Communication and networking

A network topology is the arrangement of computers and devices on a network. Two common topologies for Local Area Networks (LANs) are bus and star. A bus topology connects all devices to a single central cable, called the bus or backbone. Data sent by one device travels along the bus and is seen by all other devices. A star topology connects every device to a central hub or switch. If a device sends data, it goes to the central device which then forwards it to the destination. To build a LAN, you need specific hardware. A Network Interface Card (NIC) is installed in each computer to provide a physical connection to the network. Devices are connected using cables (like twisted-pair Ethernet cables) or wirelessly. A hub is a simple central device for a star network that broadcasts any incoming data to all connected devices. A switch is a more intelligent device that learns the addresses of connected devices and sends data only to the intended recipient, improving efficiency and reducing traffic.

There are two fundamental methods for sending data across a network: circuit switching and packet switching. In circuit switching, a dedicated, end-to-end communication path is established between two devices for the duration of the conversation. This is like booking a private road from your house to a friend's; no one else can use it while you are. The traditional telephone system is a classic example. In contrast, packet switching breaks a message down into smaller, manageable blocks called packets. Each packet is given a header containing the destination address and a sequence number. These packets are then sent independently across the network. They can take different routes to the destination and may arrive out of order. The receiving device uses the sequence numbers to reassemble the original message. The internet is built on packet switching, which is highly efficient as it allows multiple users to share the same network resources.

A protocol is a set of rules that governs how data is transmitted and received between devices. Without protocols, communication would be chaotic, like two people trying to talk without a shared language. To manage the complexity of network communication, protocols are organised into a 'stack' or 'suite' of layers. The most common model for the internet is the TCP/IP suite, which has four layers.

1. Application Layer: This is the top layer, where user-facing applications operate. Protocols here include HTTP (for web browsing) and SMTP (for email).
2. Transport Layer: This layer ensures data is delivered reliably and in the correct order. It breaks large messages into smaller segments. The two main protocols are TCP (Transmission Control Protocol), which is reliable and connection-oriented, and UDP (User Datagram Protocol), which is faster but less reliable.
3. Network Layer (or Internet Layer): This layer is responsible for addressing and routing packets across the network from the source to the destination. Its main protocol is IP (Internet Protocol), which uses IP addresses.
4. Link Layer (or Network Interface Layer): This is the lowest layer, responsible for the physical transmission of data over the network medium (e.g., Ethernet cable, Wi-Fi). It deals with physical addresses (MAC addresses).

The Application Layer of the TCP/IP stack contains the protocols that we interact with most directly. They provide the rules for specific internet services.

• HTTP (Hypertext Transfer Protocol): The foundation of the World Wide Web. When you enter a URL in your browser, it uses HTTP to send a request to the web server and receive the webpage files (HTML, CSS, images) in response.
• FTP (File Transfer Protocol): A protocol designed specifically for transferring files between computers over a network. It can be used to upload files to a server or download them from it. It often requires a username and password for authentication.
• SMTP (Simple Mail Transfer Protocol): The standard protocol for sending email. When you click 'send' on an email, your email client uses SMTP to push the message to your mail server, which then uses SMTP to forward it to the recipient's mail server.
• POP3 (Post Office Protocol version 3): A protocol used for retrieving email. When your email client checks for new mail, it can use POP3 to connect to the mail server and download messages to your computer. Once downloaded, the messages are typically deleted from the server.

Ethernet is the dominant technology for wired Local Area Networks (LANs), operating at the Link Layer of the TCP/IP stack. Every device that can connect to a network has a Network Interface Card (NIC) with a unique, built-in address called a MAC (Media Access Control) address. This is a 48-bit hexadecimal number that is permanent and globally unique, like a device's fingerprint. On a local network, switches use MAC addresses to forward data frames to the correct device. In older or simpler networks that used a shared medium (like a bus topology or a hub), a method was needed to manage how devices shared the cable without their transmissions colliding. This method is CSMA/CD (Carrier Sense Multiple Access with Collision Detection). The process is:

1. Carrier Sense: Listen to the network cable to check if it's busy. If it's clear, proceed. If not, wait.
2. Multiple Access: All devices on the network have permission to transmit.
3. Collision Detection: While transmitting, listen to see if another device started transmitting at the same time, causing a 'collision'. If a collision is detected, both devices stop transmitting, wait for a random, short period, and then go back to step 1.

Most internet activity uses a client-server model, where a central server holds data and multiple clients request it. Peer-to-Peer (P2P) networking offers an alternative. In a P2P network, each computer, or 'peer', can act as both a client (requesting data) and a server (providing data). The BitTorrent protocol is a popular example of P2P used for sharing large files. Here's how it works: A large file is broken into many small pieces. A central server, called a 'tracker', doesn't hold the file itself but keeps a list of which peers have which pieces. When you want to download the file, your BitTorrent client contacts the tracker to get a list of peers. Your client then downloads different pieces of the file from many different peers simultaneously. At the same time, your client uploads the pieces you already have to other peers who need them. This distributed approach is highly efficient for large files, as the download speed is not limited by a single server's bandwidth.

Wireless networking allows devices to connect and communicate using radio waves instead of physical cables. The most common technology for wireless LANs is Wi-Fi. A typical setup involves a Wireless Access Point (WAP), which broadcasts a radio signal. Devices like laptops and smartphones with wireless NICs can connect to the WAP, which in turn is usually connected to a wired network and the internet. Advantages: Mobility and convenience (no cables), easy to add new devices. Disadvantages: Generally slower and less reliable than wired connections, susceptible to interference from other devices, and poses greater security risks as the signal can be intercepted by anyone in range. To manage access, Wi-Fi uses CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance). Unlike wired Ethernet where collisions can be detected, it's difficult in radio. So, CSMA/CA tries to avoid collisions in the first place, often by having a device send a short 'request to send' signal before transmitting its data. Security is vital. WPA2 (Wi-Fi Protected Access 2) and the newer WPA3 are security protocols that encrypt the data sent over the air, preventing eavesdropping. Using a strong password is essential.