in which time interval between each character is not the same is known as asynchronous transmission.
  • In this transmission, each character is transmitted with additional control information which consists of additional start and stop bits.
  • Start bit is generally 0 and stop bit is 1.
  • This transmission is slow. Therefore suitable for low speed connection between system unit and keyboard mouse.


Synchronous Transmission:

  • The transmission mode, in which time interval between the characters is always the same, is known as synchronous transmission.
  • In this transmission, there is no control information added with the characters.
  • Data consisting of 0s and 1s is transmitted as one long stream of bits.
  • This transmission is faster than asynchronous transmission because it does not require extra start and stop bits. Therefore, it is used for fast data communication between computers in computer networks.


Describe the following guided media.


  1. a) Twisted pair cable:
  • It is the most commonly used cable for data communication. It consists of pairs of copper wires twisted around one another.
  • The purpose of twisting the cables is to reduce cross talk and electromagnetic interference.
  • Its transmission speed is from 2-10 million bits per second.


  1. b) Coaxial cable:
  • It is used for local networks and cable television systems. It consists of copper wire surrounded by insulating layer.
  • Insulation reduces interference and distortion.
  • Its transmission speed is from 200-500 million bits per seconds.


  1. c) Fiber optic cable:
  • It consists of smooth hair-thin strands of transparent material.
  • The transmitter has a converter that converts electrical signals into light waves.
  • These light waves are transmitted over the Fiber optic cable.
  • Another converter is placed at the receiving end that converts the light waves back to electrical signals.
  • A single Fiber optic cable can carry up to 50,000 communication lines.
  • It provides high quality transmission at extremely fast speed.
  • It can transmit trillions of bits per second.
  • It is not affected by electromagnetic fields.
  • Fiber optic cable is more expensive than twisted pair and coaxial cables.


Describe types of unguided media.

Radio Waves:

  • Radio waves are electromagnetic waves that are propagated by antennas.
  • Radio transmission consists of a transmitter and a receiver.
  • A transmitter transmits a radio signal to a receiver, which receives it.
  • Radio waves are used to transmit music, conversation, pictures and data.
  • These waves are invisible and undetectable to human beings.
  • The following are some applications of radio waves.
    • Radio and television broadcast.
    • Cell phones communication
    • Satellite communication
    • Wireless networks and wireless internet


  • Microwave signals travel through open space like radio waves.
  • Microwaves provide much faster transmission rate than telephone lines or coaxial cables.
  • Microwave antennas are installed on high buildings or high towers.
  • The transmitting and the receiving sites must be within sight of one another.
  • Microwaves are used for satellite communication and other long distance wireless communications.


  • Infra-red waves are light energy that we cannot see.
  • It travels through space at the speed of light.
  • It is used for short distance communication.
  • Infra-red waves are usually used in remote controls for television, DVD players and other similar devices.
  • It is also used in industrial, scientific and medical appliances and night-vision devices.


  • Bluetooth is a wireless communication technology that uses radio wave to connect portable electronic devices over short distance.
  • It supports networking of wide range of portable devices. For example mobile phone, mouse, keyboard, wireless speaker, wireless headset, tablet, laptop, computer and personal computer.
  • The most common use of Bluetooth is connecting a mobile phone to a wireless headset or to a laptop computer.


  • A satellite is an object that is placed in an orbit around the earth and revolves around it with speed that is slightly faster than Earth’s average orbital speed for communication.
  • It is a wireless Receiver and Transmitter used for transmitting data over long distance at high speed.
  • Ground stations beam signals through antennas to satellite.
  • Satellites amplify and retransmit the signals to another ground station which can be located many thousands of miles away.
  • The main drawback of satellite communication is the high cost of placing the satellite into its orbit.
  • Satellite are launched by rockets or space shuttles and precisely positioned in the space with an orbit speed that exactly matches with the rotation speed of the earth.


Describe the functions of the following communication devices.


  • Router is a communication device that is used when two or more networks have to be connected for communication.
  • They send information from one network to another by selecting the best pathway available.
  • There are two types of routers i.e. wired and wireless.

Network Interface Card (NIC):

  • A Network Interface Card (NIC) or simply network card is used to connect computers together to create computer network.
  • It is a card that is installed on the motherboard.
  • In modern computers, it is integrated on the motherboard.
  • There are two types of network cards. Wired network card and wireless network card.

Switch/Access Point:

  • A switch/access point is used for connecting computers together in local area network (LAN).
  • Switch is used in wired networks whereas access point is used in wireless networks.
  • A switch/access point receives information from a computer in the network inspects it and then transmit it appropriately to the destination computer.

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