How unit 1 is examined
This unit covers network basics, topologies, layering, service types, the OSI and TCP/IP models and the physical layer; connection-oriented vs connectionless services carry the most marks, then network classification, topology cost and the OSI/TCP-IP layers.
Computer Network: Definitions, goals, components
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Definition. <mark>A computer network is a collection of autonomous computers and devices interconnected by transmission media so that they can exchange data and share resources.</mark>
Key points.
- The primary goals are resource sharing (printers, files, internet), fast communication (email, video calls), reliability through duplicate resources, and scalability by adding nodes easily.
- Hardware components are hosts (end systems), the NIC that connects a host to the medium, hub, switch, router, modem and the transmission media (cable, fibre, wireless).
- A hub repeats bits to all ports, a switch forwards frames by MAC address, and a router forwards packets between networks by IP address.
- Software components are protocols (TCP/IP), the network operating system, and network services and applications such as web, email and file transfer.
Asked: [7 marks] (May 2024) What is a computer network and what are its primary goals? Identify and explain the key components of a computer network including hardware and software elements.
Architecture, Classifications & Types
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Definition. <mark>Network architecture is the design of a network: its physical layout (topology), its layers and protocols, and its scale.</mark>
Key points.
- By scale, a PAN covers about 1-10 m around one person (Bluetooth), a LAN covers a building or campus (Ethernet, Wi-Fi), a MAN covers a city (cable TV, WiMAX) and a WAN covers countries (the Internet, leased lines).
- By transmission, networks are broadcast (one shared channel, as in a LAN) or point-to-point (dedicated links, as in a WAN).
- By relationship, networks are client-server (central server) or peer-to-peer (all nodes equal).
- Topology is the physical arrangement of nodes: bus (one backbone), ring (closed loop), star (central hub or switch) and mesh (every node linked to every other).
- A mesh gives the highest redundancy but the highest cost, a bus is cheapest but one cable break stops everything, and a star isolates faults but depends on the central device.
Formula. Links: bus $N-1$, ring $N$, star $N$ (N end nodes plus the central hub), mesh $\dfrac{N(N-1)}{2}$. Cost $=$ links $\times$ cost per link.
Example. Given $N=20$, cost $=$100$ per link.
| Topology | Links | Cost |
|---|---|---|
| Bus | 19 | $1,900 |
| Ring | 20 | $2,000 |
| Star | 20 | $2,000 |
| Mesh | $20\times19/2=190$ | $19,000 |
Bus is the most cost-effective at $1,900 and mesh has the highest redundancy at $19,000.
Answer frame. For classification, open with the definition, list PAN, LAN, MAN, WAN in a table by range and example, then add broadcast/point-to-point, and close with service primitives from the Service primitives topic. For the numerical, write the four formulas, substitute N=20, tabulate links and cost, and close with the cost-vs-redundancy verdict.
Pitfall: If the star's hub is counted among the 20 nodes the links become 19, so state your assumption.
Asked: [7 marks] (Jun 2025) Classify computer networks. Discuss the significance of service primitives in connection-oriented and connectionless services. Asked: [7 marks] (Jun 2025) A company designs a network of 20 nodes at $100 per link. Calculate the number of links and total cost for bus, ring, star and mesh topologies, and find the most cost-effective topology and the one with the highest redundancy.
Layered Architecture: Protocol hierarchy
<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">Not asked since 2022</span>
Definition. <mark>A protocol hierarchy organises network functions into a stack of layers, where each layer offers services to the layer above and uses the services of the layer below.</mark>
Key points.
- Layer $n$ on one machine talks to layer $n$ on another machine using the layer-$n$ protocol; these are peer entities.
- Actual data travels down the stack, across the physical medium and up the peer stack; each layer adds its header on the way down.
- Layering breaks a complex problem into small independent parts, so one layer can change without touching the others.
Design Issues, Interfaces and Services
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Definition. <mark>Design issues are the problems every layer designer must solve to make communication reliable, efficient and fair.</mark>
Key points.
- Addressing: every sender and receiver needs a unique identifier so data reaches the right process or host.
- Error control: detecting and correcting errors using codes and retransmission so that data arrives correctly.
- Flow control: stopping a fast sender from swamping a slow receiver.
- Multiplexing and routing: many conversations share one link, and a path must be chosen through the network.
- Reliability, scalability, resource sharing and security must hold as the network grows; more layers give modularity but cost overhead and delay.
- An interface defines the operations and services a lower layer offers to the upper layer, while a protocol is the rule set between peers.
Asked: [7 marks] (May 2023) Discuss about the design issues of computer network.
Connection Oriented & Connectionless Services
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Definition. <mark>A connection-oriented service first sets up a connection between sender and receiver (like a telephone call), transfers data over it in order, then releases it; a connectionless service sends each packet independently with no setup (like the postal system).</mark>
Diagram.
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Key points.
- A connection-oriented service has three phases: connection establishment, data transfer and connection release; TCP is the example.
- A connectionless service has only data transfer, and each datagram carries the full destination address and is routed independently; UDP over IP is the example.
- Connection-oriented service is reliable, since it gives acknowledgements, retransmission and flow control, whereas connectionless service is unreliable with best-effort delivery.
- Connection-oriented service delivers data in order over one fixed path, whereas connectionless datagrams may arrive out of order, duplicated or lost.
- Connection-oriented service has higher overhead and delay because of setup and state at both ends, and connectionless service has low overhead and is faster.
- Connection-oriented service suits file transfer, email and web pages, and connectionless service suits DNS queries, streaming, VoIP and broadcasts.
| Basis | Connection-oriented | Connectionless |
|---|---|---|
| Setup | Required before data | None |
| Reliability | Reliable (ACK, retransmit) | Unreliable (best effort) |
| Ordering | In order | May be out of order |
| Path | Fixed for the session | Each packet independent |
| Overhead | High | Low |
| Example | TCP, virtual circuit | UDP, IP datagram |
| Use | FTP, email, HTTP | DNS, VoIP, video |
Answer frame. For the 7-mark difference, open with both definitions, give the table above with seven rows, and close with TCP vs UDP. For the 14-mark short note, write this topic as one of the two options with definition, three phases, diagram, table, and advantages and applications, then pick a second option from Hybrid ARQ, ICMP or SNMP.
Pitfall: Do not say connectionless means no error checking at all; UDP still has an optional checksum, it just does not retransmit.
Asked: [7 marks] (May 2023) Give the difference between connection oriented and connection less services. Asked: [14 marks] (May 2024) Write a short note on any two: (a) Connection Oriented and Connection less services (b) Hybrid ARQ (c) ICMP (d) SNMP
Service primitives
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Definition. <mark>Service primitives are the basic operations, such as LISTEN, CONNECT, SEND, RECEIVE and DISCONNECT, through which a user process accesses the service provided by a layer.</mark>
Key points.
- LISTEN makes the server wait for an incoming connection; CONNECT actively requests one; SEND and RECEIVE transfer data; DISCONNECT releases the connection.
- Connection-oriented service uses all five, so both sides know the state of the exchange; connectionless service needs only SEND and RECEIVE, with no setup or release.
- Primitives are the interface between a layer and its user, hiding how the service is implemented.
- Each primitive is classed as request, indication, response or confirm.
ISO-OSI Reference Model: Principle, Model
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Definition. <mark>The OSI model is a seven-layer reference model from ISO that divides network communication into layers, each with a defined function, so that different systems can interoperate.</mark>
Diagram.
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Dotted peer links are logical, only layer 1 is a real wire; data goes down A, across, and up B with a header added at each layer</figcaption></figure>
Key points.
- The principle is that a layer is created where a new level of abstraction is needed, with each layer performing a well-defined function.
- Each layer uses only the services of the layer directly below and provides services to the layer above.
- Peer layers communicate logically using a protocol, and each layer adds a header (the data link layer also adds a trailer) on the sending side, which is called encapsulation.
- Layer boundaries are chosen to minimise information flow across interfaces.
Asked: [7 marks] (May 2023) Identify the layers in OSI reference model and illustrate their functions with suitable diagram.
Descriptions of various layers and its comparison with TCP/IP
<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>The seven OSI layers, top to bottom, are Application, Presentation, Session, Transport, Network, Data Link and Physical; TCP/IP compresses them into four layers.</mark>
| Layer | Function | Protocol | Device |
|---|---|---|---|
| 7 Application | Gives user access to network services | HTTP, FTP, SMTP, DNS | Gateway |
| 6 Presentation | Translation, encryption, compression | SSL/TLS, JPEG, MPEG | Gateway |
| 5 Session | Dialogue control, synchronisation | NetBIOS, RPC | Gateway |
| 4 Transport | End-to-end delivery, segmentation, flow and error control | TCP, UDP | Gateway |
| 3 Network | Logical addressing and routing of packets | IP, ICMP, OSPF | Router |
| 2 Data Link | Framing, MAC addressing, error detection | Ethernet, PPP, HDLC | Switch, bridge |
| 1 Physical | Transmission of raw bits over the medium | RS-232, Ethernet PHY | Hub, repeater, cable |
Key points.
- TCP/IP has four layers: Application (covering OSI 5-7), Transport, Internet and Network Access (covering OSI 1-2).
- OSI is a generic reference model designed before its protocols, while TCP/IP was built around working protocols and the model came afterwards.
- OSI makes a clear distinction between services, interfaces and protocols, whereas TCP/IP does not.
- OSI supports both connection-oriented and connectionless service in the network layer but only connection-oriented in the transport layer, whereas TCP/IP supports only connectionless in the internet layer but both in transport.
- TCP/IP is more relevant today because the Internet runs on it, and OSI protocols were never widely adopted; OSI survives as a teaching model.
| Basis | OSI | TCP/IP |
|---|---|---|
| Layers | 7 | 4 |
| Approach | Model first, protocols later | Protocols first, model later |
| Session/Presentation | Separate layers | Merged into Application |
| Transport service | Connection-oriented only | TCP and UDP |
| Usage | Reference and teaching | Real internet |
Answer frame. For the seven layers, open with the top-to-bottom list, give the table row by row, and close with the encapsulation flow. For the comparison, draw the two stacks side by side, give the five-row table, and close with why TCP/IP is more relevant today.
Asked: [7 marks] (May 2024) Describe each of the seven layers of the ISO/OSI model. For each layer explain its function and provide examples of protocols and devices associated with it. Asked: [7 marks] (Jun 2025) Compare and contrast the OSI and TCP/IP reference models in detail. Which model is more relevant in today's networking environment and why?
Principals of physical layer: Media, Bandwidth, Data rate and Modulations
<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">Low weight</span>
Definition. <mark>Data rate is the number of bits per second sent over a channel, and bandwidth is the range of frequencies the channel passes, measured in hertz; more bandwidth allows a higher data rate.</mark>
Formula.
$$C = 2B\log_2 L \quad \text{(Nyquist, noiseless)}$$
$$C = B\log_2(1+\mathrm{SNR}) \quad \text{(Shannon, noisy)}$$
Key points.
- In Nyquist's formula $B$ is bandwidth in Hz and $L$ is the number of signal levels, so doubling the bandwidth or adding levels raises the maximum data rate.
- Shannon's formula gives the absolute capacity of a noisy channel, where SNR is the signal-to-noise power ratio; raising the signal levels cannot beat it.
- Noise limits the data rate because with more levels the levels lie closer together and noise makes them indistinguishable.
- Media are guided (twisted pair, coaxial, optical fibre) or unguided (radio, microwave, satellite).
- Modulation (ASK, FSK, PSK) changes amplitude, frequency or phase of a carrier to send digital data over an analog channel.
Example. For $B=3000$ Hz and SNR $=1000$: $C=3000\log_2(1001)\approx 3000\times9.97\approx$ 29.9 kbps.
Asked: [7 marks] (May 2024) Explain the concept of data rate in the context of the physical layer. How is data rate related to the bandwidth of a communication channel? Explain.
Last-minute revision
- A computer network is autonomous computers interconnected to share data and resources.
- Links for N nodes: bus $N-1$, ring $N$, star $N$, mesh $N(N-1)/2$; for N=20 these are 19, 20, 20 and 190.
- At $100 per link, N=20 costs $1,900 (bus), $2,000 (ring), $2,000 (star) and $19,000 (mesh).
- PAN is about 10 m, LAN is a building, MAN is a city, WAN is a country or larger.
- Connection-oriented has setup, data and release phases (TCP); connectionless has independent datagrams (UDP).
- The five primitives are LISTEN, CONNECT, SEND, RECEIVE and DISCONNECT.
- OSI layers from the top: Application, Presentation, Session, Transport, Network, Data Link, Physical.
- Router works at layer 3, switch at layer 2, hub at layer 1.
- TCP/IP has four layers: Application, Transport, Internet, Network Access.
- Nyquist: $C=2B\log_2 L$; Shannon: $C=B\log_2(1+\mathrm{SNR})$.
- Design issues: addressing, error control, flow control, multiplexing, routing.
Memory hooks
- OSI top-down: "All People Seem To Need Data Processing"; bottom-up: "Please Do Not Throw Sausage Pizza Away".
- Topology links: bus is N minus one, ring and star are N, mesh is half of N times N minus one.
- TCP is a phone call (setup, talk, hang up); UDP is a postcard.
- Nyquist is noiseless, Shannon is noisy: "S for stuff in the noise".
Coverage checklist
- Computer Network: Definitions, goals, components: definition, goals, hardware and software (May 2024 explain).
- Architecture, Classifications & Types: PAN/LAN/MAN/WAN, topology link and cost numerical (Jun 2025, two questions).
- Layered Architecture: Protocol hierarchy: peers, stack, encapsulation (no past question).
- Design Issues, Interfaces and Services: design issues (May 2023).
- Connection Oriented & Connectionless Services: difference table (May 2023), short note (May 2024).
- Service primitives: five primitives, significance (no own question; used in Jun 2025 classification question).
- ISO-OSI Reference Model: Principle, Model: layers with diagram (May 2023).
- Descriptions of various layers and its comparison with TCP/IP: seven layers (May 2024), OSI vs TCP/IP (Jun 2025).
- Principals of physical layer: Media, Bandwidth, Data rate and Modulations: data rate, bandwidth, Nyquist, Shannon (May 2024).