Computer Networks and Communication

A computer network is a collection of interconnected computing devices that can exchange data and share resources. Networks are classified by their geographical scope, such as a Local Area Network (LAN) within a single building, or a Wide Area Network (WAN) that spans cities or countries, like the Internet.

### Data Transmission Methods

Data is transmitted between devices as a sequence of bits. The two fundamental methods are:

* Serial Data Transmission: Bits are sent one after another over a single wire or channel. While slower than parallel, it is reliable over long distances and less prone to interference. Examples include USB and Ethernet.

* Parallel Data Transmission: Multiple bits are sent simultaneously over several parallel wires or channels. This is much faster for short distances but is susceptible to data skew (bits arriving out of sync) over longer distances. It was historically used for printers and is used internally within computer components like the system bus.

### Network Architecture

This defines how devices, known as nodes, are organized and how tasks are allocated.

* Client-Server Model: This is a centralized model where a powerful computer, the server, provides services and resources (e.g., files, web pages, database access) to other computers, known as clients. Clients request services, and the server responds. This model allows for centralized management, security, and backup, but the server can become a bottleneck or a single point of failure.

* Peer-to-Peer (P2P) Model: In this decentralized model, every node is an equal, or a peer. Each peer can act as both a client and a server, sharing its resources directly with other peers without a central server. This model is resilient and scalable but can be difficult to manage and secure. BitTorrent is a common example.

### Network Topologies

Topology refers to the physical or logical arrangement of devices in a network.

* Bus Topology: All devices are connected to a single central cable, called the backbone. Data sent by one device is broadcast to all others. It is inexpensive to set up but a break in the backbone cable disables the entire network, and data collisions can occur if multiple devices transmit simultaneously.

* Star Topology: All devices are connected to a central device, such as a hub or a switch. This is the most common topology in modern LANs. It is easy to add/remove devices and a single cable failure only affects one device. However, if the central device fails, the entire network goes down.

* Mesh Topology: Every device is connected to every other device (full mesh) or to several other devices (partial mesh). This provides high redundancy and reliability, as data can be rerouted if a path fails. However, it is expensive and complex to implement due to the extensive cabling required.

### Communication Protocols and the TCP/IP Suite

A protocol is a set of rules that governs data communication. The TCP/IP protocol suite is the foundational set of protocols for the Internet, structured in four layers:

* TCP (Transmission Control Protocol): A connection-oriented protocol that ensures reliable, ordered delivery of data. It breaks data into segments, numbers them, and reassembles them at the destination, re-transmitting any lost segments.

* UDP (User Datagram Protocol): A connectionless protocol that offers fast, low-overhead data delivery but does not guarantee it. It is used for time-sensitive applications like video streaming or online gaming where speed is more critical than perfect accuracy.

### IP Addressing and DNS

An IP address is a unique numerical label assigned to each device on a network. IPv4 uses a 32-bit address (e.g., `172.16.254.1`), while IPv6 uses a 128-bit address to accommodate the growing number of internet-connected devices. A subnet mask (e.g., `255.255.255.0`) is used to divide the IP address into a network portion and a host portion. Since remembering IP addresses is difficult, the Domain Name System (DNS) acts as the internet's phonebook, translating human-friendly domain names (e.g., `www.cambridgeinternational.org`) into machine-readable IP addresses.

### Web Technologies

Web technologies operate within the client-server model over the internet:

* HTML (Hypertext Markup Language): The standard language for creating web pages. It defines the structure and content, using tags to create elements like headings, paragraphs, and images.

* CSS (Cascading Style Sheets): A language used to describe the presentation and styling of a document written in HTML. It controls layout, colours, fonts, and an overall visual appearance.

* JavaScript (JS): A scripting language that enables interactivity and dynamic behaviour on web pages, such as validating forms, creating animations, and fetching data without reloading the page. It runs on the client's browser.