Computer Networks (AL-602) Unit 1 Notes - RGPV

How unit 1 is examined

Foundations of networks: definition, topologies, service types, the OSI and TCP/IP models, and the physical layer. Marks sit in the OSI layers, connection-oriented versus connectionless services, media, and the network definition.

Computer Network: Definitions

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Definition. <mark>A computer network is a collection of autonomous computers interconnected by a communication medium so that they can share resources and exchange data.</mark>

Key points.

Importance: many users share one printer, disk or database, so the same hardware is not bought again and again.
It gives fast communication such as email, chat and video calls between distant users.
It gives reliability, because data can be stored on several machines and a failed machine is replaced by another.
It saves cost, since one powerful server is cheaper than a full set of stand-alone systems.
It scales easily, because new computers are added without redesigning the system.
Merits: shared hardware, software and data, central backup, and easy communication.
Demerits: security threats such as hacking and viruses, high setup and maintenance cost, complex administration, and dependence on the network, since a failure stops all work.

Answer frame. Open with the definition; list importance points 1-5; give merits and demerits as two short lists or a two-column table; close with one line on the role of networks in the Internet.

Asked: [7 marks] (May 2023, Jun 2026) What is a computer network? Discuss its importance, merits and demerits. Define a computer network; explain its goals, components and applications.

Goals

Definition. Network goals are the purposes for which computers are connected.

Key points.

Resource sharing makes programs, data and devices available to every user irrespective of location.
Reliability comes from replicating files on several machines.
Cost saving comes from using small client computers with shared servers.
Communication and scalability: users exchange messages, and capacity grows by adding machines.

Components

Definition. Components are the hardware and software parts that together make a network work.

Key points.

Hosts (end systems) run applications, and each connects through a network interface card (NIC).
Transmission media (wire or wireless) carry the signals between devices.
Connecting devices, namely hubs, switches and routers, forward data between links.
Protocols are the agreed rules of communication, and network software implements them.
Applications such as email, file sharing, the WWW and remote login are the network's uses.

Architecture

Definition. Network architecture is the physical layout of nodes, called the topology, together with the layered protocol design.

Key points. The table compares the six topologies.

Topology	Advantage	Disadvantage
Bus	Cheap, little cable	One cable break stops all; collisions
Star	Easy to add nodes; a fault stays local	Hub failure kills the network
Ring	Orderly access, no collisions	One break fails the ring
Mesh	Most reliable, dedicated links	Costly, n(n-1)/2 links
Tree	Scalable, hierarchical	Depends on the root bus
Hybrid	Flexible, combines strengths	Complex and costly

<mark>A topology is chosen for cost, reliability, scalability and fault tolerance.</mark>

Asked: [7 marks] (May 2024) Explain the different types of topologies with their advantages and disadvantages.

Classifications & Types

Definition. Networks are classified mainly by geographical area.

Key points.

A PAN covers a few metres around one person, such as Bluetooth devices.
A LAN covers a building or campus, up to a few km, with high speed and private ownership.
A MAN covers a city, for example a cable TV network.
A WAN covers countries or continents and uses leased lines and satellites; the Internet is the largest.
Other classes are by transmission (broadcast or point-to-point) and by role (client-server or peer-to-peer).

Protocol hierarchy

Definition. A protocol hierarchy organizes network functions into a stack of layers, each offering services to the layer above.

Key points.

Layer n on one machine talks to layer n on the other machine using the layer n protocol; this is peer-to-peer communication.
In reality data goes down to the physical medium and up the other stack, and each layer uses only the service of the layer below.
Layering makes the design modular, so one layer can change without disturbing the others.
The set of layers and protocols is called the network architecture.

Design Issues

Definition. Design issues are the problems every layer designer must solve to make communication dependable.

Key points.

Addressing identifies the sender and receiver, since many processes and hosts exist.
Error control uses detecting and correcting codes and retransmission so that data arrives correct.
Flow control stops a fast sender from swamping a slow receiver.
Routing chooses the path when several paths exist, and multiplexing shares one channel among many conversations.
Other issues: ordering of messages, message size (fragmentation), and congestion.

Answer frame. List the five issues with one sentence each; then continue with the connection-oriented versus connectionless comparison table below.

Asked: [7 marks] (Jun 2026) Discuss design issues in computer networks and explain connection-oriented and connectionless services.

Interfaces and Services

Definition. A service is the set of operations a layer offers to the layer above; the interface is the boundary where this is offered.

Key points.

The interface defines the service access points (SAP) through which the upper layer reaches the service.
A service is what a layer does; a protocol is how it does it between peers, and protocols may change without changing the service.
Services are of two kinds: connection-oriented and connectionless.

Connection Oriented & Connectionless Services

Definition. <mark>A connection-oriented service first sets up a connection, then transfers data in order over it, and finally releases it, as in a telephone call; a connectionless service sends each message independently with no set-up, as in the postal system.</mark>

Diagram. Phases of a connection-oriented service; the sender and receiver are the two end systems.

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Key points.

A connection-oriented service works in three phases: connection establishment, data transfer and connection release; TCP is the example.
It is reliable, since data is acknowledged and lost packets are re-sent, and it delivers in order.
Packets follow the same path, so header overhead per packet is small but set-up delay exists.
A connectionless service sends datagrams, each carrying the full destination address and routed independently.
It is unreliable and unordered, with no acknowledgement, but it is fast and has low overhead; UDP/IP is the example.
Connection-oriented suits file transfer, email and web; connectionless suits DNS queries, voice, video and streaming, where speed matters more than a lost packet.

Basis	Connection-oriented	Connectionless
Set-up	Needed (handshake)	Not needed
Reliability	Reliable, acknowledged	Unreliable, best effort
Ordering	Delivered in order	May arrive out of order
Path	Fixed route	Each packet independent
Overhead	Set-up delay, more state	Low overhead, fast
Example	TCP	UDP, IP

Answer frame. Open by defining both; draw the three-phase diagram; give the table on set-up, reliability, ordering, overhead and examples; close with TCP for accuracy and UDP for speed. For the short-note version, give the definition, phases, working, and advantages and limitations.

Pitfall: Do not call IP or UDP "wrong"; connectionless is not bad, it is chosen for speed.

Asked: [7 marks] (May 2022, Dec 2024, Jun 2025) Difference between connectionless and connection-oriented communication. Asked: [14 marks] (May 2024) Short notes (any three): Connection Oriented, ARP, CSMA/CA, DNS.

Service primitives

Definition. <mark>Service primitives are the basic operations, such as request and response calls, through which a user process accesses the service provided by a layer.</mark>

Key points.

There are four classes of primitive: Request, Indication, Response and Confirm.
Request is issued by the user to ask the service to do something; Indication is delivered to the peer to tell it that the event happened.
Response is the peer's answer to an Indication; Confirm tells the original user the request is completed.
For a connection-oriented service the usual primitives are LISTEN, CONNECT, ACCEPT, RECEIVE, SEND and DISCONNECT.
LISTEN blocks the server until a client arrives; CONNECT starts the connection and waits; ACCEPT completes it.
SEND and RECEIVE exchange data, and DISCONNECT releases the connection from either side.
A confirmed service uses all four classes, while an unconfirmed service uses only Request and Indication.

Diagram. Time sequence of a connection: CONNECT.request at the client, CONNECT.indication at the server, CONNECT.response from the server, and CONNECT.confirm back to the client.

Answer frame. Open with the definition; list the four classes; show the CONNECT sequence; list the six socket-style primitives with one line each; close with the comment that they hide protocol details from the user.

Asked: [14 marks] (May 2023) Short note on any two: Service Primitives, CSMA/CD, IEEE Standards 802 series, Fragmentation and reassembly.

ISO-OSI Reference Model: Principle

Definition. <mark>The OSI reference model, proposed by ISO, divides network communication into seven layers, each with a defined function, so that different systems can interoperate.</mark>

Key points.

The layers from top are Application, Presentation, Session, Transport, Network, Data Link and Physical.
Each layer at the sender communicates logically with the same layer at the receiver; this is peer-to-peer communication with the layer's protocol.
Encapsulation: as user data moves down, each layer adds its header (H7 to H2), and the data link layer also adds a trailer T2.
The physical layer transmits raw bits; on the receiver side each layer strips its header, which is decapsulation.
Importance: standardization, interoperability among vendors, and modular design where one layer can be changed alone.
It is a reference for network architectures such as TCP/IP; it is not itself a protocol suite.

Diagram.

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Answer frame. Open with the definition and the seven layers; draw the two stacks with headers; explain encapsulation and decapsulation; give the importance; close with one line on its use as a reference.

Asked: [7 marks] (May 2022, Jun 2025) Explain the OSI model and its importance; show headers and trailers added to user data.

Descriptions of various layers and its comparison with TCP/IP

Definition. <mark>OSI has seven layers, while the TCP/IP model has four (Network Access, Internet, Transport, Application) and was built from working protocols.</mark>

Layer	Function	Protocols	Data unit
7 Application	Gives user services such as mail and file transfer	HTTP, FTP	Data
6 Presentation	Translation, encryption, compression	SSL, JPEG	Data
5 Session	Dialogue control, synchronization	NetBIOS, RPC	Data
4 Transport	End-to-end delivery, error and flow control	TCP, UDP	Segment
3 Network	Logical addressing, routing	IP, ICMP	Packet
2 Data Link	Framing, MAC addressing, error detection	Ethernet, PPP	Frame
1 Physical	Bit transmission, signals, cables	RS-232, DSL	Bits

Key points.

TCP/IP maps Application, Presentation and Session of OSI into one Application layer.
Transport maps to Transport, Network to Internet, and Data Link with Physical to Network Access.
OSI is a generic, protocol-independent model made before the protocols; TCP/IP was made from the protocols and is what the Internet uses.
OSI has a clear split of service, interface and protocol; TCP/IP does not.
OSI Transport is only connection-oriented in one mode of the standard, while TCP/IP Transport offers both TCP and UDP.
Merit of OSI: general and clean. Criticism: too complex, bad timing and slow implementations. Merit of TCP/IP: simple and proven. Criticism: not generic and describes no other stack.

Diagram.

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Answer frame. For the explain question: draw the seven-layer stack with a function and two protocols per layer, then add peer-to-peer and encapsulation. For the comparison question: draw both stacks side by side, give a table on layers, type, approach, reliability and usage, then merits and demerits.

Asked: [7 marks] (May 2023, May 2024) Explain the OSI reference model with the function of each layer; name two protocols per layer. Asked: [7 marks] (Dec 2024, Jun 2026) Compare the OSI and TCP/IP models with suitable diagrams.

Principals of physical layer: Media

Definition. <mark>Transmission media are the physical paths that carry signals from sender to receiver, and they are guided (wired) or unguided (wireless).</mark>

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Key points.

Twisted pair is two insulated copper wires twisted to cut interference; it is cheap and used for telephones and LANs, but has limited bandwidth and distance.
Coaxial cable has a central conductor, insulation and a metal shield; it has higher bandwidth and less noise, and is used in cable TV.
Optical fibre carries light by total internal reflection in a glass core inside a cladding; it has very high bandwidth and no electrical interference, but costs more.
Radio waves travel in all directions and pass through walls; microwaves need line-of-sight; infrared cannot pass walls.
Satellite links cover very wide areas with a delay of about 250 ms.
Local loop broadband options: DSL uses the existing telephone copper for up to a few Mbps over about 5 km; cable modem uses shared coaxial TV cable at tens of Mbps; FTTH uses fibre for hundreds of Mbps to Gbps at the highest cost.

Local loop	Medium	Speed	Cost
DSL	Telephone copper	1-25 Mbps, distance limited	Low
Cable	Coax, shared	10-100 Mbps	Medium
FTTH	Fibre	100 Mbps - 1 Gbps	High

Answer frame. For the media question: define, draw the classification with cross-section sketches, cover each medium in a sentence, compare on bandwidth, cost and interference. For the local loop question: give the table above with properties.

Asked: [7 marks] (May 2022) Differentiate broadband local loop technologies stating their properties. Asked: [7 marks] (May 2023) Guided and unguided transmission media with a sketch. Asked: [7 marks] (Jun 2026) Explain physical media, bandwidth, data rate and data modulation.

Bandwidth

Definition. <mark>Bandwidth is the range of frequencies a channel can pass, measured in Hz; data rate is the number of bits sent per second, measured in bps.</mark>

Key points.

Bandwidth in Hz is the difference between the highest and lowest frequency, for example a telephone line from 300 Hz to 3300 Hz has 3000 Hz.
In digital usage bandwidth also means the maximum bit rate of a link, such as 100 Mbps Ethernet.
Data rate is the actual bits per second achieved, for example sending 1 MB in 8 s gives 1 Mbps.
They are related: by Nyquist the maximum rate is $2B\log_2 L$ for a noiseless channel with L levels, and by Shannon it is $B\log_2(1+SNR)$ for a noisy one.

Asked: [7 marks] (May 2024) Define data rate and bandwidth with a suitable example.

Data rate and Modulations

Definition. Modulation changes a carrier wave (amplitude, frequency or phase) in step with the data so that it suits the channel.

Formula. $C = B\log_2(1+SNR)$ with $SNR = (V_s/V_n)^2$ as a power ratio.

Example. Given B = 4 kHz, signal 20 V, noise 6 mV.

Step	Working
SNR	$(20/0.006)^2 = (3333.3)^2 = 1.111\times10^7$
$\log_2(1+SNR)$	$\log_2(11111112) = 23.41$
C	$4000 \times 23.41 = 93{,}622$ bps

The maximum data rate is about 93.6 kbps.

Key points.

ASK varies amplitude, FSK varies frequency and PSK varies phase.
Modulation lets signals travel far over a medium and share it through different carriers.
Data rate in bps equals baud rate multiplied by bits per signal element.

Pitfall: Convert both to the same unit (mV to V) and square the ratio for power.

Asked: [7 marks] (Dec 2024) Line of 4 kHz bandwidth, signal 20 V, noise 6 mV: maximum data rate.

Last-minute revision

A computer network is autonomous computers interconnected to share resources.
Topologies: bus, star, ring, mesh, tree, hybrid; mesh has n(n-1)/2 links.
Connection-oriented: set-up, data, release; TCP. Connectionless: datagrams; UDP.
Primitives: Request, Indication, Response, Confirm.
OSI order from top: Application, Presentation, Session, Transport, Network, Data Link, Physical.
TCP/IP has four layers; Application, Presentation, Session collapse into one.
Encapsulation adds headers H7 to H2 and the trailer T2 at Data Link.
Guided: twisted pair, coax, fibre; unguided: radio, microwave, infrared, satellite.
Nyquist $2B\log_2 L$; Shannon $B\log_2(1+SNR)$.
4 kHz, 20 V, 6 mV gives about 93.6 kbps.

Memory hooks

OSI top to bottom: "All People Seem To Need Data Processing"; bottom to top: "Please Do Not Throw Sausage Pizza Away".
Connection-oriented is a telephone call; connectionless is a postcard.
TCP is trust, UDP is unreliable but urgent.
Four primitives: RIRC, Request Indication Response Confirm.

Coverage checklist

Computer Network: Definitions: covers May 2023, Jun 2026 definition question.
goals: covered in the Jun 2026 definition question.
components: covered in the Jun 2026 definition question.
Architecture: covers the May 2024 topologies question.
Classifications & Types: PAN, LAN, MAN, WAN.
Protocol hierarchy: layers, peers.
Design Issues: covers Jun 2026 design issues question.
Interfaces and Services: SAP.
Connection Oriented & Connectionless Services: covers the May 2022, Dec 2024, Jun 2025 comparison and May 2024 short note.
Service primitives: covers the May 2023 short note.
ISO-OSI Reference Model: Principle: covers May 2022, Jun 2025.
Descriptions of various layers and its comparison with TCP/IP: covers May 2023, May 2024, Dec 2024, Jun 2026.
Principals of physical layer: Media: covers May 2022, May 2023, Jun 2026.
Bandwidth: covers May 2024.
Data rate and Modulations: covers Dec 2024.

Account

Computer Networks (AL-602) - Unit 1 Short Notes

How unit 1 is examined

Computer Network: Definitions

Goals

Components

Architecture

Classifications & Types

Protocol hierarchy

Design Issues

Interfaces and Services

Connection Oriented & Connectionless Services

Service primitives

ISO-OSI Reference Model: Principle

Descriptions of various layers and its comparison with TCP/IP

Principals of physical layer: Media

Bandwidth

Data rate and Modulations

Last-minute revision

Memory hooks

Coverage checklist