Introduction to Information Technology

Communication protocol. A communication protocol is a set of rules and conventions that governs how two or more devices exchange data over a network. It defines the data format, addressing, error control, flow control, timing and sequencing of messages, so that devices from different vendors can communicate. Examples are HTTP, FTP, SMTP, TCP, IP and Ethernet.

OSI model. The Open Systems Interconnection (OSI) reference model was developed by ISO. It divides network communication into seven layers, each with a defined function and each serving the layer above it. Data moves down the layers at the sender (each layer adds a header) and up at the receiver.

A common way to remember the order from top to bottom is "All People Seem To Need Data Processing". The model's benefits are standardization, interoperability, and ease of troubleshooting, because a problem can be isolated to one layer.

Objectives of an operating system

An operating system (OS) is system software that manages the computer hardware and provides services to application programs and users. Its objectives are:

• Convenience: make the computer easy to use through an interface (GUI or command line).
• Efficiency: use CPU, memory and I/O devices in the best possible way.
• Resource management: allocate and protect resources among competing programs and users.
• Ability to evolve: allow new hardware and services to be added without disturbing existing programs.

Process management. A process is a program in execution. The OS:

• creates and terminates processes;
• schedules the CPU among ready processes (FCFS, SJF, Round Robin, priority) using process states (new, ready, running, waiting, terminated) and the process control block;
• provides synchronization and inter-process communication;
• detects and handles deadlocks.

Memory management. The OS:

• keeps track of which parts of main memory are in use and by whom;
• allocates memory when a process starts and reclaims it when it ends;
• uses techniques such as contiguous allocation, paging, segmentation and virtual memory so that programs larger than physical memory can run;
• protects one process's memory from others.

File management. The OS:

• creates, deletes, opens, reads, writes and renames files and directories;
• organizes files in a hierarchical directory structure and maps them to storage blocks (allocation methods and free-space management);
• controls access through permissions and protection;
• provides backup and recovery.

Data model. A data model is a collection of concepts and tools for describing the data, the relationships between data, the semantics and the constraints in a database. It gives a way to represent the structure of a database at different levels of abstraction. Common data models are the hierarchical, network, relational and object-oriented models, and the entity-relationship model for conceptual design.

Conceptual data model using the ER model. A conceptual data model describes the data requirements of an organization independently of any DBMS or physical storage. The entity-relationship (ER) model builds it from three main concepts:

1. Entity: a real-world object that can be distinguished from others, such as STUDENT or COURSE (drawn as a rectangle). A weak entity depends on another entity (double rectangle).
2. Attribute: a property of an entity, such as roll_no, name, address (drawn as an ellipse). There are simple, composite, multivalued (double ellipse), derived (dashed ellipse) and key attributes (underlined).
3. Relationship: an association between entities, such as a student ENROLLS in a course (drawn as a diamond). Its cardinality is one-to-one, one-to-many, many-to-one or many-to-many, and participation is total or partial.

Steps: identify the entities, identify their attributes and key attributes, identify the relationships and their cardinalities, then draw the ER diagram.

Example (college database).

• Entities: STUDENT(roll_no, name, address), COURSE(course_id, title, credit), TEACHER(t_id, name).
• Relationships: STUDENT enrolls in COURSE (many-to-many, since a student takes many courses and a course has many students); TEACHER teaches COURSE (one-to-many).

 [STUDENT] ---M---<enrolls>---N--- [COURSE] ---N---<teaches>---1--- [TEACHER]
   |  |  |                           |  |                           |   |
 roll_no name address           course_id title                  t_id name

This diagram is the conceptual model; it is later mapped to relational tables STUDENT, COURSE, TEACHER and ENROLLS(roll_no, course_id).

Computer system. A computer system is an electronic system made up of hardware and software that accepts data as input, processes it according to stored instructions, stores it, and produces useful information as output. A complete computer system consists of hardware, software and people (users).

Components

1. Hardware: the physical parts of the computer.
   • Input unit: keyboard, mouse, scanner; it accepts data and instructions.
   • Central Processing Unit (CPU): the brain of the computer. It has the ALU (arithmetic and logic operations), the control unit (directs and coordinates all operations) and registers (fast temporary storage).
   • Memory: primary memory (RAM, ROM, cache) holds the data and programs in use; secondary storage (hard disk, SSD, optical disk) holds data permanently.
   • Output unit: monitor, printer, speaker; it presents the results.
2. Software: the programs that tell the hardware what to do.
   • System software: operating system, utilities, language translators.
   • Application software: word processors, spreadsheets, browsers.
3. People (liveware): programmers, operators and end users who operate the system.
4. Data / procedures: the raw facts processed and the instructions for using the system.

The flow is: Input unit, then CPU (with memory), then Output unit.

Why a bus is used. A bus is a set of parallel wires (a communication pathway) that carries data, addresses and control signals between the CPU, memory and I/O devices. Instead of a separate connection between every pair of components, all components share the bus. This reduces the number of connections, makes the system cheaper and simpler, allows new devices to be added easily, and allows standard interfaces.

The system bus has three parts: the address bus, the data bus and the control bus.

Control bus versus data bus

For example, to write to memory the address bus gives the location, the data bus carries the value, and the control bus carries the WRITE signal.

Bytes in a kilobyte. 1 kilobyte (KB) = $2^{10}$ = 1024 bytes (in the binary convention used for memory; in the decimal SI convention 1 kB = 1000 bytes).

Magnetic tape. A magnetic tape is a sequential-access secondary storage medium: a long, narrow plastic strip coated with a magnetizable material such as iron oxide, wound on a reel or in a cartridge. Data is recorded in tracks along its length.

Working mechanism

• The tape passes at constant speed over a read/write head inside a tape drive, moved by two reels, a take-up reel and a supply reel, with a capstan and motor.
• Writing: the data to be stored is converted into electric current pulses in the head's coil. These produce a magnetic field that magnetizes tiny regions of the tape, with the direction of magnetization representing 1 or 0.
• Reading: as the magnetized tape moves past the head, the changing magnetic field induces a small current in the head's coil, which is converted back into bits.
• Data is arranged in blocks (records) separated by inter-block gaps, and in parallel tracks, usually 9 (8 data bits plus a parity bit per byte).
• Access is sequential, so to reach a record the tape must wind past all the records before it. Access is slow, but the cost per bit is very low.

Magnetic tapes are used for backups and archiving of large data.

Use of a plotter. A plotter is an output device that draws high-quality, large, vector graphics by moving one or more pens (or an inkjet head) over the paper, along both X and Y directions. It is used for:

• engineering and architectural drawings, maps and blueprints (CAD/CAM output);
• circuit and PCB layouts;
• posters, banners and large charts;
• cartography and GIS outputs.

How the quality of a printer is determined. The quality of a printer is judged mainly by:

1. Resolution: measured in dots per inch (dpi). The more dots per inch, the sharper the text and images. For example, 300 dpi is draft quality and 1200 dpi or more is photo quality.
2. Speed: measured in pages per minute (ppm) for page printers or characters per second (cps) for impact printers. This affects productivity rather than print clarity.
3. Colour capability: the number of colours and the accuracy of reproduction (monochrome, CMYK).
4. Type of print technology: laser and inkjet give better quality than dot-matrix (impact).
5. Paper handling and ink or toner quality: the paper sizes supported, and the print durability.

In short, the resolution in dpi is the main measure of quality.

Internet of Things (IoT). The Internet of Things is a network of physical objects (things) such as sensors, appliances, vehicles and machines that are embedded with electronics, software and network connectivity so that they can collect data, exchange it over the Internet and be monitored or controlled remotely, usually with little human intervention. A typical IoT system has sensors/devices, a connectivity layer (Wi-Fi, Bluetooth, LoRa, mobile data), cloud data processing, and a user interface.

Possible applications in Nepal

• Smart agriculture: soil moisture and temperature sensors for irrigation control, greenhouse monitoring, and livestock tracking.
• Smart home: remotely controlled lights, fans, CCTV and security alarms through mobile apps.
• Smart metering and energy: smart electricity meters for NEA, and monitoring of hydropower plants and solar systems.
• Transportation: GPS vehicle tracking for public transport and fleet management, and smart parking.
• Health care: remote patient monitoring and wearable health devices, which are especially useful in remote hilly areas.
• Disaster management: flood and landslide early-warning sensors on rivers, and earthquake monitoring.
• Water and environment: monitoring water tank levels, air pollution in Kathmandu valley.
• Industry and retail: machine monitoring and inventory tracking.

The challenges are high cost, limited Internet coverage in remote areas, power supply, and security.

Confidentiality. Ensuring that information is accessible only to those authorized to see it, and is not disclosed to unauthorized persons or processes. It is achieved by encryption, access control and passwords. For example, encrypting an e-mail so that only the intended receiver can read it.

Integrity. Ensuring that data is accurate and complete and has not been altered, modified or destroyed by unauthorized parties, either in storage or during transmission. It is achieved by hashing, digital signatures, checksums and access permissions. For example, the receiver can detect that the amount in a bank transfer was not changed in transit.

Authentication. The process of verifying the identity of a user, device or message source before granting access, that is, confirming that the claimed identity is genuine. It is achieved by something you know (password, PIN), something you have (smart card, OTP token) or something you are (fingerprint, face), and by digital certificates. For example, logging in to an e-banking account with a password and an OTP.

Together with availability, confidentiality and integrity form the core goals of information security (the CIA triad), and authentication is the basis of access control.

Multimedia. Multimedia is the integrated use of multiple forms of content, namely text, graphics, still images, audio, video and animation, delivered through a computer or other electronic device, usually with interactivity. The elements are combined and controlled by software so that the user can navigate and interact with the content.

Applications of multimedia

• Education and training: e-learning, computer-based tutorials, simulations, virtual classrooms and educational CDs.
• Entertainment: video games, movies, animation, music and video streaming.
• Business: presentations, advertisements, product demonstrations, kiosks and video conferencing.
• Medicine: medical imaging, simulated surgery training, telemedicine.
• Engineering and science: visualization, CAD, 3D modelling and simulation.
• Publishing and journalism: online newspapers, e-books, digital magazines.
• Public places: information kiosks at airports, museums and tourist sites.
• Virtual reality and web: interactive websites, social media, virtual tours.
• Government and e-governance: public awareness campaigns and online services.

Big Data. Big data refers to data sets that are so large, fast-growing and varied that traditional database tools cannot capture, store, manage or analyse them efficiently. It is described by the "V"s: Volume (very large amounts, terabytes to petabytes), Velocity (speed at which data is generated and processed), Variety (structured, semi-structured and unstructured data such as text, images, logs and video), and often Veracity (quality) and Value. Sources are social media, sensors, e-commerce transactions and web logs. Technologies include Hadoop, Spark and NoSQL databases.

Centralized versus distributed database

(a) Converting $(10.4)_{10}$ to binary

Integer part: $10=8+2=(1010)_2$.

Fractional part: multiply by 2 repeatedly and record the integer parts:

The remainder 0.4 repeats, so the fraction is $0.0110\,0110\ldots$, a recurring binary fraction. Therefore

$(10.4)_{10}=(1010.0110\,0110\ldots)_2\approx(1010.0110)_2$

(b) Binary addition of $(10111)_2+(01111)_2$

  carries: 1 1 1 1 1
           1 0 1 1 1     (23)
         + 0 1 1 1 1     (15)
         -----------
         1 0 0 1 1 0     (38)

Bit by bit from the right: $1+1=0$ carry 1; $1+1+1=1$ carry 1; $1+1+1=1$ carry 1; $0+1+1=0$ carry 1; $1+0+1=0$ carry 1; final carry 1.

$(10111)_2+(01111)_2=(100110)_2$

Check: $23+15=38=32+4+2=(100110)_2$.

Fixed-point number representation

In fixed-point representation the position of the binary (radix) point is fixed and assumed to be at a definite place in the word, so the number of bits for the integer part and for the fractional part is fixed beforehand. A word of $n$ bits is divided into a sign bit, an integer field and a fraction field. If the point is at the right end, only integers are represented; if it is placed after the sign bit, only fractions are represented. The advantages are simple hardware and fast arithmetic; the disadvantages are a limited range and precision.

Negative numbers may be represented in sign-magnitude, 1's complement or 2's complement form. The leftmost bit is the sign bit (0 for positive, 1 for negative).

Fixed-point representation of signed 8

Using an 8-bit word (1 sign bit and 7 magnitude bits, point at the right end):

• $+8$: $0\,0001000$ in all three forms.
• $-8$ in sign-magnitude: $1\,0001000$; in 1's complement: $1\,1110111$; in 2's complement: $1\,1111000$.

Converting $(14.14)_{10}$

Integer part: $14=(1110)_2$.

Fraction $0.14$ by repeated multiplication by 2:

So $0.14\approx0.00100011110101_2$ and

$(14.14)_{10}\approx(1110.00100011110101)_2$

(The fraction does not terminate, so the result is approximate.)

For octal, convert the integer part by dividing by 8: $14=1\times8+6=(16)_8$. Fraction by repeated multiplication by 8:

$0.14\times8=1.12\to1$; $0.12\times8=0.96\to0$; $0.96\times8=7.68\to7$; $0.68\times8=5.44\to5$; $0.44\times8=3.52\to3$.

$(14.14)_{10}\approx(16.10753)_8$

Check by grouping the binary in threes: $1110\to001\,110=16$ and $.001\,000\,111\,101\,01\to.10752$ (the last digit differs only because of truncation).

Switching. In a network with many nodes, it is not practical to connect every pair of nodes directly. Switching is the technique of moving data from a source to a destination through intermediate nodes (switches) of the network, setting up the path along which the data travels. The main techniques are circuit switching, packet switching and message switching.

Circuit switching versus packet switching

Advantages of optical fibers

• Very high bandwidth and data rate (Gbps to Tbps).
• Very low signal attenuation, so fewer repeaters are needed over long distances.
• Immunity to electromagnetic and radio-frequency interference and crosstalk.
• Greater security, since tapping is difficult.
• Light weight and small diameter compared with copper cable.
• No electrical hazards, such as sparks or short circuits, and no corrosion.
• Low error rate and reliable long-term transmission.

Why system software is needed

Hardware understands only machine language (binary). A user or an application program cannot control the CPU, memory, disk and I/O devices directly. System software is the layer between hardware and application software that:

• manages and allocates hardware resources (CPU time, memory, files, devices);
• hides hardware complexity and gives a convenient interface (GUI/command line);
• translates programs written in high-level languages into machine code (compilers, assemblers, interpreters);
• provides device drivers so that peripherals work with the computer;
• loads, links and runs programs, and provides utilities for maintenance, security and error handling.

Without system software every application would have to manage the hardware itself, and the computer would be practically unusable.

Types of operating system

1. Batch operating system. Similar jobs are collected into a batch and run one after another without user interaction. Good for large repetitive jobs (payroll, billing); CPU idle time is reduced but there is no interactivity and turnaround time is long.
2. Multiprogramming / multitasking system. Several programs are kept in memory and the CPU switches among them, so the CPU is rarely idle. Time-sharing is the interactive form: each user gets a small time slice, giving the impression of a dedicated machine (UNIX, Windows).
3. Multiprocessing (parallel) system. Two or more CPUs share memory and I/O and execute processes simultaneously, giving higher throughput and reliability.
4. Real-time operating system (RTOS). Responds to inputs within a guaranteed time limit. Hard real-time (missile control, pacemaker) never misses a deadline; soft real-time (multimedia streaming) tolerates occasional delay.
5. Distributed operating system. A group of independent computers connected by a network appears to the user as a single system; resources and load are shared (e.g. LOCUS, Amoeba).
6. Network operating system. Runs on a server and lets clients share files, printers and users over a network (Windows Server, Novell NetWare).
7. Single-user and multi-user systems. Single-user (MS-DOS, Windows 98) supports one user at a time; multi-user (UNIX, Linux) lets many users work at once with separate accounts and protection.
8. Mobile / embedded operating system. Designed for phones and devices with limited resources (Android, iOS).

Role of the Control Unit (CU)

The control unit is the part of the CPU that directs the operation of the whole computer. It does not process data itself; it:

• fetches each instruction from memory using the program counter;
• decodes the instruction to find the operation and operands;
• generates timing and control signals that activate the ALU, registers, memory and I/O devices in the correct sequence;
• controls the flow of data between CPU, memory and peripherals;
• handles interrupts and keeps all components synchronised with the clock.

Analog vs digital computer

RISC (Reduced Instruction Set Computer) uses a small set of simple instructions, each normally executed in one clock cycle. CISC (Complex Instruction Set Computer) uses a large set of powerful instructions, many of which take several cycles.

Purpose of cache memory

Cache is a small, very fast memory placed between the CPU and main memory (RAM). The CPU is much faster than RAM, so without cache it would wait for data. Cache keeps copies of the most recently and frequently used instructions and data (principle of locality). When the CPU finds what it needs in the cache (a hit) it avoids the slow RAM access; otherwise (a miss) the block is fetched from RAM into the cache. This reduces the average access time and increases overall system speed. Cache is organised in levels L1 (inside the CPU, fastest), L2 and L3.

Sequential access vs direct access

A keyboard is an input device that converts key presses into binary codes the computer can understand. It contains a key matrix, a keyboard controller (microprocessor) and a cable or wireless interface.

Working mechanism

1. The keys are arranged in a grid of rows and columns, with a switch at each crossing point. Pressing a key closes the circuit (or changes capacitance) at its row-column intersection.
2. The keyboard controller continuously scans the matrix by sending a signal along each row and sensing the columns. When a key is pressed, the controller detects which row and column are connected and so identifies the key.
3. The controller produces a scan code for that key. A separate code is produced when the key is released, so the system can tell press from release and handle combinations such as Shift, Ctrl and Alt.
4. The scan code is stored in the keyboard's small buffer and sent serially through the cable (PS/2 or USB) or by Bluetooth/radio to the computer.
5. The keyboard driver and the operating system translate the scan code into the character code (ASCII/Unicode) according to the current layout and shift state, and pass it to the active program, which shows the character on the screen.
6. If a key is held down, the controller repeats the code after a delay (typematic rate).

Key switch types are mechanical (spring contact), membrane (rubber dome pressing two conductive layers) and capacitive.

Why an IP address is used

Every device on the Internet must be uniquely identifiable so that data packets can be sent to the correct destination and the reply can come back. The IP (Internet Protocol) address is a numeric logical address (for example 192.168.1.10 in IPv4, 32 bits) assigned to each host. It serves two purposes:

• Identification of the host (and of its network);
• Location addressing, which lets routers decide the path along which packets are forwarded.

It is similar to a postal address for a house: without it packets could not be routed.

Significance of domain names

Numeric addresses are hard for people to remember. A domain name (for example www.tu.edu.np) is a human-readable name mapped to an IP address by the Domain Name System (DNS).

• Easy to remember and type, and suitable for branding (.com, .edu, .np show type or country).
• The IP address of a website can change (new server or host) while the domain name stays the same, so users are not affected.
• Several domain names can point to one server and one name can map to several servers (load balancing).
• Supports email addresses (user@domain) and hierarchical organisation of the Internet.

Computer network

A computer network is a collection of two or more computers and other devices (printers, servers, switches) interconnected by wired or wireless transmission media so that they can share data, files, hardware resources and communicate with each other.

Network for the building

The building has 30 computers (15 on each floor) within a single small area, so I would create a Local Area Network (LAN) using a star topology with Ethernet.

Design

• One switch on each floor; every computer connects to its floor switch by UTP (Cat 5e/6) cable.
• The two floor switches are linked by a backbone cable (preferably fibre or Cat 6), forming an extended star (tree) topology.
• A router at the top connects the LAN to the Internet, and a server may provide file/print sharing, with IP addresses given by DHCP.

Justification

• The area is small (one building), which is exactly the range of a LAN (up to a few kilometres), so MAN or WAN would be unnecessary and costly.
• LAN gives high speed (100 Mbps to 1 Gbps) at low cost, and all data, printers and Internet connection can be shared.
• In a star topology, a failure of one cable or computer affects only that computer; faults are easy to find and new computers are easy to add.
• Using one switch per floor limits cable length and traffic, and the whole network can be managed centrally.

Malicious software (malware)

Malware is any program deliberately written to damage, disrupt, steal data from, or gain unauthorised access to a computer system without the owner's knowledge. Types include viruses, worms, trojan horses, spyware, adware, ransomware, rootkits and logic bombs.

Virus vs worm

Multimedia is the integrated use of several media (text, graphics, audio, video and animation) in a single computer-controlled application. Its main characteristics are:

1. Multiple media. Combines two or more media types, such as text with images, sound and video, to present information.
2. Digital representation. All media are converted to digital form, so they can be stored, processed, compressed and transmitted by a computer.
3. Integration. The different media elements are combined into one coordinated presentation, synchronised in time (for example, lip movement matches audio).
4. Interactivity. The user can control what is shown, in what order and when, through menus, buttons and links (as in educational software and games).
5. Computer controlled. The sequence and presentation is managed by a computer program.
6. Hypermedia / non-linear navigation. Information is connected by links, so users can jump freely instead of reading in a fixed order.
7. Large storage and bandwidth need. Audio and video files are big, so compression and fast storage and networks are needed.
8. Real-time operation. Audio and video must be played continuously without delay (streaming).
9. Applications. Education, entertainment, advertising, training, simulation, web pages and video conferencing.

Database system

A database is an organised collection of related data stored so that it can be easily accessed, managed and updated. A database system consists of the database, the database management system (DBMS) software that creates, manipulates and controls access to it, the hardware, and the users (DBA, programmers, end users). Examples of DBMS are MySQL, Oracle and SQL Server.

Storing data using the relational model

In the relational model (proposed by E. F. Codd), data is stored in relations, which are two-dimensional tables:

• Each table (relation) represents one entity, e.g. STUDENT.
• Each row (tuple) is one record; each column (attribute) is a property with a name and a domain (data type).
• A primary key uniquely identifies each row; a foreign key is an attribute that refers to the primary key of another table and links the tables.
• Every cell holds a single atomic value, and no two rows are identical.

Example:

STUDENT

DEPARTMENT

The tables are joined through DeptID, so a department name is stored only once, reducing redundancy. Data is created and retrieved with SQL, e.g. SELECT Name, DeptName FROM STUDENT, DEPARTMENT WHERE STUDENT.DeptID = DEPARTMENT.DeptID;.

Computer software

Computer software is the set of programs, procedures and related data that instruct the hardware what to do. It is intangible and is broadly divided into system software and application software.

System software vs application software

Types of system software

1. Operating system. Controls and coordinates the hardware and gives users an interface. Functions: process management, memory management, file management, device management, security. Examples: Windows, Linux, macOS, Android.
2. Language translators. Convert programs into machine code. Assembler translates assembly language; compiler translates the whole high-level program at once and reports all errors; interpreter translates and executes line by line.
3. Device drivers. Small programs that let the OS communicate with a peripheral such as a printer, graphics card or scanner.
4. Utility programs. Maintenance tools such as antivirus, disk defragmenter, backup, file compression and disk cleanup.
5. Linker and loader. The linker combines object modules and library routines into one executable; the loader places the executable in memory and starts it.
6. Firmware / BIOS. Permanent instructions in ROM that start the computer, run the power-on self test and load the operating system.

Benefits of a computer network

• Resource sharing. Printers, scanners, storage and software can be shared, which reduces cost.
• Data and file sharing. Users can quickly exchange and access the same files, with centralised data.
• Communication. Email, chat, video conferencing and messaging between people anywhere.
• Reliability. Data can be backed up or duplicated on several machines, so a failure of one does not lose everything.
• Cost saving and scalability. Sharing hardware and licences is cheaper, and new computers are easy to add.
• Centralised administration and security. Users, backups and updates are managed from the server.
• Access to the Internet and remote working from many places.

Types of system software

System software manages the hardware and supports application software. Its main types are:

• Operating system (Windows, Linux): manages the processor, memory, files and devices and provides the user interface.
• Language translators (assembler, compiler, interpreter): convert programs into machine code.
• Device drivers: let the operating system communicate with peripherals.
• Utility programs: antivirus, backup, disk defragmenter and compression tools.
• Linker and loader: combine object code into an executable and load it into memory.
• Firmware/BIOS: start-up instructions stored in ROM.

Network topology

Network topology is the physical or logical arrangement of the nodes and links of a network.

• Bus: all nodes share a single backbone cable; cheap but a break stops the network.
• Star: every node connects to a central hub or switch; easy to manage and fault-tolerant, but depends on the hub.
• Ring: each node is connected to two neighbours forming a closed loop; data travels in one direction.
• Mesh: every node is linked with others; very reliable but expensive.
• Tree (hybrid): star networks joined along a bus; scalable and hierarchical.

Database

A database is an organised collection of logically related data stored electronically so that it can be easily accessed, managed and updated. It is created and managed by a database management system (DBMS) such as MySQL, Oracle or SQL Server. Example: a college database storing students, courses and results.

Benefits of the database system

• Reduced data redundancy. Data is stored once and shared, saving space.
• Data consistency and integrity. Updating at one place keeps all copies correct, and constraints enforce valid data.
• Data sharing. Many users and applications can use the same data concurrently.
• Security. Access rights and passwords restrict who can see or change data.
• Backup and recovery. The DBMS restores data after a failure.
• Data independence. Programs are not affected when the storage structure changes.
• Standards and query facilities. Quick retrieval through query languages such as SQL, and easier decision making.

Database system architecture

1. One-tier (single-user). The database and the user interface are on the same machine; used by a single developer or a personal database such as MS Access.
2. Two-tier (client-server). The client application (user interface and business logic) communicates directly with the database server through ODBC/JDBC. It is simple and fast but the client is heavy and scaling is limited.
3. Three-tier. The presentation tier (browser/GUI), the application tier (business logic on an application server) and the database tier (DBMS) are separate. It gives better security, scalability and maintenance; web applications use it.

ANSI/SPARC three-level architecture (logical view)

• External level (view level): the different user views of parts of the data.
• Conceptual level: the logical structure of the whole database (entities, relationships, constraints).
• Internal level: the physical storage of data (files, indexes).

The mappings between levels give logical and physical data independence.

Characteristics of a computer

• Speed. Performs millions or billions of operations per second.
• Accuracy. Gives precise results; errors arise mainly from wrong input or programs (GIGO).
• Diligence. Works for long hours without tiredness or loss of concentration.
• Versatility. Can do many different kinds of tasks (calculation, graphics, communication).
• Storage. Stores huge quantities of data and retrieves it quickly.
• Automation. Executes a stored program without human intervention.
• No intelligence (no IQ). It does only what it is instructed to do.
• Reliability and repeatability of results.

Digital vs analog computer

Primary memory

Primary (main) memory is the memory that is directly accessed by the CPU. It holds the programs and data currently in use. It is fast and made of semiconductor chips (RAM, ROM, cache). RAM is volatile (contents are lost when power is off), ROM is non-volatile.

Primary vs secondary memory

Memory hierarchy

Memory hierarchy is the arrangement of the different memories of a computer in levels according to speed, cost and capacity. Going upward, memory is faster, costlier per bit and smaller; going downward it is slower, cheaper and larger. The order is:

1. CPU registers (fastest, smallest)
2. Cache memory (L1, L2, L3)
3. Main memory (RAM)
4. Secondary storage (SSD, hard disk)
5. Tertiary/offline storage (optical disks, magnetic tape)

The hierarchy works because of the principle of locality: most accesses go to the fast levels, giving an average access time near that of the fast memory at a cost near that of the slow one.

Types of ROM

ROM (Read Only Memory) is non-volatile memory whose contents are retained without power; it stores the BIOS and firmware.

• Mask ROM (MROM). Programmed by the manufacturer during fabrication; cannot be changed. Economical for mass production.
• PROM (Programmable ROM). Blank when made; the user writes it once with a PROM programmer by blowing fuses. It cannot be erased.
• EPROM (Erasable PROM). Can be erased by exposing the chip to ultraviolet light through a quartz window and then reprogrammed.
• EEPROM (Electrically EPROM). Erased and rewritten electrically, byte by byte, without removing the chip; used for BIOS and settings.
• Flash memory. A form of EEPROM erased in blocks; faster and cheaper, used in pen drives, SSDs and memory cards.

1. Keyboard

The keyboard is the most common input device for entering text, numbers and commands. Under the keys is a matrix of rows and columns. Pressing a key closes a switch at one crossing. A keyboard controller chip continuously scans the matrix, finds the row and column of the pressed key and generates a scan code. This code is sent to the computer through a USB/PS/2 port or wirelessly; the operating system converts it to a character (ASCII/Unicode) and passes it to the running program. Keys include alphanumeric, function, control and navigation keys. The common layout is QWERTY.

2. Mouse

The mouse is a pointing device that moves a pointer on the screen and selects objects. An optical mouse has an LED (or laser) that lights the surface and a small camera/sensor that takes thousands of pictures per second; a processor compares successive images to find the direction and distance moved, and sends the movement to the computer. It has buttons (left, right) and a scroll wheel; clicking sends a signal that the operating system uses to select or open items. It connects by cable, USB receiver or Bluetooth. (An older mechanical mouse used a rolling ball and sensor rollers.)

Convert $(110101.101)_2$ to decimal by multiplying each bit by its place value $2^k$.

Integer part

$110101_2 = 1\cdot2^5 + 1\cdot2^4 + 0\cdot2^3 + 1\cdot2^2 + 0\cdot2^1 + 1\cdot2^0 = 32 + 16 + 0 + 4 + 0 + 1 = 53$

Fractional part

$0.101_2 = 1\cdot2^{-1} + 0\cdot2^{-2} + 1\cdot2^{-3} = 0.5 + 0 + 0.125 = 0.625$

Result

$(110101.101)_2 = (53.625)_{10}$

IP address

An IP (Internet Protocol) address is a unique numerical logical address assigned to every device (host) connected to a network that uses the Internet Protocol. It has a network part and a host part. IPv4 example: 192.168.1.1.

Why it is needed

• To uniquely identify each device on a network or the Internet.
• To locate the device: routers use the destination IP address to forward packets across networks.
• To enable two-way communication, since the sender's address tells the receiver where to reply.

IPv4 vs IPv6

Multimedia combines different forms of content in a single application. The elements are:

1. Text. The basic element, used for titles, menus, headings and explanations. Its appearance depends on font, size, colour and style. Text can be plain or hypertext, with links to other content.
2. Graphics (images). Still pictures, either bitmap (raster; made of pixels, e.g. JPEG, PNG, BMP) or vector (made of lines and shapes defined by equations, e.g. SVG). They illustrate ideas and make a page attractive.
3. Audio. Speech, music and sound effects, stored as digital samples (WAV, MP3, MIDI). Quality depends on sampling rate and bit depth.
4. Video. Recorded moving pictures played as a rapid sequence of frames (typically 24 to 30 fps), with sound. Files are large and are compressed (MPEG, MP4, AVI).
5. Animation. Creating the illusion of motion by displaying a series of drawn or computer-generated images in succession (2D, 3D, GIF, Flash).
6. Interactivity. Buttons, menus and links that let the user control the flow of the presentation.

Cryptography

Cryptography is the science of securing information by converting it into an unreadable form so that only the intended recipient can read it. Original data is plaintext; the scrambled form is ciphertext. Encryption converts plaintext to ciphertext using an algorithm and a key; decryption reverses it.

Types

• Symmetric-key cryptography: the same secret key encrypts and decrypts (DES, AES). It is fast, but the key must be shared securely.
• Asymmetric (public-key) cryptography: a public key encrypts and a different private key decrypts (RSA). No secret needs to be shared beforehand.

How it provides security

• Confidentiality. Only someone with the proper key can read the data, even if it is intercepted on the network.
• Integrity. Hash functions (SHA) produce a digest; any change in the data changes the digest, so tampering is detected.
• Authentication. Digital certificates and passwords prove the identity of the sender or server.
• Non-repudiation. A digital signature (hash encrypted with the sender's private key) proves who sent the message, and the sender cannot deny it.

Uses: HTTPS/SSL, online banking, e-mail, password storage, digital signatures.

(a) Central Processing Unit (CPU)

The CPU is the brain of the computer; it interprets and executes program instructions and controls all other parts. It is a microprocessor chip with three main components:

• Arithmetic Logic Unit (ALU): performs arithmetic (addition, subtraction, etc.) and logical (AND, OR, NOT, comparison) operations.
• Control Unit (CU): fetches and decodes instructions and sends control signals to coordinate the ALU, memory and I/O.
• Registers: small, very fast storage inside the CPU (program counter, instruction register, accumulator).

Its performance depends on clock speed (GHz), number of cores, word length and cache size. It works through the fetch-decode-execute cycle.

(b) IoT (Internet of Things)

IoT is a network of physical objects ("things") such as sensors, appliances, vehicles and wearable devices that have embedded electronics, software and network connectivity so that they can collect and exchange data over the Internet, often without human intervention. A typical system has sensors/devices, connectivity (Wi-Fi, Bluetooth, LoRa, 5G), a cloud platform to process data, and a user interface or application. Applications: smart homes (lights and thermostats), smart agriculture, healthcare monitoring, smart cities and industrial automation. Challenges include security, privacy and standardisation.

Analog vs digital computer

CPU and how it works

The Central Processing Unit (CPU) is the part of the computer that executes instructions and controls all operations. Its parts are:

• ALU: arithmetic and logical operations.
• Control unit: fetches, decodes and sequences instructions, and issues control signals.
• Registers: high-speed storage (program counter PC, instruction register IR, memory address register MAR, memory data register MDR, accumulator).

Working: the program and data are loaded into main memory. The CPU repeatedly takes the next instruction from the address in the PC, decodes it, fetches any operands, lets the ALU perform the operation, stores the result in a register or memory and goes on to the next instruction. The control unit synchronises everything with the system clock.

Instruction cycle

The instruction cycle (fetch-decode-execute cycle) consists of:

1. Fetch. The address in PC is placed in MAR; the instruction is read from memory into MDR and then into IR; PC is incremented to point to the next instruction.
2. Decode. The control unit decodes the opcode in IR to find the operation and the addressing of the operands.
3. Execute. The ALU or other unit performs the operation, which may need an operand fetch from memory or register.
4. Store (write back). The result is written to a register or memory.

Interrupts are checked at the end of each cycle, and then the cycle repeats.

Computer network

A computer network is a set of two or more computers and devices connected by communication links (cables or wireless) to share data, resources and information. Examples are a school LAN and the Internet.

Transmission media

Media are of two kinds: guided (wired) and unguided (wireless).

A. Guided media

1. Twisted pair cable (UTP/STP): pairs of insulated copper wires twisted to reduce interference.
   • Advantages: cheap, flexible, easy to install; widely used in LANs and telephones.
   • Disadvantages: limited distance (about 100 m), low bandwidth, susceptible to noise (UTP) and tapping.
2. Coaxial cable: central copper conductor with insulation, a metal shield and an outer jacket.
   • Advantages: higher bandwidth and better noise immunity than twisted pair, longer distances, used in cable TV and older Ethernet.
   • Disadvantages: bulkier and costlier than twisted pair, harder to install, a break affects the bus.
3. Optical fibre: glass or plastic core carrying light signals by total internal reflection (single-mode, multi-mode).
   • Advantages: very high bandwidth and speed, very low attenuation so long distance, immune to electromagnetic interference, hard to tap, secure.
   • Disadvantages: expensive, difficult to install and splice, fragile, needs special equipment.

B. Unguided (wireless) media

1. Radio waves: omnidirectional, pass through walls; used for FM radio, Wi-Fi, Bluetooth, mobile.
   • Advantages: no cables, mobility, cheap to cover large areas.
   • Disadvantages: interference, less secure, limited by spectrum.
2. Microwaves: line-of-sight terrestrial links using towers.
   • Advantages: high data rate, no cable laying over rough terrain.
   • Disadvantages: need line of sight, affected by rain and weather, towers cost money.
3. Satellite: signals relayed by a satellite in orbit.
   • Advantages: global coverage, reaches remote areas.
   • Disadvantages: expensive, propagation delay, weather effects.
4. Infrared: short-range, line-of-sight (TV remote). Cheap and secure inside a room but cannot pass walls and covers a short distance.

Database management system (DBMS)

A DBMS is software that lets users define, create, store, retrieve, update and control a database. It acts as an interface between the database and users or application programs. Examples are MySQL, Oracle, PostgreSQL, SQL Server and MS Access.

Why a DBMS is used

• It reduces data redundancy and prevents inconsistency, since data is stored once and shared.
• It enforces integrity constraints (primary key, foreign key, data types) so data stays valid.
• It provides security through user accounts and access privileges.
• It supports concurrent access by many users with transaction control (locking).
• It offers backup and recovery after crashes.
• It gives data independence: changes in storage do not affect applications.
• It provides fast querying through SQL and indexing, and eases data sharing and decision making compared with file-based systems.

Database system architecture

Three-schema (ANSI/SPARC) architecture

• External level (view level): the individual views of different users, showing only the data they need.
• Conceptual level: the logical design of the whole database (entities, attributes, relationships, constraints), independent of storage.
• Internal level: how the data is physically stored (files, indexes, record formats).

The mappings between levels provide logical data independence (changing the conceptual schema without changing views) and physical data independence (changing storage without changing the conceptual schema).

Tiered (application) architecture

• Two-tier (client-server): the client application talks directly to the DBMS server.
• Three-tier: presentation tier (user interface), application/business-logic tier, and the database tier. It improves security, scalability and maintenance, and is used in web applications.

Components of the DBMS: DDL compiler, DML/query processor and optimiser, storage manager, transaction manager, buffer manager, and the data dictionary, which stores metadata.

Cache memory is a small, very fast memory placed between the CPU and main memory (RAM). It holds copies of the instructions and data that the CPU has used recently or is likely to need next, so the CPU does not have to wait for the slower RAM. It is built from SRAM, and is arranged in levels (L1 inside the core, L2 and L3 larger and slower). When the needed item is found in cache it is a cache hit; otherwise it is a cache miss and the item is fetched from RAM.

Cache memory versus register

A source data entry device captures data directly at the point where it originates (the source) and converts it into machine-readable form without a person retyping it. This reduces transcription errors and speeds up entry.

Common source data entry devices:

• Barcode reader. Reads the black and white bars on a product label with a laser or light sensor and sends the product code to the computer. Example: supermarket billing counters.
• QR code scanner. Reads two-dimensional codes through a camera; used in mobile payments and ticketing.
• Optical Character Recognition (OCR). Scans printed text and converts the characters into editable text. Example: digitising old books and number plates.
• Optical Mark Recognition (OMR). Detects marks made in marked positions on a sheet. Example: multiple-choice answer sheets in entrance examinations.
• Magnetic Ink Character Recognition (MICR). Reads characters printed in magnetic ink. Example: the cheque number and account number on bank cheques.
• Magnetic stripe / smart card reader. Reads data stored on ATM, debit and ID cards.
• Biometric devices. Fingerprint, iris or face scanners that capture a person's physical traits for attendance and security.
• RFID reader. Reads tags using radio waves, for example toll gates and library books.
• Point-of-sale (POS) terminal. Captures sales data at the time of sale and updates stock records.

Binary addition follows four basic rules, working from the rightmost bit with a carry moving to the left:

• $0+0=0$
• $0+1=1$
• $1+0=1$
• $1+1=10$, that is sum $0$ and carry $1$

With a carry-in, $1+1+1=11$, that is sum $1$ and carry $1$.

Addition of $11001_2$ and $11011_2$

$$\begin{array}{rrrrrr} \text{carry:} & 1 & 1 & 0 & 1 & 0 \\ & & 1 & 1 & 0 & 0 & 1 \\ + & & 1 & 1 & 0 & 1 & 1 \\ \hline & 1 & 1 & 0 & 1 & 0 & 0 \end{array}$$

Bit by bit from the right:

1. $1+1=0$, carry $1$
2. $0+1+1=0$, carry $1$
3. $0+0+1=1$, carry $0$
4. $1+1=0$, carry $1$
5. $1+1+1=1$, carry $1$
6. The final carry $1$ is written as the leftmost bit.

Result: $11001_2 + 11011_2 = 110100_2$.

Check: $25 + 27 = 52$ and $110100_2 = 32+16+4 = 52$.

Software is a set of programs, procedures and related data that instruct the computer hardware what to do and how to do it. It is intangible: it can be stored and run but not touched. Examples are Windows, Linux, MS Word and web browsers.

Software versus hardware

Why software is needed

• Hardware alone cannot do anything useful; software tells it which operations to perform.
• The operating system manages the CPU, memory, files and devices, and provides an interface for the user.
• Application software lets users carry out tasks such as writing documents, accounting, designing and browsing.
• It hides the complexity of hardware, so users need not know machine language.
• The same hardware can perform many different jobs simply by loading different software.

The operating system (OS) is system software that manages the computer hardware and provides services to users and application programs. Its main functions are:

1. Process management. Creates, schedules and terminates processes, shares CPU time among them (multitasking), and handles synchronisation and deadlock.
2. Memory management. Allocates and frees main memory for programs, keeps track of which parts are in use, and uses virtual memory/paging when RAM is insufficient.
3. File management. Creates, stores, names, organises (in directories), protects, and retrieves files on secondary storage.
4. Device management. Controls input/output devices through device drivers, handles buffering, spooling and interrupts.
5. Secondary storage management. Allocates disk space, schedules disk requests and manages free space.
6. Security and protection. Authenticates users with passwords, and controls access to files, memory and other resources.
7. User interface. Provides a command line interface (CLI) or graphical user interface (GUI) so the user can interact with the machine.
8. Networking and communication. Supports communication between processes and between computers.
9. Error detection and resource accounting. Detects hardware and program errors and records resource usage.

The internet is a worldwide network of interconnected computer networks that use the TCP/IP protocol suite to exchange data. It provides services such as the World Wide Web, email, file transfer, video calls and social media. It is not owned by any single organisation.

Internet architecture

The internet is organised as a hierarchy of networks joined by routers:

• End systems (hosts). Computers, phones and servers at the edge. Each is identified by an IP address.
• Access networks. Connect hosts to the internet through dial-up, DSL, cable, fibre, Wi-Fi or mobile (4G/5G) links.
• Internet Service Providers (ISPs). Tier-3 (local) ISPs serve homes and businesses and buy connectivity from regional Tier-2 ISPs, which in turn connect to Tier-1 ISPs, the large global backbone carriers.
• Backbone. High-capacity fibre links among Tier-1 ISPs and across continents.
• Internet Exchange Points (IXPs). Places where ISPs interconnect directly to exchange traffic.
• Routers and switches. Forward packets from the source to the destination along a suitable path.
• Protocols. It is built on layered TCP/IP: the application layer (HTTP, SMTP, DNS), transport layer (TCP, UDP), internet layer (IP), and link/physical layer (Ethernet, Wi-Fi). Data is divided into packets, which can travel by different routes and are reassembled at the destination.
• Servers. Web, mail and DNS servers provide services; DNS converts names such as example.com to IP addresses.

The client-server model is the usual pattern: a client sends a request and a server replies.

Multimedia is the integrated use of more than one type of media, namely text, graphics, images, audio, video and animation, delivered and controlled through a computer or digital device, usually with some interactivity.

Applications of multimedia

• Education and training. E-learning, interactive tutorials, simulations and virtual classrooms make lessons easier to understand.
• Entertainment. Video games, movies, music streaming, animation and special effects.
• Business. Presentations, product demonstrations, advertising and online catalogues.
• Medicine and health. 3D anatomy models, medical imaging, telemedicine and surgical training simulators.
• Communication. Video conferencing, social media and multimedia messaging.
• Engineering and science. Visualisation and simulation of designs, structures and experiments.
• Journalism and publishing. Online newspapers, e-books and digital magazines with audio and video.
• Virtual and augmented reality. Immersive tours, gaming and training.
• Public places. Information kiosks, museums and tourism guides.

Security attack versus security threat

In short, a threat is the possibility of harm, and an attack is the threat being carried out.

How a virus affects a computer

A computer virus is a malicious program that attaches itself to other files or programs and spreads when they are run or copied. It can:

• Corrupt, delete or modify files and data, and sometimes erase the hard disk.
• Slow down the system and use up memory and CPU time.
• Cause frequent crashes, unexpected restarts or error messages.
• Fill storage space by replicating itself.
• Damage the boot sector or operating system so the computer cannot start.
• Steal personal information, passwords and banking details.
• Spread to other computers through email attachments, pen drives and downloads.
• Display unwanted pop-ups and open back doors for attackers.

Protection comes from updated antivirus software, regular patches, avoiding unknown attachments and keeping backups.

GIS (Geographic Information System)

A GIS is a computer-based system for capturing, storing, analysing, managing and displaying data that is linked to locations on the earth. It combines maps with a database, so each map feature (road, river, building) has attributes attached to it.

• Components: hardware, software, spatial and attribute data, people, and methods.
• Data types: raster (grids/images such as satellite pictures) and vector (points, lines and polygons).
• Applications: urban and land-use planning, disaster management (flood and landslide mapping), agriculture, forestry, navigation and route finding, utilities such as water and electricity, and public health.

E-commerce

E-commerce is buying and selling of goods and services, and the transfer of money, over the internet or other electronic networks.

• Types: B2B (business to business), B2C (business to consumer), C2C (consumer to consumer) and G2C (government to citizen).
• Advantages: available 24 hours, wider market, lower operating costs, easy comparison of products and quick transactions.
• Disadvantages: security and fraud risks, no physical inspection of goods, dependence on internet, and delivery delays.
• Examples: Daraz, Amazon, eSewa and Khalti payments.

Memory hierarchy

Computer memory is arranged as a hierarchy so that a good balance of speed, size and cost is obtained. Going up the hierarchy memory is faster, smaller and costlier per byte; going down it is slower, larger and cheaper. The CPU accesses the upper levels first.

            /\
           /  \        Registers        (fastest, smallest, costliest)
          /----\
         /      \      Cache memory (L1, L2, L3)
        /--------\
       /          \    Main memory (RAM)
      /------------\
     /              \  Secondary storage (HDD, SSD)
    /----------------\
   /                  \ Tertiary / offline (optical discs, magnetic tape)
  /--------------------\   (slowest, largest, cheapest)

1. Registers. Tiny storage inside the CPU holding the data and instructions being processed now. Fastest access.
2. Cache memory. SRAM between the CPU and RAM holding frequently used data and instructions.
3. Main memory (RAM). Volatile memory (DRAM) holding the programs and data in use. Directly addressed by the CPU.
4. Secondary storage. Non-volatile, large devices such as hard disk, SSD and flash drives for permanent storage.
5. Tertiary/offline storage. Optical discs and magnetic tapes used for backup and archives.

Types of read only memory (ROM)

ROM is non-volatile memory whose contents are retained when power is off. It stores the start-up program (BIOS/firmware).

• MROM (Masked ROM). Contents are written permanently by the manufacturer during fabrication and cannot be changed.
• PROM (Programmable ROM). Blank when made; the user can write it once using a special device and cannot change it afterwards.
• EPROM (Erasable PROM). Can be erased by exposing it to ultraviolet light through a quartz window and then reprogrammed.
• EEPROM (Electrically Erasable PROM). Erased and rewritten electrically, byte by byte, without removing the chip. Used for BIOS in older machines.
• Flash memory. A form of EEPROM erased in blocks; fast and widely used in USB drives, SSDs and modern BIOS chips.

System software versus application software

Types of system software

1. Operating system. Manages hardware resources, processes, memory, files and devices, and gives a user interface. Examples: Windows, Linux, macOS, Android.
2. Device drivers. Small programs that let the operating system communicate with a hardware device such as a printer, graphics card or scanner.
3. Language translators. Convert programs into machine code:
   • Assembler converts assembly language to machine code.
   • Compiler translates the whole high-level program into machine code at once.
   • Interpreter translates and executes the program line by line.
4. Utility programs. Perform maintenance tasks: antivirus, disk cleanup, defragmenter, backup, file compression.
5. Linkers and loaders. A linker combines object modules and libraries into one executable file; a loader places the executable in memory for running.
6. Firmware/BIOS. Permanent software in ROM that starts the computer and performs the power-on self test.

Benefits of computer networks

• Resource sharing. Printers, scanners, storage and software can be shared by many users, reducing cost.
• Data and information sharing. Files and databases are accessible to all authorised users from different places.
• Communication. Email, chat, video conferencing and messaging between users.
• Centralised management and backup. Data, security and updates are managed from a server.
• Cost saving. Shared hardware and licences, and lower travel costs.
• Reliability. Data can be replicated on several machines, so a failure does not stop work.
• Scalability. Computers and users can be added easily.
• Remote access and collaboration. Users can work together on the same project from different locations.

Data transmission media

Transmission media are the physical or wireless paths carrying signals between devices. They are of two kinds.

A. Guided (wired) media

1. Twisted pair cable. Two insulated copper wires twisted together to reduce interference. UTP (unshielded) is cheap and used in LANs (Cat5e, Cat6); STP (shielded) has a metal shield for better protection. Limit is about 100 m per segment. Easy to install but susceptible to noise.
2. Coaxial cable. A central copper conductor surrounded by insulation, a metal shield and an outer jacket. Higher bandwidth and better noise immunity than twisted pair; used for cable television and older Ethernet (10Base2, 10Base5).
3. Optical fibre cable. A thin glass or plastic core carrying light pulses, surrounded by cladding. Very high bandwidth and speed, long distances, immune to electromagnetic interference and hard to tap. It is more costly and harder to install. Types are single-mode and multi-mode.

B. Unguided (wireless) media

1. Radio waves. Omnidirectional and pass through walls; used in Wi-Fi, FM radio, Bluetooth and mobile networks.
2. Microwaves. Line-of-sight, high-frequency, directional signals using towers or dishes; used for point-to-point links, TV and mobile backhaul.
3. Satellite communication. Signals relayed by orbiting satellites; covers wide and remote areas (GPS, TV broadcasting, VSAT), but has high cost and delay.
4. Infrared. Short-range line-of-sight, cannot pass through walls; used in remote controls.

A digital computer is a computer that processes data represented as discrete values, namely the binary digits 0 and 1. It performs calculations by counting and uses logic circuits to give exact results. Personal computers, laptops, smartphones and servers are digital computers.

Digital versus analog computers

The Central Processing Unit (CPU) is the brain of the computer. It has three main components.

1. Arithmetic Logic Unit (ALU). Performs all arithmetic operations (addition, subtraction, multiplication, division) and logical operations (AND, OR, NOT, comparisons such as greater than or equal to). Results are passed to registers or memory.
2. Control Unit (CU). Directs and coordinates all the operations of the computer. It fetches instructions from memory, decodes them, and sends control signals to the ALU, memory and input/output devices so that each instruction is executed in the correct sequence. It does not process data itself.
3. Registers. Very fast, small storage locations inside the CPU that hold data, instructions and addresses temporarily. Important ones are:
   • Program Counter (PC): address of the next instruction.
   • Instruction Register (IR): the instruction being executed.
   • Memory Address Register (MAR): address of memory location to be accessed.
   • Memory Data/Buffer Register (MDR/MBR): data being read from or written to memory.
   • Accumulator (AC): holds intermediate results of the ALU.

Modern CPUs also contain a small amount of cache memory, and communicate with other parts through buses.

Hard copy devices are output devices that produce a permanent, physical record of the output, usually on paper, which can be touched and kept. Examples: printers (laser, inkjet, dot-matrix) and plotters.

Soft copy devices are output devices that show the output temporarily in electronic form, which disappears when the display is turned off or changed. Examples: monitor (LCD/LED), projector and speakers.

Six human data entry devices (devices operated by a person to enter data):

1. Keyboard
2. Mouse
3. Joystick
4. Light pen
5. Touch screen
6. Microphone (voice input)

Trackball, touchpad and digitising tablet are also human data entry devices.

ASCII (American Standard Code for Information Interchange) is a character-encoding standard that assigns a numeric code to each letter, digit, punctuation mark and control character so that computers can store and exchange text. Standard ASCII uses 7 bits, giving $2^7 = 128$ characters (extended ASCII uses 8 bits for 256). For example, 'A' is 65 ($1000001_2$), 'a' is 97 and '0' is 48.

Converting $0.2345_{10}$ to binary

Multiply the fractional part by 2 repeatedly and record the integer part of each product.

Reading the integer parts from top to bottom:

$0.2345_{10} \approx 0.0011110000_2$

The conversion does not terminate exactly; the result is correct to about 10 binary places (error less than $2^{-10}$). Check: $2^{-3}+2^{-4}+2^{-5}+2^{-6} = 0.125+0.0625+0.03125+0.015625 = 0.234375 \approx 0.2345$.

Wearable computing refers to small computing devices worn on the body, or embedded in clothing and accessories, that perform computing tasks while the user moves about, often hands-free. They usually have sensors, a processor, wireless connectivity (Bluetooth/Wi-Fi) and are linked to a smartphone or the cloud.

Examples: smartwatches, fitness bands, smart glasses (Google Glass), VR/AR headsets, smart rings, health-monitoring patches and smart clothing.

Features

• Always on and always accessible.
• Sensors collect data such as heart rate, steps, sleep and location.
• Small, lightweight and comfortable to wear.
• Provide notifications, and often voice control.

Applications

• Healthcare and fitness monitoring.
• Navigation and communication.
• Industry and military (hands-free information, augmented reality).
• Entertainment and gaming.
• Payments and identification.

Limitations: limited battery life, small screens, privacy and security concerns, and cost.

A database is an organised collection of related data stored electronically so that it can be easily accessed, managed and updated. It is usually managed by a Database Management System (DBMS) such as MySQL, Oracle or SQL Server. Data is arranged in tables of rows (records) and columns (fields). Example: a student database with name, roll number and marks.

Benefits of using databases

• Reduced data redundancy. Data is stored once, avoiding duplicate copies.
• Data consistency and integrity. Rules and constraints keep data accurate and valid.
• Data sharing. Many users and applications can use the same data at the same time.
• Security. Access can be restricted by user permissions and passwords.
• Backup and recovery. The DBMS can restore data after failure.
• Data independence. Programs are not affected by changes in how data is stored.
• Fast retrieval. Queries (SQL) and indexes quickly find and report information.
• Concurrent access control. Prevents conflicts when several users update the same data.
• Standards enforcement and better decisions through up-to-date, organised information.

The main characteristics of multimedia are:

1. Integration of multiple media. Combines text, graphics, images, audio, video and animation in one product.
2. Interactivity. The user can control what is shown and in what order, through menus, buttons and links (not just passive viewing).
3. Digital representation. All media are stored and processed as digital data, so they can be edited and copied without loss.
4. Computer control. A computer or digital device presents and controls the media.
5. Hypermedia/non-linear navigation. Information is linked, so users move freely between topics rather than in a fixed sequence.
6. Synchronisation. Time-dependent media such as audio, video and animation are presented together in time (e.g. lip-sync).
7. Large storage and bandwidth need. Audio and video files are large and need compression, storage and fast networks.
8. Real-time delivery. Streaming media must be delivered continuously without delay.
9. Richness and engagement. Using several senses improves understanding and retention.

Cryptography is the science and practice of protecting information by converting it into an unreadable form so that only the intended receiver can read it. The original message (plaintext) is converted to ciphertext by encryption using an algorithm and a key; decryption reverses it. It provides confidentiality, integrity, authentication and non-repudiation.

• Symmetric-key cryptography uses one shared key for both operations (e.g. AES, DES).
• Asymmetric (public-key) cryptography uses a public key to encrypt and a private key to decrypt (e.g. RSA).

Why we use a firewall

A firewall is a hardware or software security system placed between a trusted internal network and an untrusted network such as the internet. It monitors and filters incoming and outgoing traffic according to security rules. It is used to:

• Block unauthorised access and hacking attempts.
• Prevent malware and malicious traffic from entering the network.
• Control which applications, ports, addresses and websites may be used.
• Log and monitor network activity for auditing.
• Protect private data and hide internal network details (e.g. using NAT).

Instruction cycle

The instruction cycle (fetch-decode-execute cycle) is the sequence of steps that the CPU repeats to run each instruction of a program.

1. Fetch. The address in the Program Counter (PC) is placed in the Memory Address Register; the instruction is read from memory into the Instruction Register (IR), and the PC is incremented to point to the next instruction.
2. Decode. The Control Unit interprets the instruction in the IR to find the operation and the operands.
3. Execute. The operation is carried out: the ALU computes, data is moved, or a jump is made. If the operands are in memory they are fetched first.
4. Store (write back). The result is written to a register or memory location.

The cycle then repeats for the next instruction until the program ends.

Big Data

Big Data means extremely large and complex data sets that cannot be stored or processed by traditional database tools. It is characterised by the "Vs":

• Volume: huge amounts of data (terabytes to petabytes).
• Velocity: data is generated and must be processed at high speed.
• Variety: structured, semi-structured and unstructured data (text, images, video, logs).
• Veracity: uncertainty and quality of data (sometimes also Value).

Sources include social media, sensors/IoT, transactions and web logs. Technologies include Hadoop, Spark and NoSQL databases. Applications are in business analytics, healthcare, fraud detection, recommendation systems and weather forecasting.

Primary memory versus secondary memory

Why primary memory is needed

The CPU is much faster than secondary storage, and it can execute only the instructions and use only the data that are in primary memory. Programs and data must therefore be loaded into RAM from secondary storage before they can be processed. Primary memory also holds intermediate results, supplies the operating system's working space, and ROM stores the boot program that starts the computer. Without it the CPU would waste most of its time waiting for the disk.

Types of secondary memory

1. Magnetic storage.
   • Hard disk drive (HDD): rotating platters with magnetic coating read by heads; large capacity and low cost.
   • Magnetic tape: sequential access, used for backup and archives.
   • Floppy disk: old, small removable disk.
2. Optical storage. Data is read and written by laser: CD (about 700 MB), DVD (4.7 GB or more) and Blu-ray (25 GB or more). Portable and cheap, but slower and easily scratched.
3. Flash/solid-state storage. No moving parts: SSD, USB pen drive, memory cards (SD). Fast, durable and silent, but costlier per GB than HDD.
4. Cloud storage. Data stored on remote servers over the internet (Google Drive, OneDrive), accessible from anywhere.

Application software

Application software is a program designed to carry out a specific task for the user. It runs on top of the operating system. Types and examples:

• General-purpose (packaged) software: word processors (MS Word), spreadsheets (Excel), presentation software (PowerPoint), browsers (Chrome), media players.
• Specific-purpose (custom) software: made for one organisation or function, such as payroll, hospital management, banking and school management systems.
• Graphics and multimedia software: Photoshop, Premiere.
• Entertainment and communication: games, email clients, Zoom.

Why system software is needed

• It manages hardware resources (CPU, memory, storage and I/O devices) so users do not need to.
• It provides a platform on which application software can run.
• It hides hardware complexity through the operating system and device drivers.
• Translators (compiler, assembler, interpreter) convert programs into machine code.
• Utilities keep the system secure and efficient (antivirus, backup, disk management).
• Firmware starts the computer and tests the hardware at power-on.

Without system software, the hardware could not be used by ordinary users and applications could not run.

Types of operating system

1. Batch operating system. Similar jobs are collected and executed in batches without user interaction. Example: early mainframe payroll processing.
2. Multiprogramming/multitasking OS. Several programs are kept in memory and the CPU switches among them, giving a smooth sharing of time. Example: Windows, Linux.
3. Time-sharing OS. Each user gets a small time slice of the CPU, so many users can work at once. Example: UNIX.
4. Multiprocessor (parallel) OS. Uses more than one CPU to run tasks together for higher speed and reliability.
5. Real-time OS (RTOS). Responds within strict time limits. Hard real-time (missile control, pacemakers) and soft real-time (multimedia streaming).
6. Distributed OS. Controls a group of independent computers connected by a network and makes them appear as a single system.
7. Network OS. Runs on a server to provide file, printer and user sharing in a network. Example: Windows Server.
8. Mobile OS. Designed for phones and tablets. Example: Android, iOS.
9. Single-user and multi-user OS. Single-user supports one user at a time (MS-DOS); multi-user allows several users together (Linux, UNIX).

Network topology is the geometric arrangement or layout of the devices (nodes) and the links connecting them in a computer network. It can be physical (actual cabling layout) or logical (the way data flows).

Types of topology

1. Bus topology. All devices are connected to a single common cable (the backbone) with terminators at both ends.

• Merits: simple and cheap; needs little cable; easy to add a node.
• Demerits: a break in the backbone stops the entire network; performance drops with many nodes; collisions occur; hard to troubleshoot.

2. Star topology. Every device is connected to a central hub or switch.

• Merits: easy to install and manage; failure of one node or cable does not affect others; easy fault detection; easy to add devices.
• Demerits: if the central hub fails the whole network fails; more cable is needed; hub cost.

3. Ring topology. Each device connects to exactly two neighbours forming a closed loop; data travels in one direction (often with a token).

• Merits: orderly access to the network and no collisions with token passing; performs well under heavy load.
• Demerits: failure of one node or link can break the ring; adding or removing devices disrupts the network; troubleshooting is difficult.

4. Mesh topology. Every device is connected directly to every other device (a full mesh needs $n(n-1)/2$ links).

• Merits: highly reliable and fault tolerant since there are alternative paths; dedicated links give privacy and no traffic problem.
• Demerits: very costly; complex installation and many cables and ports.

5. Tree (hierarchical) topology. Star networks are connected to a main bus or root in a hierarchy.

• Merits: scalable; easy to extend and manage in groups; fault isolation of branches.
• Demerits: the failure of the root/backbone affects the whole network; needs more cable; difficult to configure.

6. Hybrid topology. A combination of two or more topologies.

• Merits: flexible and scalable; can be designed to suit needs.
• Demerits: complex design, installation and cost.

The main characteristics of computers are:

1. Speed. A computer performs millions or billions of operations per second; tasks that take people hours are done in moments.
2. Accuracy. The results are precise; errors arise only from wrong input or faulty programs (garbage in, garbage out).
3. Diligence. A computer does not get tired, bored or lose concentration, and can work continuously at the same level.
4. Versatility. It can perform many kinds of tasks such as calculation, word processing, communication and design.
5. Storage capacity. It can store huge amounts of data and retrieve it quickly.
6. Automation. Once a program is loaded, it executes the instructions without human intervention.
7. Reliability. It gives consistent results for the same input and can run for long periods.
8. Memory (remembering power). Data stored can be kept for as long as required.
9. No intelligence (no IQ). It cannot think by itself and follows only the instructions given to it.
10. Resource sharing and connectivity. Computers can be networked to share data and devices.

Instruction format. An instruction is a binary word that tells the CPU what operation to perform and on which data. Its layout is called the instruction format. It is divided into fields:

Common formats, by number of address fields:

• Zero-address: PUSH, POP, ADD (operands are taken from the stack).
• One-address: ADD X means AC = AC + M[X].
• Two-address: ADD R1, R2 means R1 = R1 + R2.
• Three-address: ADD R1, R2, R3 means R1 = R2 + R3.

Example. A 16-bit instruction with a 4-bit opcode, 2-bit mode and 10-bit address:

| 4 bits | 2 bits | 10 bits |
| Opcode | Mode   | Address |
| 0001   | 01     | 0000001010 |

Here opcode 0001 is LOAD, mode 01 is direct addressing, and the address is 10, so the instruction means "load the contents of memory location 10 into the accumulator".

Instruction set. The instruction set is the complete collection of instructions that a particular CPU can understand and execute. It defines the operations, the data types, the registers, the addressing modes and the instruction formats of the processor. Its main groups are:

• data transfer (MOV, LOAD, STORE),
• arithmetic and logic (ADD, SUB, AND, OR, NOT),
• control transfer (JMP, CALL, RET, conditional branches),
• input/output instructions.

Instruction sets are of two broad kinds: CISC (many complex instructions, e.g. Intel x86) and RISC (few simple, fixed-length instructions, e.g. ARM).

I/O port. An I/O port is the connection point through which the CPU exchanges data with an input or output device. It consists of a socket or connector and the controller and register circuitry behind it. Each port has an address (a port number) that the CPU uses to select it. Examples are USB, HDMI, serial (COM), parallel, PS/2 and audio jack ports.

Working of the I/O system. Input/output devices are much slower than the CPU and use different data formats and speeds, so each device is attached through an I/O interface (controller), which sits between the system bus and the device.

1. The CPU places the address of the required port or device on the address bus and issues a read or write command on the control bus.
2. The interface decodes the address and, if it matches, becomes active. The data moves between the CPU (or memory) and the interface's data and status registers over the data bus.
3. The controller converts the data into the form the device needs (for example parallel to serial, or digital to analogue) and passes it to the device. Input travels the opposite way.
4. The status register tells the CPU whether the device is ready or busy, or has an error.

The data-transfer techniques are:

• Programmed I/O: the CPU polls the status register and waits until the device is ready. It is simple but wastes CPU time.
• Interrupt-driven I/O: the device sends an interrupt when it is ready, and the CPU runs an interrupt service routine and then resumes its work.
• Direct Memory Access (DMA): a DMA controller transfers a block of data directly between the device and memory, and interrupts the CPU only when the whole block is done.

The I/O addressing may be memory-mapped (ports share the memory address space) or isolated (separate I/O address space with special IN/OUT instructions).

Unicode. Unicode is the international character-encoding standard that assigns a unique number (code point) to every character of almost all the world's writing systems, including Devanagari, Latin, Chinese and Arabic, plus symbols and emoji. It overcomes the limit of ASCII, which has only 128 (or 256 in extended form) characters. Code points are written as U+0041 (A) or U+0915 (the Devanagari letter ka). Unicode is stored using encodings such as UTF-8 (1 to 4 bytes, compatible with ASCII), UTF-16 (2 or 4 bytes) and UTF-32 (4 bytes). Because one code is used on every platform, text can be exchanged and displayed correctly in any language.

Conversion of $0.865_{10}$ to base 16. Multiply the fractional part repeatedly by 16 and take the integer parts in order:

The fraction 0.84 has appeared before, so the digits D, 7, 0, A, 3 repeat from step 2 onward.

$0.865_{10} = 0.DD70A3D70A3\ldots_{16} \approx 0.DD70A_{16}$

Cloud computing is the delivery of computing resources (servers, storage, databases, networking, software and analytics) over the Internet on demand, on a pay-as-you-go basis. Users do not buy and maintain their own hardware; they rent what they need from a provider such as AWS, Microsoft Azure or Google Cloud.

Main characteristics: on-demand self-service, broad network access, resource pooling, rapid elasticity (scale up or down quickly) and measured service (pay for what is used).

Service models:

• IaaS (Infrastructure as a Service): virtual machines, storage and networks, e.g. Amazon EC2.
• PaaS (Platform as a Service): a platform to develop and deploy applications, e.g. Google App Engine.
• SaaS (Software as a Service): ready-made applications used through a browser, e.g. Gmail, Google Docs.

Deployment models: public cloud, private cloud, hybrid cloud and community cloud.

Advantages: low upfront cost, scalability, accessibility from anywhere, automatic backup and updates.

Disadvantages: depends on Internet connection, security and privacy concerns, and the user depends on the provider (vendor lock-in).

Data warehouse. A data warehouse is a large, central repository that stores integrated historical data collected from many different sources (operational databases, files, external systems) and is designed for query, reporting and analysis, not for day-to-day transactions. According to W. H. Inmon, it is "a subject-oriented, integrated, time-variant and non-volatile collection of data in support of management's decision making".

Characteristics:

• Subject-oriented: organised around major subjects such as sales, customers or products.
• Integrated: data from different sources is cleaned and made consistent (same formats, units and names).
• Time-variant: it holds data over a long period, so trends can be studied.
• Non-volatile: data is only added and read; it is not changed or deleted once loaded.

Working (ETL). Data is extracted from the source systems, transformed (cleaned, converted, summarised) and loaded into the warehouse. Users then analyse it with OLAP tools, reports, dashboards and data mining.

Difference from an operational database: an operational (OLTP) database handles current, detailed daily transactions, whereas a warehouse (OLAP) holds historical, summarised data for decisions.

Uses: sales and trend analysis, customer behaviour analysis, forecasting and business intelligence. Data marts are smaller warehouses for a single department.

Multimedia means the combined use of several forms of content in one application or presentation. Its elements are:

1. Text: the written words, in various fonts, sizes and styles. It is the most basic element and is used for titles, menus and explanations.
2. Graphics / images: still pictures, drawings, photographs, charts and icons (bitmap or vector, such as JPEG, PNG, SVG).
3. Audio: sound, including speech, music and sound effects (MP3, WAV).
4. Video: recorded moving pictures with sound (MP4, AVI), giving a realistic presentation of events.
5. Animation: a sequence of drawn or computer-generated images displayed rapidly to give the illusion of movement (GIF, Flash, 2D and 3D animation).
6. Interactivity: the control the user has over the content, through links, buttons, menus and hyperlinks, which makes the product interactive rather than passive.

Examples of multimedia are educational software, computer games, websites, video conferencing and e-learning material.

Security awareness. Security awareness is the programme of educating users and employees about security threats and the safe use of information systems. People are often the weakest link, since attacks such as phishing, social engineering and malware depend on human mistakes. Awareness training teaches users to:

• use strong, unique passwords and keep them secret,
• recognise phishing e-mails and suspicious links or attachments,
• keep software and antivirus up to date,
• protect devices and data, and avoid unsafe use of public Wi-Fi and removable media,
• report incidents promptly.

It is done through training sessions, posters, newsletters, simulated phishing tests and regular reminders.

Security policy. A security policy is a formal, written document that states the rules, procedures and responsibilities for protecting an organisation's information and IT assets. It defines what is to be protected, who may access what, what users may and may not do, and what happens when rules are broken. Its typical contents are:

• the purpose and scope,
• access control and password rules,
• acceptable use of the Internet, e-mail and devices,
• data backup and recovery,
• incident response procedures,
• penalties for violations.

A good policy is approved by management, clear, enforceable and reviewed regularly. It aims to protect confidentiality, integrity and availability of information.

Awareness makes sure people know and follow the policy, so the two work together.

Cache memory. Cache memory is a small, very fast memory placed between the CPU and main memory (RAM). It stores copies of the instructions and data that the CPU uses most often, so the CPU does not need to wait for the slower RAM. It is made of SRAM and is more expensive than DRAM, so it is small (KB to a few MB).

• Working: when the CPU needs data it first looks in the cache. If found, it is a cache hit and is served quickly. If not, it is a cache miss, and the block is fetched from RAM and placed in the cache. This works because of locality of reference (recently used and nearby data tend to be used again).
• Levels: L1 (smallest, fastest, inside each core), L2 and L3 (larger, slower, often shared).
• Benefit: it reduces the average memory access time and raises overall system speed.

IoT (Internet of Things). IoT is a network of physical objects ("things") that have sensors, software and network connectivity built in, so that they can collect data and exchange it over the Internet with little human intervention.

• Components: sensors and actuators, connectivity (Wi-Fi, Bluetooth, Zigbee, cellular), data processing in the cloud or at the edge, and a user interface or application.
• Applications: smart homes (lights, thermostats), wearable health devices, smart agriculture, smart cities and traffic control, industrial monitoring, and connected vehicles.
• Challenges: security and privacy, device compatibility, and managing large amounts of data.