Computer Science Class 9 Chapter 1: Computer Systems

• RAM (Random Access Memory)
• Cache memory
• Registers

Volatile memory loses its data when the power is switched off.

A word processor is application software used to create, edit, format, save and print text documents (e.g. Microsoft Word, Google Docs).

Why it replaced the typewriter:

• Mistakes can be corrected on the screen before printing, while on a typewriter the page must be retyped or corrected by hand.
• The document can be saved, reused and printed any number of times, and it can be formatted (fonts, size, spell check, tables, pictures) easily.

Synchronous is faster because fewer control bits are used.

• Recycle Bin: stores deleted files and folders temporarily; they can be restored or deleted permanently.
• Computer icon (This PC): shows the drives and storage devices and the computer's information, so files can be opened and managed.
• Folder icon: a container used to store and organise files and other folders.

Data rate (bit rate) is the number of bits transmitted per second (bps).

Baud rate is the number of signal changes (signal units) per second on the channel.

Relationship: Data rate = Baud rate x number of bits per signal. If each signal carries only 1 bit, both are equal; if more bits per signal, data rate is higher than baud rate.

• Resource sharing: printers, files, internet connection and software can be shared, which saves cost.
• Communication: users can exchange messages, emails and video quickly.
• Data sharing and centralised control: data is stored in one place on a server, easy to access, back up and secure.

• Very high cost of building, launching and maintaining satellites and earth stations.
• Propagation delay (latency), because signals travel a very long distance to the satellite and back.
• Signals can be affected by bad weather (heavy rain, storms) and interference.
• Data can be intercepted easily, so security is a concern (any three).

• Data travels as light signals, so there is no electromagnetic interference or noise, unlike copper cables.
• Signal loss (attenuation) is very low, so data can travel long distances with fewer repeaters.
• It is very hard to tap, so data is more secure and error-free; it also gives very high bandwidth.

An interface is the way the user communicates with the operating system. Two main types:

• Command Line Interface (CLI): the user types text commands on the keyboard (e.g. MS-DOS).
• Graphical User Interface (GUI): the user uses icons, windows, menus and a mouse/touch (e.g. Windows, Android).

Objectives of an operating system:

• Convenience: to give users an easy, user-friendly interface to use the computer.
• Efficiency: to manage and use hardware resources (CPU, memory, I/O devices, files) in the best way, and to provide security.

Data transmission is the process of sending data from one device (sender) to another (receiver) through a transmission medium.

Example: sending an email from a computer to another computer, or sending data from a computer to a printer.

DSL (Digital Subscriber Line) sends digital data over ordinary copper telephone lines. The line is divided into different frequency ranges: one for voice and a higher one for data. So a person can use the telephone and the internet at the same time. A DSL modem at the user's end converts the signals, and the connection is always on.

A communication device is a hardware device used to transmit data between locations in a network or telecommunication system. Examples: Hub, Switch, Router and Gateway.

• The user must remember and type exact commands; it is difficult for beginners.
• A small typing mistake gives an error, and there are no icons, windows or mouse support, so it is slow and less user-friendly than GUI.

The computer will still switch on and the BIOS in ROM will run, but the operating system and programs cannot be loaded because they are stored on storage devices, so the computer cannot be used normally. Also no data can be saved permanently.

When cells are selected, Excel automatically shows their Sum, Average, Count, etc. on the status bar. The user gets quick results without typing any formula, which saves time and avoids mistakes.

In star topology every device is connected to a central hub or switch. All data must pass through this central device, so it controls and directs all communication, and devices can be added, removed or managed easily from one point. A cable fault affects only one device.

Microwave (line-of-sight microwave link) is the best choice. It gives high bandwidth and high transmission rate, has less delay than satellite links, and suffers little impairment when towers are in clear line of sight.

Synchronous mode is faster and more efficient because data is sent as a continuous block with no start and stop bits, so fewer control bits are used and there are no gaps between characters. It is suitable for sending large amounts of data at high speed. Asynchronous mode adds start and stop bits to every character, which wastes time.

A Real-Time Operating System (RTOS) processes input and gives the output within a strict, fixed time limit. It is used where timing is very important. Examples: industrial control systems, robots, missile/airplane control, medical systems.

• Integrated circuits (ICs) were used instead of transistors.
• Computers became smaller, faster, more reliable and efficient.
• Keyboards and monitors were introduced, and several applications could run at the same time.

Conditional formatting automatically changes the appearance (colour, font, border) of cells when their values meet a given condition. Example: showing marks below 33 in red.

The data communication model has five components connected in this order:

Sender (transmitter) --message--> Medium --message--> Receiver, with a Protocol (set of rules) governing both sender and receiver.

Draw boxes for Sender and Receiver, joined by a line labelled Medium, the arrow labelled Message, and Protocol written between/under them.

A computer network is a group of two or more computers and devices connected together to share data and resources.

Types by distance: LAN (Local Area Network), MAN (Metropolitan Area Network) and WAN (Wide Area Network).

• Delivery: data must reach the correct destination.
• Accuracy: data must arrive without errors/change.
• Timeliness: data must be delivered on time.

The operating system manages all resources of the computer: it allocates the CPU to tasks, allocates and frees memory for programs, controls input/output and peripheral devices, and organises files. It shares these resources among programs fairly and efficiently.

Cache is a small, extremely fast memory inside or near the CPU. It stores frequently used data and instructions, so the CPU can get them quickly without waiting for slower RAM. This reduces fetch time and increases processing speed.

• A plotter prints large-size graphics such as engineering designs, maps and banners on large sheets, which an ordinary printer cannot.
• It gives high-quality, accurate drawings and lines.

• Typing and composing Urdu documents, books, newspapers and magazines.
• Formatting text and designing pages, e.g. cards and posters, in Urdu (Nastaliq) script.

• Every device is connected directly to every other device, so there are many paths; if one link fails, data uses another path and the network keeps working.
• It is very reliable and secure, since links are dedicated, unlike bus (one cable fails) or star (hub fails) or ring (break stops all).

Satellite communication sends signals using an orbiting satellite. A sending earth station transmits the signal up (uplink) by microwaves to the satellite, which amplifies it and sends it down (downlink) to the receiving earth station. Used for satellite TV, GPS and wide-area communication.

Diagram: Earth station A --uplink--> Satellite --downlink--> Earth station B.

The teacher should use the Filter (AutoFilter) feature on the Grade column and select A+ only. Only students with A+ are displayed; the other rows are hidden.

Data rate = Baud rate x bits per signal = 56 x 2 = 112 Kbps.

Fibre optic cable is the best: very high bandwidth and transmission rate, light signals give minimum delay, and no electromagnetic interference so minimum impairments.

A LAN is best. Two buildings in the same area are a small area; a LAN covers a building or group of nearby buildings, gives high speed, is reliable and cheaper than a WAN. (A MAN could also be used if buildings are far apart within a city.)

• It manages hardware resources (CPU, memory, devices) and runs other programs.
• It gives a user-friendly interface and provides security; without it the computer cannot be used.

Packet switching divides data into small packets. Each packet is sent independently, possibly through different routes, and the packets are reassembled at the destination. Example: email over the internet.

Monitor: displays information visually on a screen. (Or printer: produces hard copy; speaker: produces sound.)

A keyboard is an input device. Its purpose is to enter text, numbers and symbols into the computer (for document creation, programming and data entry).

Difference between asynchronous and synchronous transmission:

Difference between simplex and half-duplex mode:

In circuit switching a dedicated communication path is first established between the two devices for the whole session. Data flows continuously over this path until the session ends, and then the path is released. Example: a traditional telephone call.

• Page break: ends the current page and starts the following text on a new page (the same section continues).
• Section break: divides the document into sections, so each section can have its own formatting (e.g. page orientation, margins, columns, headers/footers, page numbering). It can also start a new page.

Attenuation is the loss of strength (amplitude) of the signal as it travels along the medium, so the signal becomes weaker. Distortion is a change in the shape/form of the signal (its frequency components arrive at different speeds), so the signal is no longer the same as the original even if it is strong enough.

• Resource sharing: devices such as printers and files/software can be shared by many users, saving cost.
• Fast communication and data sharing between users (e.g. email, messaging, file transfer).
• Centralised control, backup and security (in a client/server network).

• Header: text/graphics placed in the top margin of every page (e.g. document title, company name, chapter name).
• Footer: text/graphics placed in the bottom margin of every page (e.g. page number, date, author name).

• The server provides shared resources (files, applications, printers) and controls access/security centrally.
• The clients request and use these resources from the server.

• Data rate (bit rate) is the number of bits transmitted per second, measured in bps.
• Baud rate is the number of signal changes (signal units) per second. If one signal change carries more than one bit, the data rate is higher than the baud rate.

• Non-impact printers (inkjet, laser) are faster than impact printers.
• They give higher-quality output (better text and graphics).
• They are much quieter because there is no mechanical hammering on the paper.

Cache is a small, extremely fast memory located inside or very close to the CPU. Because it is so close to the processor and stores frequently used data and instructions, the CPU does not have to wait for the slower RAM, so access time is very short.

Analog and digital signals:

• Each device has its own cable to the central hub/switch, so a break in one cable affects only that device, while in bus or ring a break can bring down the whole network.
• A faulty device or cable is easy to find and replace without stopping the other devices.

• GUI is easy to use: the user works with icons, menus and mouse clicks, so there is no need to remember commands.
• GUI is more user-friendly and visual (windows, pictures, multitasking), while CLI needs typed commands and is hard for beginners.

A real-time system responds to inputs within a fixed, very short time. Applications: industrial control and robotics in factories, aerospace/flight control, medical monitoring in hospitals, and traffic or banking systems where immediate response is required.

• Text can be edited, corrected and deleted easily without retyping the whole page.
• Formatting (font, size, colour), spell check, copy/paste, saving and printing many copies are possible.

• Integrated circuits (ICs) were used, which combine many transistors on a single chip.
• Computers became smaller, faster, more efficient and highly reliable.
• They introduced keyboards and monitors and could run multiple applications.

• Very high bandwidth: it can carry a large amount of data at very high speed over long distances.
• Reliable: light signals are not affected by electrical noise/interference and have low signal loss.

• Guided media use a physical path (wire/cable) to carry the signal, e.g. twisted pair, coaxial cable, fibre optic.
• Unguided media carry the signal through the air/space without cables, e.g. radio waves, microwave, satellite, Wi-Fi, Bluetooth.

Control buttons (Minimize, Maximize/Restore, Close) let the user quickly hide a window to the taskbar, enlarge or resize it, or close it. This makes it easy to switch between and manage several applications and improves the user experience.

A natural system occurs organically in nature without human intervention; it is self-regulating, adaptable and stable. Examples: ecosystem (forest), weather patterns, the solar system, the human body.

Difference between RAM and ROM:

Input devices are the means by which users give data and instructions to the computer. Examples: keyboard and mouse (also scanner, microphone).

The control unit manages and directs all activities of the computer. It decodes the instructions and gives timing and control signals to the ALU, memory and input/output devices, telling them how to respond to the program instructions.

Difference between data bus and address bus:

Ports are interfaces (sockets) on the computer used to connect peripheral devices such as USB drives, external hard disks, printers and HDMI cables. Examples: USB port and HDMI port (also Ethernet, audio jack).

• Used vacuum tubes as core technology.
• Were large, slow, unreliable, expensive and consumed much power (overheated).
• Programmed in machine language; punched cards for input.

Von Neumann architecture (published by John Von Neumann in 1945) is based on the stored-program concept: instructions and data are stored in the same memory. The computer consists of the CPU (CU, ALU, registers), memory unit and input/output, connected by buses (data, address and control). It is still used in most computers today.

Monitor: an output device that displays information visually on a screen. Modern LCD/LED monitors are slim, energy-efficient and give high-quality images.

• Only one user can use the system at a time.
• Only one user task/program is run at a time (e.g. MS-DOS, older Windows for personal use); it is simple and needs few resources.

CDMA (Code Division Multiple Access) is a wireless/mobile communication technology in which many users share the same frequency band at the same time. Each user's signal is given a unique code, so the receiver can separate it. It was used in mobile phone networks.

• Uses icons, windows and menus (graphical elements) instead of typed commands.
• Uses a pointing device (mouse) and supports multitasking and multimedia.

The ribbon is the strip at the top of the MS Word window that holds tabs (Home, Insert, Layout etc.). Each tab contains groups of commands and buttons, giving quick access to all tools for editing and formatting the document.

• Delivery: data must reach the correct destination.
• Accuracy: data must be delivered without errors.
• Timeliness: data must be delivered in time (without delay).

1. Open the MS Word document and click where the table is to be placed.
2. Click the Insert tab, then click Table in the Tables group.
3. Select the required number of rows and columns from the grid (or choose Insert Table and type the numbers) and click OK.

• Sharing of resources (printers, files, software) among users.
• Fast communication between users (email, messaging, internet access, online learning).

• Transistors replaced vacuum tubes.
• Computers were smaller, faster, more reliable and cost-effective than first generation.
• Used assembly and high-level languages (FORTRAN, COBOL); magnetic tapes and disks were used.

Simplex transmission is one-way communication; the receiver cannot send data back (e.g. radio/TV broadcast, computer to printer).

Duplex transmission is two-way: in half-duplex the two sides take turns, while in full-duplex both send and receive at the same time (e.g. telephone).

The Spelling and Grammar tool checks the document for spelling mistakes and grammar errors, marks them (red/blue lines) and suggests corrections.

The CPU (Central Processing Unit) is the main electronic circuit/'brain' of the computer that executes instructions and performs calculations. Its main components are the Control Unit (CU), Arithmetic Logic Unit (ALU) and registers.

Difference between system and application software:

Attenuation is the loss of strength (amplitude) of a signal as it travels through the medium. It occurs because the medium has resistance and absorbs/spreads the signal energy, so longer distances give weaker signals; amplifiers/repeaters are used to restore it.

In a peer-to-peer (P2P) network every computer acts as both a client and a server; each peer can share and access resources. It is usually small (2 to 10 computers) because management of many computers is difficult. Peers share files, printers etc.

• Font style, size and colour (e.g. bold, italic, underline).
• Alignment (left, centre, right, justify) and line spacing/bullets.

Transmission media is the path (medium) through which a message travels from sender to receiver. Three guided media: twisted pair (telephone) cable, coaxial cable and fibre optic cable.

Differences between RAM and ROM:

An operating system is fundamental system software that manages hardware resources and provides services for computer programs (process scheduling, memory, file and device management, user interface). Examples: Microsoft Windows and Linux (also macOS, Android).

Comparison of simplex and full-duplex mode:

• Keyboard: enters text, numbers and symbols.
• Mouse: controls the pointer for selecting, dragging and opening files.
• Scanner: captures images/documents and converts them into digital form.

Comparison of hard disk (HDD) and SSD:

• Abacus: an ancient tool with beads on rods used for arithmetic.
• Pascaline: Blaise Pascal's mechanical calculator (1642) that used gears and wheels in a wooden box.
• Difference Engine: Charles Babbage's steam-powered machine (1820s) for solving numerical problems mechanically.

(Any other correct three, such as Stepped Reckoner, Analytical Engine, Tabulating Machine or Mark I, also get marks.)

• They offer exceptional speed, compact size and vast storage capacity.
• Advanced GUIs and multimedia capabilities became standard.
• They support modern programming languages such as C++, Java and Python.
• Portable devices such as laptops and smartphones became widespread.

These computers use microprocessors built with LSI and VLSI technology.

• They focus on artificial intelligence (AI) and natural language processing (NLP).
• They are capable of learning, reasoning and solving problems autonomously.
• Parallel processing lets them handle complex tasks efficiently.
• Voice recognition and gesture controls are common.
• Expert systems and robots are used in fields like medicine and engineering.

A system is a collection of interconnected components working together to achieve specific goals. It processes inputs to produce desired outputs.

• Natural example: human body (or ecosystem, weather, solar system) - it occurs in nature without human input.
• Artificial example: computer (or communication network, power plant) - it is designed and built by humans.

• Forest - natural: it occurs organically in nature without human help.
• Internet - artificial: it is a communication network designed and built by humans.
• Power plant - artificial: it is an energy system made by humans.
• Solar system - natural: it exists in nature and is governed by natural laws.

The motherboard is the main circuit board of the system. It contains the microprocessor, memory (RAM slots) and slots/ports for connecting other components.

It is called the backbone because it ensures communication between all parts of the computer.

A touch screen takes input from the user through touch gestures and also shows the results on the display, so it combines input and output functions.

It is commonly used in smartphones, tablets and kiosks.

Registers are temporary storage spaces within the CPU.

During processing they hold data and instructions temporarily so that the CPU can use them quickly while it works.

Expansion slot: a socket on the motherboard where additional hardware can be installed to expand system functionality.

Expansion card: a circuit board inserted into an expansion slot, e.g. sound card, graphics card, network card.

• (a) Typing patient names: Keyboard.
• (b) Converting paper reports into digital form: Scanner.
• (c) Reading barcodes on medicine boxes: Barcode reader.

Each device gets one mark.

The Tabulating Machine was invented by Herman Hollerith in 1890.

It was a punch card-based mechanical calculator that could compute statistics and store data.

It laid the foundation for IBM.

A microprocessor integrates an entire processing unit onto a single chip using LSI and VLSI technologies. It is also known as the CPU.

It made computers exceptionally fast and compact, with large storage, and led to portable devices such as laptops and smartphones.

(Any three are enough.)

• A = Control Unit (CU): decodes instructions and gives timing and control signals to other parts.
• B = ALU: performs arithmetic (add, subtract) and logical (AND, OR, NOT) operations.
• C = Registers: high-speed storage areas in the CPU; all data must be in a register before it is processed.

The Input/Output (I/O) controller manages the flow of data between the CPU and external devices such as hard drives, USB devices and network interfaces.

• When data is sent to the printer, the I/O controller handles the transfer.
• Data from the USB drive or network reaches the CPU/memory through it, using the buses.

(a) 1 GB = 1024 MB, so 3 GB = 3 x 1024 = 3,072 MB.

(b) 1 MB = 1024 KB, so 3,072 x 1024 = 3,145,728 KB.

8 GB = 8 x 1024 = 8,192 MB.

Number of photos = 8,192 / 4 = 2,048 photos.

• Memory word: the smallest unit of data a computer can process in a single operation.
• Word size: the number of bits a CPU can process in one instruction; it can range from 16 to 64 bits.
• A 64-bit CPU processes 64-bit chunks, while a 32-bit CPU processes 32-bit chunks. So the 64-bit CPU handles larger chunks (twice as many bits) in one instruction, which helps in complex operations and larger memory spaces.

• Chip memory: built using semiconductor technology, fast and compact. Example: RAM, ROM, flash memory (USB, SSD).
• Magnetic memory: stores data on magnetized material; large capacity at low cost but slower access. Example: hard disk or magnetic tape.
• Optical memory: a laser reads or writes data stored as tiny pits and lands. Example: CD, DVD, Blu-ray disc.

• L1 cache: located directly within the processor core; the smallest and fastest.
• L2 cache: slightly larger and slower than L1; built into the CPU or placed on the motherboard.
• L3 cache: shared among the cores of a multi-core processor; the largest and slowest of the three.

The Basic Input/Output System (BIOS) is the program that controls input/output devices and the start-up (boot) process. It is stored in the ROM chip on the motherboard, which is non-volatile.

• It tests the computer's components when the computer is turned on.
• It loads the operating system into RAM so the computer is ready for use.

Pipelining is a technique in which the CPU breaks instructions into stages (such as fetch, decode and execute) and processes multiple instructions simultaneously, for example fetching one instruction while another is being decoded.

It improves the speed and efficiency of the CPU because the stages do not wait for the whole instruction to finish.

An interrupt is a signal sent by a device such as the keyboard or display to pause the CPU's current task so that an important event can be handled.

In this case, each key press sends an interrupt, the CPU pauses its current work, handles the key press (accepts the character) and then returns to the task.

• A = Registers (smallest and fastest, inside the CPU).
• B = Cache (small and extremely fast, stores frequently used data).
• C = RAM (larger but slower than cache, main memory).
• D = Secondary storage (SSD/HDD; the largest and slowest, keeps data permanently).

• Fetch: the CPU brings the instruction from memory.
• Decode: the CPU works out (understands) what the instruction means.
• Execute: the CPU carries out the operation.

The cycle repeats for the next instruction.

Software engineering is a systematic approach to the development, operation and maintenance of software. It applies engineering principles, techniques, methodologies and tools to make sure that software is of good quality, reliable and efficient.

Two types: (1) Application software engineering – develops web, mobile and desktop applications for specific user needs. (2) Enterprise software engineering – develops software for large organisations to automate processes and manage data.

• Digital hardware engineering: designs digital circuits and components such as processors and memory units.
• Analog hardware engineering: designs analog circuits such as amplifiers and sensors.
• Integrated circuit (IC) design: designs and fabricates integrated circuits such as CPUs and GPUs.
• Computer architecture: designs the structure and organisation of a computer system, including CPU architecture, memory hierarchy and I/O systems.
• Embedded systems design: designs hardware systems built into larger devices, such as microcontrollers and consumer electronics.

(Any three.)

Embedded software engineering is the field of software engineering that focuses on designing, developing, testing and maintaining software that is built into hardware devices or systems.

Examples: automotive embedded systems (software in a car), digital home appliances (washing machine, microwave oven) and industrial control systems.

Firmware is software that is permanently stored on hardware devices.

Examples: the BIOS (Basic Input/Output System) on the motherboard of a computer, and the firmware in digital appliances such as a washing machine or a microwave oven.

Compiler: converts a program written in a high-level language such as C++, Java or Python into machine code, translating the source code into an executable program.

Assembler: does a similar job for assembly language. It converts assembly language code (mnemonics such as MOV and ADD) into machine language.

Both are system software.

• A – Compiler: translates the source code into object files (machine code).
• B – Linker: combines the object files into a single executable file.
• C – Loader: loads the executable file into main memory so that it can run.

System utilities are tools that help to manage and maintain the computer system.

They perform tasks such as disk cleanup, data backup, system monitoring and virus scanning. Examples are disk defragmenters, antivirus software and system diagnostic tools.

Productivity software helps users to do tasks efficiently, organise information and create documents, presentations, spreadsheets and databases. It supports office work, document management and collaboration.

Microsoft Office Suite includes Microsoft Word (word processing), Excel (spreadsheets) and PowerPoint (presentations).

Educational software is created to support learning and skill development, from interactive learning games to digital resources for teaching and assessment.

• Learning Management Systems (LMS): a central place for content, resources, assessments and communication.
• Kahoot!: lets teachers create interactive quizzes and games.
• Duolingo: a language learning app that gamifies learning.
• Scratch: a visual programming language to teach coding to children.

Machine language consists of binary code (0s and 1s) and is directly understood by the computer hardware.

Limitations: (1) it is highly dependent on the architecture of the computer, so a program for one type of computer may not work on another; (2) it is very complex to write, so it is rarely used.

Both convert high-level language programs into machine language.

Object-oriented languages organise code around objects that represent real-world entities. Each object has properties (attributes) and methods (behaviours). This makes complex programs easier to manage because it models the real world.

Examples: Java, C++ and Python.

• Operating system development: writing the core software that manages hardware resources.
• Device drivers: creating software that enables communication with hardware devices.
• Firmware development: developing software that resides on hardware components.
• Real-time systems: precise timing in industrial control, robotics and aerospace.
• Security tools: intrusion detection, firewalls and encryption software.
• Game development: optimising game engines, physics and graphics rendering.

(Any three, with a short explanation.)

A gateway is a communication device that connects different types of networks that use different protocols.

It is needed because it acts as a translator: it converts data formats from one network to another so that both networks can understand each other (compatibility).

Example: it may connect a network of PCs with an IBM mainframe network.

• Server: a computer that provides resources such as information, software or hardware (e.g. printers) to other computers on the network.
• Client: a computer that accesses the resources provided by the server.
• Advantage: centralized control makes management easier and security stronger; a dedicated server optimizes resource sharing and keeps unauthorized users away from data.

• Allows internet and network access from any location within the coverage area.
• Easier and cheaper to install than wired networks (no cables to lay).
• Can accommodate additional devices and users easily.
• Enables work from home and access to corporate resources on the go.

(Any three.)

• Signals can be disrupted by external devices or environmental factors.
• Higher chance of unauthorized access and data breaches (security risk).
• Coverage may need extra equipment such as access points or boosters.
• Can be slower and less reliable in crowded or busy areas.

(Any three.)

A MAN is a network that can span from several buildings or a large campus to an entire city and connects a number of LANs with high-speed lines.

• It is larger than a LAN and smaller than a WAN; it covers about 5 to 50 km in diameter.
• It uses fibre optic cable or microwave transmission.
• It provides high-speed communication.
• It is used by telephone companies, Internet Service Providers and cable TV companies.

In asynchronous transmission the time gap between characters is variable. The start bit tells the receiver that a character is coming and the stop bit tells it that the character has ended. They provide timing (synchronization) between sender and receiver.

Synchronous transmission sends data as one continuous block at a fixed time interval, so sender and receiver are already synchronized and no start or stop bits are needed.

Diagram: devices (nodes) joined in a circle with arrows showing data moving in one direction.

• Advantage: cost-effective.
• Disadvantage: a break anywhere in the ring can bring down the entire network, because data must pass round the whole loop.

A protocol is a set of rules that governs data communication. If the two devices do not follow the same protocol, they are connected but cannot communicate or understand each other's data.

Example: two people are connected by phone but speak different languages; the line works but they cannot understand each other.

• Wired: telephone cable, coaxial cable, fibre optics.
• Wireless: Bluetooth, Wi-Fi, microwave, radio wave, satellite.

(Any three of each; 1/2 mark each.)

A router connects multiple networks, often linking a local network to the internet. It finds the best path for data packets to travel from one network to another and forwards them accordingly.

Example: a wireless router connecting a home network to the internet. Routers are essential because they direct internet traffic and make network communication between networks possible.

Email (packet switching): the data is divided into small packets which are sent independently, may take different routes and are reassembled at the destination, so no line is blocked for a long time.

Phone call (circuit switching): a dedicated path is set up between the two devices for the whole call, giving a continuous flow of data (voice) until the call ends.

• (a) All devices are connected to the central hub or switch, so if it fails the whole network goes down.
• (b) Each device has its own cable to the hub, so a broken cable cuts off only that one device; the rest of the network keeps working.

Data communication standards are rules and protocols that define how data is transmitted and processed in networks.

• They make devices and systems from different manufacturers compatible.
• They allow interoperability, so communication is reliable and efficient across different networks.

The OSI (Open Systems Interconnection) model is a conceptual framework that explains how different networking protocols and technologies work together for proper data transmission.

It was developed by the International Standards Organization (ISO) in the 1970s and has seven layers: Application, Presentation, Session, Transport, Network, Data Link and Physical.

Transport layer: ensures reliable data transfer and maintains the order and integrity of data packets.

Network layer: decides the best (optimal) path for data to travel from source to destination, using devices like routers.

TCP/IP stands for Transmission Control Protocol / Internet Protocol. It is the protocol suite used for communication on the Internet.

• It provides reliable, connection-oriented data transfer.
• TCP and IP headers are added to data at the layers, which helps data to be packaged, addressed and transmitted correctly.
• Its development in the 1970s allowed different networks to communicate.

• (a) SMTP - used for sending email messages.
• (b) HTTP - transfers web pages on the World Wide Web.
• (c) FTP - transfers files between computers on a network.

• The user types the domain name (www.ncc.gov.pk), which is easy to remember.
• The browser asks the DNS server for the address of this name.
• The DNS server translates the domain name into an IP address and sends it back.
• The browser uses the IP address to contact the web server and receive the page as packets.

Encapsulation: as data goes down through the layers at the sender, each layer adds its own header (for example, a TCP header and then an IP header).

De-capsulation: when the data reaches the destination, these headers are removed layer by layer until the original data reaches the application.

• 1960s: began as ARPANET, a U.S. Department of Defense project for research institutions and military installations.
• 1970s: development of TCP/IP protocols allowed different networks to communicate.
• 1990s: World Wide Web and web browsers appeared.
• (Also: 2000s broadband and social media; 2010s mobile internet and IoT.)

• The Internet is a network of interconnected networks. Data is divided into packets.
• Routers (and switches) forward the packets from one network to another along a suitable path.
• An ISP (Internet Service Provider) gives homes, schools and offices access to the Internet and connects them to the larger network.
• Packets reach servers that host the content, and the replies come back the same way.

File size in bits = 50 × 8 = 400 megabits

Time = size ÷ speed = 400 ÷ 10 = 40 seconds

An operating system (OS) is system software that manages the computer hardware and software resources and provides an interface between the user and the computer. It controls memory, files, processes and input/output devices.

Examples: Microsoft Windows, Linux, macOS, Android.

• Keyboard: used to enter text, numbers and commands.
• Mouse: used to point, click and select items on the screen.
• Scanner: converts printed documents and pictures into digital form.

(Others: microphone for voice input, webcam for video, touch screen.)

(a) Short notes on two GUI operating systems

1. Microsoft Windows: It is the most widely used GUI operating system on personal computers. The user works with windows, icons, menus and a mouse pointer. It has a Start menu, taskbar and desktop. It supports multitasking, and many types of application software and hardware run on it. Different versions are Windows 7, 10 and 11.

2. macOS: It is the GUI operating system made by Apple for Mac computers. It has a Dock, menu bar and attractive icons. It is known for its smooth graphics, good security and easy use, and it is popular for design, video and music work.

(Other correct examples: Linux with a desktop such as Ubuntu, and Android on mobile phones.)

(b) Four advantages of a GUI operating system

• Easy to use: the user clicks icons and menus; commands need not be remembered or typed.
• Easy to learn: even a beginner or a child can start working quickly.
• Multitasking: many programs can be opened in different windows at the same time and the user can switch between them.
• Attractive and clear display: pictures, colours, fonts and graphics make the work pleasant; file and folder operations (copy, move, delete) can be done by dragging and dropping.
• Saves time: shortcut icons open programs and files quickly.

In data transmission, the sender and receiver can be coordinated in two ways.

System software manages and controls the hardware and provides a platform for running other software. Application software is designed for users to perform specific tasks.

Diagram: a pyramid with Registers at the top, then Cache, RAM and Secondary storage at the bottom; speed and closeness to the CPU increase towards the top, size increases towards the bottom.

Computer memory is organised in a hierarchy balancing speed, size and cost. Faster memory is smaller and closer to the CPU; slower memory is larger and used for storage.