3. Data transmission and networking

Data is broken into small units called packets before it is transmitted.

• If one packet is lost or corrupted, only that packet needs to be sent again, not the whole file.
• Packets can take different routes, so traffic avoids busy or broken links.
• Many users can share the same network links at the same time.

1. The data is split into packets, each given a header with the destination address and a packet number.
2. Each packet is sent separately; packets can take different routes across the network.
3. At each router, the router reads the destination IP address and makes a forwarding decision, sending the packet on along the best available route at that moment.
4. Packets may arrive out of order (or some may be delayed or lost).
5. At the destination the packets are reassembled into the correct order using the packet numbers. Missing or damaged packets are requested again.

Suitability: simplex is the simplest and cheapest but nothing can be sent back (no acknowledgement). Half-duplex lets replies be sent but devices must take turns, so it is slower than full duplex. Full duplex is fastest for two-way communication but needs more complex (more expensive) equipment.

Direction and bit-arrangement combine: e.g. a network cable is serial full duplex.

A NIC is the hardware that allows a device to connect to a network (wired or wireless).

• It converts the computer's data into signals that can travel across the network, and back.
• Each NIC has a unique identifier: its MAC address, set when it is made.

A media access control (MAC) address uniquely identifies a device's NIC on a network.

• Format: 48 bits, written as 12 hexadecimal digits in six pairs, e.g. 00:1A:2B:3C:4D:5E (or with hyphens).
• The first half identifies the manufacturer; the second half is the serial number of the device/NIC.
• Purpose: used to make sure data on a local network reaches the correct device.
• Normally fixed (it does not change when the device moves to another network).

A switch connects devices on a local network. It stores the MAC addresses of the devices connected to each of its ports in a table, so when data arrives it can be sent only to the device it is meant for, rather than to every device.

An internet protocol (IP) address uniquely identifies a network, and a device on that network, on the internet. It is used to send data to the right place — every packet carries the source and destination IP addresses.

Unlike a MAC address, an IP address can change, e.g. when a device joins a different network.

Each IPv4 number is one byte (8 bits), so it can only be 0–255. Each IPv6 group is 16 bits (four hex digits). IPv6 was introduced because there are not enough IPv4 addresses for every device; leading zeros in an IPv6 group may be left out.

A router connects networks together, e.g. a home network to the internet.

• It receives packets and reads the destination IP address.
• It uses a routing table to decide where to forward each packet — to a device on its own network or on to the next router.
• It assigns (dynamic) IP addresses to devices that join its local network.
• It connects the local network to the internet through one public IP address.

A uniform resource locator (URL) is a text-based web address that people can remember, used to locate a web page or resource.

Structure of https://www.example.com/learn/index.html:

HTTP (hypertext transfer protocol) is the set of rules for requesting and delivering web resources: the browser sends an HTTP request to the web server, and the server sends an HTTP response containing the page, image or file (or an error code such as 404).

HTTPS is the secure version of HTTP. The data is encrypted (using TLS, see §4.2) before it is sent, so if it is intercepted it cannot be understood. The server's identity is checked with a digital certificate. Browsers show a padlock. Use it for log-ins, payments and personal data.

1. The user types a URL into the web browser (or clicks a link).
2. The browser sends the domain name to a DNS server.
3. The DNS server looks up the domain name and returns the matching IP address of the web server (if it does not know it, it asks another DNS server).
4. The browser uses that IP address to send an HTTP(S) request to the web server.
5. The web server finds the page and sends back its HTML file (and other files: images, CSS, scripts).
6. The browser renders (interprets) the HTML and displays the web page.

The role of DNS: it translates human-readable domain names into IP addresses, so browsers can find the web server and make HTTP requests — people remember names, but the network needs numbers.

Cookies are small text files containing pieces of data that a website asks the web browser to store on the user's device.

Cloud storage and services are provided over the internet from remote servers in data centres, owned by a cloud provider. Users access them from anywhere with an internet connection, on any device.

Features: data held on many servers (often copied to several locations); scalable — buy more space or power when needed; usually pay as you go or subscription; the provider manages maintenance, security and backups.

Cloud services include file storage, online office software, email, streaming, and renting computing power.

Local storage (on the device or your own servers) is fast and works offline, and you control it — but you must buy, back up and maintain it yourself.