Unit 1: Network models and topologies
Computer Networks-I notes · PTU syllabus (UGCC2517)
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Unit summary
A computer network connects devices so they can share data and resources. This unit covers the objectives, components and architecture of networks, the OSI and TCP/IP reference models, and the physical and logical topologies used to arrange devices.
After this unit you can
- Define a computer network and list its objectives and components
- Explain the seven OSI layers and their functions
- Explain the TCP/IP model and compare it with OSI
- Compare bus, star, ring, mesh and hybrid topologies
PTU syllabus topics
- Definition and objectives of computer networks
- network components and architecture
- OSI model layers and functions
- TCP/IP model layers and functions
- comparison of OSI and TCP/IP
- physical vs logical topologies (star, ring, bus, mesh, hybrid) and their trade-offs
- 7 Application
HTTP, FTP, SMTP
- 6 Presentation
Encryption, compression
- 5 Session
Opening and closing sessions
- 4 Transport
TCP, UDP
- 3 Network
IP addressing and routing
- 2 Data link
MAC addressing, frames
- 1 Physical
Cables, signals, bits
Topic 1
Computer networks: definition, objectives and components
A computer network is a set of interconnected autonomous computers and devices that exchange data and share resources. Objectives: resource sharing (printers, files, internet), communication (email, chat), reliability (backups and alternatives), cost reduction and scalability. Components: end devices (computers, phones), network interface cards, transmission media (cables, wireless), networking devices (switches, routers) and protocols (rules for communication). Types by size: PAN (personal), LAN (building/campus), MAN (city) and WAN (country or world).
Topic 2
The OSI reference model
The OSI (Open Systems Interconnection) model, from ISO, divides communication into seven layers.
| Layer | Function | Example |
|---|---|---|
| 7 Application | Network services to users | HTTP, FTP, SMTP, DNS |
| 6 Presentation | Translation, encryption, compression | JPEG, SSL/TLS, ASCII |
| 5 Session | Establishes, manages and ends sessions | NetBIOS, RPC |
| 4 Transport | End-to-end delivery, segmentation, error and flow control | TCP, UDP |
| 3 Network | Logical addressing and routing | IP, ICMP, routers |
| 2 Data link | Framing, MAC addressing, error detection | Ethernet, switches |
| 1 Physical | Transmission of raw bits | Cables, hubs, signals |
Exam tip
Mnemonic (top to bottom): "All People Seem To Need Data Processing".
Topic 3
The TCP/IP model
The TCP/IP model is the practical model used by the internet. It has four layers: Application (OSI layers 5–7), Transport, Internet (network) and Network Access (data link + physical).
Layers
7
4
Developed by
ISO, as a reference model
US Department of Defense, from real protocols
Approach
Model first, protocols later
Protocols first, model describes them
Use
Teaching and reference
Used on the internet
Transport
Connection-oriented and connectionless
TCP and UDP
Topic 4
Network topologies
Physical topology is the actual layout of cables and devices; logical topology is how data actually flows.
Bus
Cheap, little cable
A cable break stops the network
Star
Easy to add devices and find faults
Hub/switch failure stops everything
Ring
Orderly access, no collisions
One break can stop the ring
Mesh
Highly reliable, many paths
Costly cabling: n(n − 1)/2 links
Hybrid
Flexible, combines strengths
Complex to design
Example
A full mesh of 5 devices needs 5 × 4 / 2 = 10 links.
Key terms
- Computer network
- Interconnected devices sharing data and resources
- Protocol
- A set of rules for communication
- OSI model
- A seven-layer reference model for networking
- TCP/IP model
- The four-layer model used by the internet
- Topology
- The arrangement of devices and links in a network
Quick revision
- PAN < LAN < MAN < WAN.
- OSI: physical, data link, network, transport, session, presentation, application.
- TCP/IP: network access, internet, transport, application.
- Star is most common in LANs; mesh needs n(n − 1)/2 links.
Important exam questions
Practice questions written to the PTU exam pattern for this unit's syllabus: short answers (Section A style) and long answers (Sections B and C style).
Short-answer questions
- Q1.Define a computer network and list two objectives.
- Q2.Name the seven layers of the OSI model.
- Q3.Which OSI layer handles routing?
- Q4.Compare OSI and TCP/IP models.
- Q5.Differentiate between physical and logical topology.
- Q6.How many links does a full mesh of 6 devices need?
Long-answer questions
- Q1.Explain the OSI reference model with the functions of each layer.
- Q2.Explain the TCP/IP model and compare it with the OSI model.
- Q3.Explain the different network topologies with their merits and demerits.
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