SS 2 First Term

Learning Objectives

• Define a network topology and name the main types.
• Compare bus, star, ring, mesh, tree and hybrid topologies.
• Describe the OSI and TCP/IP reference models.
• Classify networks as PAN, LAN, MAN or WAN.

What is a Network Topology?

A network topology is the arrangement of the devices in a network and the connections between them. The physical topology is the actual wiring, while the logical topology is the path data follows, and the two do not always match; the school lab is wired in a star, but the switch decides the logical route the data takes.

Bus Topology

In a bus topology all devices connect to a single central cable called the backbone. Data travels along the bus in both directions, and terminators at each end absorb the signal so it does not bounce back. Because all devices share one wire, only one can transmit at a time, so the whole bus is a single collision domain and two simultaneous transmissions clash. The bus is cheap and simple, but the backbone is a single point of failure; one cut takes the whole network offline.

Star Topology

In a star topology every device connects to a central switch or hub with its own cable, and data passes through the centre first. A switch forwards data only to the intended device, which is why stars are efficient and common in school labs, offices and homes. Its strength is isolation; one damaged cable affects only that device. Its weakness is the centre; if the switch fails, the whole network stops.

Ring Topology

In a ring topology each device connects to two neighbours to form a closed circle, and data travels in one direction, each station checking and then forwarding the signal. Older token ring networks circulated a token and allowed a device to transmit only while it held it, so no collisions occurred and access was fair. The ring is fragile because every device is part of the path; one broken device or cable breaks the loop. The idea survives in the dual fibre rings that protect telephone backbones.

Mesh Topology

In a mesh topology devices are joined by many direct links. A full mesh connects every device to every other, so four nodes need six cables (4 x 3 / 2), while a partial mesh connects only the key devices fully. Because data reroutes automatically when a link fails, the mesh is the most reliable but also the most expensive topology, so it is reserved for internet backbones and core bank networks.

Tree (Hierarchical) Topology

A tree, or hierarchical, topology joins several star networks onto one central backbone, combining bus and star. It branches like a family tree from a root down through switches to the workstations and scales well; a new department can be added as one star. A failure near the root brings down everything beneath it, so large campuses use a tree with one star per building.

Hybrid Topology

A hybrid topology combines two or more topologies in one network, such as stars inside offices joined by a mesh backbone. Real networks, including the internet, are almost always hybrids, because each section can use the layout that suits its cost and reliability needs.

The table below compares them.

Why Network Models Exist

Computers from different manufacturers use different hardware, so networking needed common rules called protocols. A network model organises the protocols into layers; each layer serves the layer above it and uses the layer below, so one layer can be upgraded without rewriting the whole system.

The OSI Reference Model

The Open Systems Interconnection (OSI) model, published by the ISO in 1984, is a seven-layer reference model for networks. A memory aid for the layers, from the bottom, is 'Please Do Not Throw Sausage Pizza Away'. When data is sent, it starts at the top layer and moves down, each layer wrapping it in its own information (encapsulation); the receiver unwraps it on the way back up.

The Seven Layers of the OSI Model

The Physical layer transmits raw bits as electrical signals, light pulses or radio waves. Common devices include hubs, repeaters and cables. The data unit is a bit. The analogy is the road that carries a letter.

The Data link layer groups bits into frames, moves them between neighbouring devices and checks for errors using MAC addresses. Common devices include switches. The data unit is a frame. The analogy is the local postman.

The Network layer routes packets across networks using IP addresses. Common devices include routers. The data unit is a packet. The analogy is the sorting centre that picks the next city.

The Transport layer delivers data end to end; TCP numbers segments so all arrive in order, UDP sends quickly without guarantees, and port numbers identify the receiving application. The data unit is a segment. The analogy is a guaranteed courier.

The Session layer opens, manages and closes a conversation between two applications. Common protocols include NetBIOS and RPC. The analogy is a telephone call.

The Presentation layer translates, compresses and encrypts data into a common format, such as ASCII or JPEG. The analogy is a translator.

The Application layer provides the services users actually use, such as browsing, email and file transfer. Common protocols include HTTP, HTTPS, FTP, SMTP and DNS. The analogy is the person writing the letter.

The table below summarises the layers.

The TCP/IP Model

The TCP/IP model is the practical model used on the internet and has four layers. The application layer merges the top three OSI layers (HTTP, HTTPS, FTP, SMTP, DNS); the transport layer holds TCP and UDP; the internet layer uses IP to address and route packets; and the network access layer covers the media such as Ethernet and Wi-Fi. When you open a website, data is wrapped layer by layer and unwrapped in reverse at the server.

The table below maps them to the OSI model.

VPN means virtual private network, giving secure remote access.

Comparing OSI and TCP/IP

OSI and TCP/IP describe the same journey in different detail. OSI has seven layers and is a reference model for teaching, standardising and troubleshooting, so it is not implemented in pure form. TCP/IP has four layers and is the working model built into routers and operating systems. TCP/IP merges the top three OSI layers into one application layer and the bottom two into one network access layer. Use OSI to name a layer or locate a fault in an exam; use TCP/IP to explain how the internet actually moves data.

The table below compares them.

Client-Server and Peer-to-Peer Models

In the client-server model a central server stores resources and manages data, while client computers request those resources. The web works this way (the browser is the client, the website is the server), and a school file server works the same way. Central control makes permissions, backups and security easy to manage, but the server is a single point of failure.

In the peer-to-peer model every computer can act as both client and server, with no central authority. Two home laptops can share files directly, which is cheap and easy, but control is hard; if no machine holds the master copy, data can be lost, and without central security each machine is a potential malware entry point. Organisations choose client-server; small home groups choose peer-to-peer.

The table below compares them.

Use this glossary while you revise.

Corporate Networks

A corporate network is a private network that connects computers, servers and devices within an organisation or across branches. Its main purposes are sharing resources, providing an intranet and supporting collaboration.

An intranet is a private network accessible only to authorised members of the organisation. It hosts internal websites, staff directories, memos and policy documents that should not be on the public internet.

Shared resources: printers, scanners, internet connections

Intranet: private websites and documents for staff only

Collaboration: shared calendars, email, video conferencing, instant messaging

In Nigeria, banks and telecom companies run large corporate networks connecting offices across many states. These use LAN, MAN and WAN building blocks with added security and central management.

Network Protocols in Business

A network protocol is a set of rules that controls how data is formatted, addressed, transmitted and received across a network. Without protocols, devices from different manufacturers cannot communicate.

TCP/IP: the foundation protocol of the internet; TCP breaks data into packets and guarantees delivery, IP handles addressing and routing

HTTP and HTTPS: used by web browsers to load websites; HTTPS adds encryption for secure transactions

FTP: transfers large files between computers

SMTP, POP3 and IMAP: handle email sending and receiving

DNS: translates domain names such as google.com into IP addresses

A typical school or business network uses all of these protocols simultaneously. Understanding them helps administrators troubleshoot and choose the right tools.

Network Security Measures and Threats

Network security is the set of practices and technologies used to protect data, devices and users from unauthorised access, data theft and malicious attacks.

Malware: viruses, worms and ransomware that damage files or lock systems

Phishing: fake emails or websites that trick users into revealing passwords

Unauthorised Wi-Fi access: connecting to weakly protected networks to steal data

Denial-of-service: overwhelming a server with fake requests so legitimate users cannot access it

Protection measures include firewalls, encryption, strong passwords and multi-factor authentication, and antivirus software. Good habits like not clicking suspicious links and never sharing passwords also help.

Nigerian organisations must comply with the Nigeria Data Protection Regulation (NDPR), which requires protection of citizens personal data.

Meaning and Uses of Networks

A computer network is two or more computers joined so that they can exchange data and share resources. The joining may be by cable or by radio. What is shared includes data, printers, storage space and an internet connection.

Networks are used in schools, banks, hospitals, government offices, shops and homes. A school network lets a teacher send a document to every computer in a laboratory. A bank network lets every branch reach the same account records.

Classes of Network

A personal area network covers a very small area, usually a few metres, such as a phone connected to a headset. A local area network covers one building or a small group of buildings, such as a school, and it is owned by one organisation.

A metropolitan area network covers a town or a city, joining several local area networks, such as the branches of a bank across a city. A wide area network covers a country or the world and is formed by joining many networks together.

The comparison is by area, and the area decides the equipment. The larger the area, the more expensive the equipment and the slower the data, because the signal travels further.

Network Devices

A modem converts a signal from the provider into a signal the computer can use, and back again. A router joins two or more networks and passes data between them, choosing the best route. A switch joins computers within one network and sends data only to the computer it is meant for, which reduces traffic.

A hub is an older device that sends data to every port, so every computer receives it. A switch is the modern replacement. A network interface card allows a computer to join a network, by cable or by radio.

A repeater strengthens a weakened signal. A bridge joins two networks of the same type. A gateway joins two networks of different types. An access point provides a wireless connection for devices within its range.

Topology

Topology is the shape in which the devices are connected. In a bus topology all the devices are connected to one central cable. It is cheap and uses little cable, but if the main cable breaks the whole network stops.

In a star topology every device has its own cable to a central switch. If one cable breaks, only that device is affected, which is why this is the common layout in schools. In a ring topology the devices are joined in a closed loop and the data passes in one direction around it.

In a mesh topology each device is connected to several others, so there is more than one route for the data. It is the most reliable and the most expensive. A hybrid topology combines more than one of these shapes.

Wired and Wireless Connections

A wired connection uses a cable and is fast, stable and not affected by walls. A wireless connection uses radio waves, so no cable is needed, but the signal weakens with distance and with obstacles.

Topology may be physical, showing the actual cables, or logical, showing the path the data takes. The two may differ, which is a point examined in network questions.

Practical Work

1. Draw a star and a mesh topology and label each device.
2. Use a table to compare the strengths and weaknesses of the topologies.
3. List the seven OSI layers in order.
4. Classify five networks by the area each covers.

Scenario-Based Questions

1. A school lab is wired in a star. Explain why a cut cable affects only one computer.
2. A company needs the most reliable network and will pay for it. Recommend a topology.
3. Explain why real networks such as the internet are almost always hybrid.
4. Explain why the OSI model is arranged in layers.

LEARNING OBJECTIVES: BY THE END OF THE LESSON STUDENTS SHOULD BE ABLE TO:

Rationale: A network topology is the arrangement of the devices in a network and the connections between them. Understanding what is a Network Topology?, bus Topology, star Topology, ring Topology helps learners use digital technology confidently, safely and responsibly.

Pre-requisite Knowledge: In Week 6, learners studied Logic Gates and Digital Logic, including what is a Logic Gate?, the AND Gate, the OR Gate. The knowledge from that week is used again in this lesson.

Learning Material/Teaching Aid: Whiteboard, markers, computer or laptop where available, printed notes and charts on Computer Networks, practical task sheets, learners' notebooks and pens.

Reference Materials: Digital Technology for Senior Secondary Schools (SS 2); NERDC Digital Technology Curriculum; Digital Technology SS 2 First Term Notes (Week 7); First Term examination and Continuous Assessment question papers.

LESSON DEVELOPMENT

Assignment: i. A school lab is wired in a star. Explain why a cut cable affects only one computer. ii. A company needs the most reliable network and will pay for it. Recommend a topology.