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BT-205 · Basic Computer Engineering/Quick Revision Short Notes

Basic Computer Engineering (BT-205) - Unit 4 Short Notes

How unit 4 is examined

Covers networking models (OSI, TCP/IP), internetworking devices, the Internet and e-commerce, malware and cyber attacks, and security measures; the OSI/TCP/IP layers, firewall and malware answers carry the marks.

Computer Networking: Introduction, Goals, ISO-OSI Model, Functions of Different Layers

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Definition. <mark>A computer network is a set of autonomous computers and devices connected by a communication medium so that they can share data, hardware and software resources.</mark> The ISO-OSI model (Open Systems Interconnection, by ISO) is a seven-layer reference model that divides network communication into functional layers.

Goals. Resource sharing (printers, files, software), reliability through duplicate resources, cost saving, fast communication and centralised data management.

Diagram.

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Key points.

  1. The Physical layer (1) transmits raw bits over the medium and defines voltage levels, data rate, topology and connectors; its unit is the bit.
  2. The Data Link layer (2) groups bits into frames, adds physical (MAC) addresses, and provides error detection, flow control and access to the medium.
  3. The Network layer (3) does logical (IP) addressing and routing of packets from source to destination across networks.
  4. The Transport layer (4) gives end-to-end delivery with segmentation, port (service-point) addressing, flow control and error recovery; its unit is the segment.
  5. The Session layer (5) establishes, manages and terminates sessions between applications, and inserts synchronisation checkpoints.
  6. The Presentation layer (6) translates data formats, and does encryption, decryption and compression.
  7. The Application layer (7) is the user's interface to the network and provides services such as e-mail, file transfer and web access.
  8. Data moves down the stack at the sender, each layer adding a header, and up the stack at the receiver, each layer removing it.

Differences between OSI and TCP/IP.

Basis ISO-OSI TCP/IP
Layers 7 4 (or 5)
Nature Theoretical reference model Practical, implemented on the Internet
Development Model built before protocols Protocols built first, model described after
Top layers Application, Presentation, Session separate One Application layer covers all three
Bottom layers Data Link and Physical separate One Network Access layer
Service, interface, protocol Clearly separated concepts Not clearly separated
Transport Connection-oriented only TCP (connection-oriented) and UDP (connectionless)
Network layer Connection-oriented and connectionless Connectionless (IP) only

Answer frame. Open with "The OSI model is a seven-layer reference model for network communication"; draw the seven-layer stack with names and numbers; develop points 1-7 bottom to top, then point 8; close with "Each layer serves the layer above and uses the one below". For the comparison, draw OSI and TCP/IP side by side with the mapping (Application+Presentation+Session = Application; Transport = Transport; Network = Internet; Data Link+Physical = Network Access), then the table.

Pitfall: Do not write the layers top-down for "function of each layer" without numbering them; the examiner marks the correct sequence.

Asked: [7 marks] (Jun 2022, Dec 2024) Draw ISO-OSI model of computer networking. Explain function of each layer. Asked: [7 marks] (Nov 2022, Dec 2023, Jun 2025) Differentiate ISO-OSI with TCP/IP (structure, layering, compatibility with existing protocols).

Internetworking Concepts, Devices, TCP/IP Model

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Definition. <mark>Internetworking is the connecting of two or more distinct networks, using devices such as routers, so that they work as one unified network; the Internet is the largest example.</mark> TCP/IP (Transmission Control Protocol/Internet Protocol) is the protocol suite on which the Internet runs.

Diagram.

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Key points (TCP/IP layers).

  1. The Network Access (Link) layer moves frames over the physical network using technologies such as Ethernet and Wi-Fi, and uses MAC addresses.
  2. The Internet layer gives IP addressing and routes packets between networks; its protocols are IP, ICMP (error and control messages) and ARP (IP to MAC address).
  3. The Transport layer provides end-to-end communication with port addressing; TCP is reliable and connection-oriented, UDP is fast and connectionless.
  4. The Application layer supports user services through HTTP, FTP, SMTP and DNS.

Difficulties in joining networks.

  1. Networks use different hardware, frame sizes and protocols, which must be made compatible.
  2. Addressing schemes differ, so a common logical address (IP) is needed.
  3. Routing must scale as the number of networks grows.
  4. Joined networks create traffic congestion.
  5. Security is harder because more entry points exist.

Devices.

Device Layer Role
Hub / repeater 1 Regenerates and broadcasts signals; one collision domain
Switch / bridge 2 Forwards frames by MAC address; splits collision domains
Router 3 Forwards packets by IP address between subnets and networks
Gateway Up to 7 Converts between different protocol suites

Answer frame. For TCP/IP, open with "TCP/IP is the protocol suite of the Internet with four layers"; draw the four-layer stack with protocols; describe layers bottom to top; close with its significance: it is the standard the Internet runs on, it is open and independent of hardware. For internetworking, define it, list the difficulties, then explain switch (Layer 2, MAC, divides collision domains) and router (Layer 3, IP, joins subnets).

Asked: [7 marks] (Jun 2022, Jun 2023, Dec 2024) Draw TCP/IP model of computer networking. Explain function of each layer. Why is it significant? Asked: [7 marks] (Dec 2023) Define internetworking and the difficulties in joining numerous networks. Discuss the importance of routers and switches within the internetwork.

Introduction to Internet, World Wide Web, E-commerce

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Definition. <mark>The Internet is a worldwide network of interconnected networks using TCP/IP; the World Wide Web (WWW) is a collection of linked web pages accessed on it through a browser using HTTP.</mark> E-commerce is buying and selling of goods and services over electronic networks, chiefly the Internet.

Key points.

  1. The Internet is the infrastructure (cables, routers, servers); the Web is one service on it, using URLs, HTTP and HTML, and e-mail and FTP are others.
  2. E-commerce types include B2B, B2C, C2C and C2B.
  3. Advantage: shops are available 24 hours a day, 7 days a week.
  4. Advantage: global reach and a wide choice of products with easy price comparison.
  5. Advantage: lower cost, as there is no physical shop and less staff.
  6. Disadvantage: security concerns such as card fraud and data theft.
  7. Disadvantage: no personal touch or physical inspection of goods, and delivery delays.
  8. Disadvantage: dependence on technology and Internet access.

Social media in e-commerce. Platforms such as Instagram, Facebook and LinkedIn build brand awareness and reach; they allow direct engagement, feedback and real-time support; they enable targeted advertising and customer data analytics; and social commerce features (shop buttons, influencer marketing) allow direct purchase.

Answer frame. Open with the e-commerce definition; list advantages then disadvantages in two short numbered groups; close with "E-commerce saves cost and time but needs strong security". For social media, open with "Social media links marketing with sales" and follow the four points above.

Asked: [7 marks] (Nov 2022) What is E-Commerce? Write the advantages and disadvantages. Asked: [7 marks] (Jun 2024) What role do social media platforms play in e-commerce marketing and customer engagement?

Computer Security Basics: Viruses, Worms, Malware, Trojans, Spyware and Anti-Spyware

<span style="display:inline-block;padding:.16em .6em;border:1.5px solid currentColor;border-radius:999px;font-size:.68em;font-weight:700;letter-spacing:.06em;text-transform:uppercase;opacity:.75">Medium weight</span>

Definition. <mark>Computer security protects a system and its data through the CIA triad: Confidentiality (only authorised access), Integrity (data not altered) and Availability (services usable when needed).</mark> Malware is malicious software designed to damage, disrupt or steal from a system.

Key points.

Type Property Propagation Effect
Virus Attaches to a host program or file; needs it to run Infected files, downloads, pen drives Corrupts files, slows the system
Worm Self-replicating, standalone Spreads over networks by itself, using vulnerabilities and e-mail Consumes bandwidth and resources
Trojan Disguised as useful software; does not replicate User installs it willingly Opens a back door, steals data
Spyware Secretly monitors the user Bundled free software, downloads Steals passwords, browsing data
  1. Malware also includes ransomware, which locks files and demands payment.
  2. Anti-spyware software scans for, detects and removes spyware and blocks new installs.

Protection procedure.

  1. Install antivirus and anti-malware software and keep its definitions updated.
  2. Apply operating system and application updates (patches) regularly.
  3. Enable the firewall and scan e-mail attachments before opening them.
  4. Browse safely, avoid pirated software and unknown links, and use only trusted downloads.
  5. Take regular backups so data can be restored.

Answer frame. For the fundamentals question, open with the CIA triad, define malware, give the table (virus, worm, Trojan, spyware) with propagation and effects, and close with anti-spyware and antivirus. For the protection question, define malware, virus and worm in one line each, then give the five steps in order and close with "Prevention is cheaper than recovery".

Pitfall: A virus needs a host and user action; a worm spreads on its own. Do not mix them.

Asked: [7 marks] (Dec 2023) Describe fundamentals of computer security and malware types (viruses, worms, Trojans, spyware): properties, propagation and effects. Asked: [7 marks] (Jun 2025) Explain the procedure involved in protecting devices from malware, virus and worms.

Types of Cyber Attacks: Money Laundering, Information Theft, Cyber Pornography, Email Spoofing, DoS, Cyber Stalking, Logic Bombs, Hacking, Spamming, Cyber Defamation, Pharming

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Key points.

  1. <mark>E-mail spoofing is forging the header of an e-mail so that it appears to come from a trusted sender, used for phishing and social engineering; it is countered by SPF and DKIM authentication and by spam filters.</mark>
  2. Denial of Service (DoS) floods a server or network with requests until it cannot serve legitimate users, which attacks availability; when many machines attack together it is a DDoS.
  3. DoS types include SYN flood (half-open TCP connections), ping flood and Ping of Death (oversized ping packets); mitigation uses firewalls, rate limiting, traffic filtering and extra bandwidth.
  4. A logic bomb is malicious code hidden in a program that stays dormant until a condition (a date, or an employee's name missing from payroll) triggers it, and then deletes or corrupts data; it is prevented by code review and antivirus.
  5. Phishing sends fraudulent messages to steal passwords and card details; pharming redirects a user to a fake website by corrupting DNS or the hosts file.
  6. Information theft is stealing confidential data; hacking is unauthorised access to a system; money laundering hides the source of illegal money through digital transactions.
  7. Cyber stalking is repeated online harassment, cyber defamation is publishing false statements that harm reputation, cyber pornography is publishing obscene material online, and spamming is sending bulk unsolicited messages.

Answer frame. Open with a one-line definition of each attack asked; for e-mail spoofing give purpose then SPF/DKIM; for DoS give mechanism, types, impact on availability and mitigation; for logic bombs give trigger, action and prevention; close with "Layered defences reduce these threats".

Asked: [7 marks] (Jun 2022) Short notes on (i) E-mail spoofing (ii) Denial of service. Asked: [7 marks] (Jun 2023) Describe Denial of Service (DoS) and logic bombs in detail. Asked: [7 marks] (Jun 2024) Explain common types of cyber threats such as malware, phishing and DoS attacks.

Security Measures: Firewall, Computer Ethics, Cyber Laws on Internet Fraud, Good Security Habits

<span style="display:inline-block;padding:.16em .6em;border:1.5px solid currentColor;border-radius:999px;font-size:.68em;font-weight:700;letter-spacing:.06em;text-transform:uppercase;opacity:.75">High weight</span>

Definition. <mark>A firewall is a network security system, in hardware, software or both, that monitors incoming and outgoing traffic and allows or blocks it according to predefined security rules.</mark>

Diagram.

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Key points (firewall).

  1. The firewall sits at the boundary between a trusted internal network and an untrusted network such as the Internet, and every packet must pass through it.
  2. It compares each packet's source, destination, port and protocol with rules and permits, drops or rejects it.
  3. Packet-filtering firewalls check headers only and are fast but simple.
  4. Stateful inspection firewalls track the state of connections and allow only packets that belong to a valid session.
  5. Application proxy (gateway) firewalls inspect content at the application layer and hide internal hosts, so they are the most secure but slower.
  6. Benefits are blocking unauthorised access, preventing attacks and malware, and logging traffic for audit.

Internet fraud (cyber law). Under the IT Act, the elements to prove are: a false representation or deception; the accused's knowledge that it was false; intent to deceive; reliance on it by the victim; and resulting financial or material loss. Digital evidence such as logs, e-mails and transaction records is used to prove them.

Computer ethics. Respect privacy, do not pirate software, do not access others' data without permission, and do not spread malware.

Good security habits.

  1. Use strong, unique passwords and multi-factor authentication (MFA).
  2. Update the operating system and software regularly to patch vulnerabilities.
  3. Beware of phishing: do not click unknown links or attachments.
  4. Browse safely (HTTPS sites), keep antivirus running and back up data regularly.

Answer frame. For the firewall, open with the definition, draw the firewall between Internet and LAN, give working then the three types, and close with benefits. For fraud, define it, list the five elements in order, add digital evidence. For habits, open with "Cyber hygiene means routine safe practice", give the four habits and close with backup.

Asked: [7 marks] (Nov 2022, Jun 2025) What is a firewall? How can it provide security to the system? Illustrate its working principle. Asked: [7 marks] (Jun 2023) Write a short note on good computer security habits. Asked: [7 marks] (Dec 2024) Explain the elements required to prove internet fraud under the law.

Last-minute revision

  • OSI layers bottom to top: Physical, Data Link, Network, Transport, Session, Presentation, Application (7).
  • TCP/IP layers: Network Access, Internet, Transport, Application (4).
  • OSI is a theoretical model; TCP/IP is the practical protocol suite of the Internet.
  • Units: bit (Physical), frame (Data Link), packet (Network), segment (Transport).
  • Switch works at Layer 2 with MAC addresses; router at Layer 3 with IP addresses.
  • Internetworking joins distinct networks into one; a gateway converts between protocol suites.
  • TCP is reliable and connection-oriented; UDP is connectionless.
  • CIA triad: Confidentiality, Integrity, Availability.
  • Virus needs a host; worm self-replicates; Trojan is disguised; spyware monitors.
  • DoS attacks availability; logic bomb triggers on a condition; pharming redirects DNS.
  • Firewall types: packet filtering, stateful inspection, application proxy.
  • Fraud elements: false representation, knowledge, intent, reliance, loss.

Memory hooks

  • OSI top to bottom: "All People Seem To Need Data Processing" (Application, Presentation, Session, Transport, Network, Data Link, Physical).
  • TCP/IP: "A T I N", Application, Transport, Internet, Network Access.
  • Virus needs a host, Worm walks alone, Trojan is a gift horse, Spyware spies.
  • Firewall: "Filter, Track, Proxy" for packet, stateful, application.
  • Fraud: "False, Know, Intend, Rely, Lose".

Coverage checklist

  • Computer Networking: Introduction, Goals, ISO-OSI Model, Functions of Different Layers: draw OSI with layer functions (Jun 2022, Dec 2024); differentiate ISO-OSI and TCP/IP (Nov 2022, Dec 2023, Jun 2025).
  • Internetworking Concepts, Devices, TCP/IP Model: TCP/IP diagram and layers (Jun 2022, Jun 2023, Dec 2024); internetworking, difficulties, routers and switches (Dec 2023).
  • Introduction to Internet, World Wide Web, E-commerce: e-commerce pros and cons (Nov 2022); social media in e-commerce (Jun 2024).
  • Computer Security Basics: Introduction to viruses, worms, malware, Trojans, Spyware and Anti-Spyware Software: security fundamentals and malware types (Dec 2023); protection procedure (Jun 2025).
  • Different types of attacks like Money Laundering, Information Theft, Cyber Pornography, Email spoofing, Denial of Service (DoS), Cyber Stalking, ,Logic bombs, Hacking Spamming, Cyber Defamation , pharming: e-mail spoofing and DoS (Jun 2022); DoS and logic bombs (Jun 2023); malware, phishing, DoS (Jun 2024).
  • Security measures Firewall, Computer Ethics & Good Practices, Introduction of Cyber Laws about Internet Fraud, Good Computer Security Habits: firewall (Nov 2022, Jun 2025); security habits (Jun 2023); internet fraud elements (Dec 2024).
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