Chapter 1: Computer Network and Communication

Whenever you talk to a friend, write a letter, or send a message on a phone, communication is taking place. There must always be at least two parties: a sender and a receiver, and a way for the message to travel from one to the other.

Basic forms of communication

• Verbal communication: speaking, conversations.
• Non-verbal communication: gestures, facial expressions, body language.
• Written communication: letters, emails, books.
• Visual communication: pictures, diagrams, videos.

The word "tele" means distant. So telecommunication literally means "communication at a distance". It allows people who are far apart to share voice, video, text, or data instantly.

Common examples

Whenever you send a WhatsApp message, download a file, or watch a YouTube video, data communication is happening between your device and a remote computer.

Data communication has five essential elements. Without any one of them, the communication cannot take place.

Based on the direction of data flow, transmission can be of three types:

1. Simplex

Data flows in only one direction. The sender can only send and the receiver can only receive.

Examples: Radio broadcast, television, keyboard to computer.

2. Half-Duplex

Data can flow in both directions, but only one direction at a time. When one side is sending, the other must wait.

Examples: Walkie-talkie, push-to-talk systems.

3. Full-Duplex

Data flows in both directions simultaneously. Both parties can send and receive at the same time.

Examples: Telephone call, video call, modern (full-duplex) Ethernet connections.

The word "broadband" comes from "broad bandwidth", meaning a wide range of frequencies is used to carry a large amount of data quickly. Broadband can carry voice, video and data at the same time.

It is usually measured in bits per second (bps), Kbps (kilobits/second), Mbps (megabits/second), or Gbps (gigabits/second).

Throughput is always less than or equal to bandwidth because of real-world factors such as network traffic, errors, signal loss, and protocol overhead.

Low latency means very little delay; high latency means a noticeable lag. Latency and bandwidth are different: you can have high bandwidth but still feel lag if latency is high.

Many home connections are asymmetric: download speed is higher than upload speed, because users usually download more than they upload.

Mobile networks have evolved through several "generations" (G). Each generation brought faster speeds and new features.

Features of Data Packets

• Each packet contains a piece of the actual data (called the payload).
• Each packet has a header with information like sender address, receiver address and sequence number.
• Packets can take different paths to reach the destination.
• At the destination, packets are reassembled in the correct order using their sequence numbers.

1 Hz = 1 cycle per second. A wider range of frequencies (more bandwidth) can usually carry more data. Common units:

• kHz (kilohertz) = 1,000 Hz
• MHz (megahertz) = 1,000,000 Hz
• GHz (gigahertz) = 1,000,000,000 Hz

For example, Wi-Fi commonly uses the 2.4 GHz and 5 GHz frequency bands (and newer Wi-Fi also uses 6 GHz).

Without transmission media, data cannot move from one device to another. It can be a physical cable or open air carrying electromagnetic waves.

Communication media is broadly classified into two main types:

The main types of wired cable are:

Twisted-pair cables also come in categories (standards) based on their speed and quality. A category is not a separate cable type: a CAT cable can be built as UTP or STP.

Wireless media carry signals through open air as electromagnetic waves. The main kinds are radio waves, microwaves and infrared; technologies like Wi-Fi, Bluetooth and satellite are built on top of these.

It looks similar to a telephone connector (RJ-11) but is wider because it uses 8 positions instead of 4 or 6. These connectors are used to plug Ethernet cables into computers, switches, routers and modems.

Features

• Connects two different types of media into one network.
• Extends the distance of a network beyond the limit of copper cable (typically 100 m).
• Operates at the Physical layer of the OSI model.
• Available in different speeds (10/100/1000 Mbps).
• Compact, low-cost and easy to install.

1. Bus / Linear Topology

All devices are connected to a single central cable called the backbone (bus). Cheap and easy to install, but if the main cable fails, the whole network fails.

2. Star Topology

All devices are connected to a central hub or switch. If one cable fails, only that device is affected. Most widely used in offices and schools today.

3. Ring Topology

Each device is connected to two others, forming a closed ring. Data may travel in one direction (unidirectional) or both directions (bidirectional, as in dual-ring networks). A break in the ring can affect the whole network.

4. Mesh Topology

Every device is connected to many (or all) other devices, giving multiple paths for data. Very reliable: if one link fails, data uses another path. Used in the Internet backbone and modern Wi-Fi mesh systems. Drawback: needs a lot of cabling/links.

5. Tree (Hierarchical) Topology

Devices are arranged like a tree, with a root node and branches of star-connected groups. Common in large organisations and school networks because it is easy to expand and manage.

6. Hybrid Topology

A combination of two or more different topologies (e.g., star + bus). Flexible and scalable but more complex to design.

Computer networks make it possible to share files, printers, internet connections and applications. Networks can be small (between two computers in a room) or huge (millions of computers across the world, like the Internet).

Advantages

• Resource sharing: printers, scanners, internet, files.
• Fast and easy communication (email, chat, video call).
• Centralised data storage and easy backup.
• Saves money by sharing hardware and software.
• Centralised software management and updates.

Limitations

• Installation cost can be high for large networks.
• Security risks: virus, hacking, data theft.
• If the server fails, many users are affected.
• Needs skilled technicians to maintain.
• Heavy traffic can slow down the network.

1. Client-Server Architecture

One powerful computer (the server) provides resources, files or services to many other computers (clients). All requests go through the server.

2. Peer-to-Peer (P2P) Architecture

All computers in the network are equal; there is no dedicated server. Each computer can act as both client and server, sharing files directly with others.

DNS (Domain Name System)

DNS works like the Internet's phonebook. When you type www.google.com, DNS finds the matching IP address so your device can connect. Example: a domain name maps to an address such as 142.250.x.x.

DHCP (Dynamic Host Configuration Protocol)

DHCP automatically gives a device its IP address, subnet mask, gateway and DNS settings when it connects. Example: your phone gets an IP address automatically when it joins your home Wi-Fi, without you typing anything.

It is a 48-bit address written as six pairs of hexadecimal digits, for example 00:1A:2B:3C:4D:5E. It is used to identify devices within a local network.

Just like your home has a unique postal address so the postman can deliver letters, every computer has a unique IP address so data packets can find their way to it.

IPv4 (Internet Protocol version 4)

The original and still most widely used IP version. It uses a 32-bit address written as four decimal numbers separated by dots (each from 0 to 255).

Example: 192.168.1.1

IPv4 provides about 4.3 billion unique addresses.

IPv6 (Internet Protocol version 6)

The newer version that uses a 128-bit address written as eight groups of four hexadecimal digits separated by colons.

Example: 2001:0db8:85a3:0000:0000:8a2e:0370:7334

IPv6 provides an extremely large number of addresses: approximately 340 undecillion (3.4 × 10³⁸).

Internet

A global network of interconnected networks ("network of networks") that links millions of computers worldwide. It is public and can be accessed by anyone with an internet connection.

Intranet

A private network inside an organisation (school, company) that uses internet technologies. It is accessible only to authorised members.

Extranet

An extension of an intranet that allows limited access to outside users like suppliers, partners or customers, usually through a secure login.