If networks spread viruses, why does every school connect its PCs?

Why connect computers?

• A computer network is two or more computers (or other devices) connected so they can share data and resources.
• Joining them brings gains, and it brings costs and risks:

Types of network

• A LAN (local area network) covers a small area, such as one school, and is usually owned and run by one organisation.
• A WAN (wide area network) covers a wide geographical area and links LANs. It is often under collective or shared ownership.
• The internet is the biggest WAN.
• A PAN (personal area network) links one person's devices over a few metres, e.g. phone and earbuds, using Bluetooth.

Client-server and peer-to-peer

• On a client-server network, servers provide services and clients request and use them.
• On a peer-to-peer network there is no server: every computer is equal and shares its own files.

Wired or wireless?

• Wired connections use cables; wireless connections use radio waves.
• The main wired standard is Ethernet; common wireless ones are Wi-Fi and Bluetooth.

Measuring network performance

• Bandwidth is the most data a connection can carry each second. Speeds are measured in bits per second (bps), not bytes.
• Speeds are given in Mbps (million bits per second) or Gbps (billion bits per second).
• Time (s) = file size in bits ÷ rate in bits per second. 1 MiB = 1,024 × 1,024 × 8 bits.

Mobile generations

• Mobile phones connect to the internet through a mobile network of cell masts. Its rules are set by mobile communication standards, released as generations.
• 3G made mobile internet browsing, email and video calls practical.
• 4G is much faster, carrying all data, including voice, as internet traffic; streaming HD video became normal.
• 5G gives higher speeds, lower latency and room for far more connected devices, such as sensors and self-driving vehicles.

One box fails and a whole office goes offline. How?

The star topology

• A topology is the layout of how the devices on a network are connected.
• In a star topology, every device has its own cable to one central device, usually a switch. All data passes through the centre, which sends it on to the right device.

The mesh topology

• In a mesh topology, devices connect directly to several others, so there are many paths between them.
• In a full mesh every device connects to every other one. With 4 devices that is 6 links.
• If a link fails, data is sent along another path: the network self-heals.

The bus topology

• In a bus topology, every device connects to one shared main cable, the backbone, with a terminator at each end.
• Data sent by one device travels along the whole backbone; only the device it is addressed to keeps it.
• It is cheap and needs little cable, but if the backbone breaks the whole network fails, and it slows down as devices are added because of data collisions.

The ring topology

• In a ring topology, each device connects to exactly two others, forming a closed loop.
• Data travels round the ring in one direction, passing through each device until it reaches its destination.
• There are no collisions, so it copes well with heavy traffic. But one broken link or device can stop the whole ring, and adding a device means breaking the loop.

How do two computers that have never met agree how to talk?

Protocols

• A protocol is a set of rules for transferring data between devices.
• It sets things like how data is formatted, how devices are addressed and how errors are spotted.
• Both devices must follow the same protocol, whoever made them. Different jobs use different protocols.

Protocols for the web and email

• HTTP: a browser requests web pages from a web server, which sends them.
• HTTPS: HTTP with encryption, so intercepted data can't be read.
• FTP: transfers files between computers.
• SMTP: sends email from an app to a mail server, and between mail servers.
• IMAP: reads email kept on the server, so every device shows the same inbox.
• POP3: downloads email to one device, usually deleting it from the server.

Protocols that move the data

• TCP splits data into numbered packets, checks they all arrive, has missing ones resent and puts them back in order.
• IP gives each packet its source and destination IP addresses and routes it across networks.
• TCP and IP work together, so they are often named as one: TCP/IP.
• Ethernet is the set of rules for wired LANs; Wi-Fi (802.11) for wireless LANs.

Why use layers?

• Sending data is split into layers. Each layer does one part of the job and each protocol belongs to one layer.
• Sent data passes down the layers, each adding its own information. Received data passes back up.
• Benefits: each layer is a smaller problem; one layer can change without changing the others (swap a cable for Wi-Fi and the browser still works); makers can build for one layer.

The 4-layer TCP/IP model

How does a video arrive in order if its pieces take different routes?

Splitting data into packets

• Before data is sent across a network, it is broken into small pieces called packets.
• Each packet is sent on its own and the receiving computer joins them back together.
• Small packets share the network's links fairly: no single huge file blocks a link. If one packet is lost or damaged, only that packet needs to be sent again.

Inside a packet

• The header holds the destination address, the source address (where it came from) and the packet number, used to put the data back in order.
• The payload is the actual data being carried.

Packet switching

• In packet switching, each packet is sent across the network separately.
• Routers read each packet's destination address and send it on towards its destination, one hop at a time.
• Each packet may take a different route, depending on which links are free or working.
• So packets can arrive out of order. Once the last packet arrives, the receiver uses the packet numbers to reorder them.

Routers only use numbers. How does a typed web address find its site?

Reading a web address

• The internet is a worldwide collection of computer networks: the infrastructure.
• The world wide web is the collection of websites and web pages you reach using the internet.
• A URL (uniform resource locator) is the text address of a web page.
• Its parts: the protocol (https), the domain name (www.example.org) and the web page / file name (/physics/waves.html).

From name to number

• Every device on the internet is found by its IP address, e.g. 203.0.113.25. Names are easier to remember.
• The DNS (domain name service) matches domain names to IP addresses.
• The browser sends the domain name to a DNS server, which returns the IP address.
• If that server doesn't hold the name, it asks another DNS server. If none does, an error is returned.

Fetching the page

• A web server stores web pages and sends them to clients, such as your browser, that request them.
• The browser sends an HTTP request to the server's IP address. The server replies with the page's HTML, which the browser renders into the page you see.
• HTTPS is the secure version: data is encrypted, so intercepted data can't be read.
• An ISP (internet service provider) connects users to the internet.

How does your data find one computer among billions online?

IP addresses

• An IP address identifies a device on a network, so data can be sent to it. Every packet carries a destination IP address.
• An IPv4 address is 32 bits, written as four denary numbers from 0 to 255 separated by dots, e.g. 192.168.0.12. Each number is one byte (8 bits).

Routers

• A router connects networks together, for example a home or school LAN to the internet.
• The internet is a vast number of networks joined by routers.
• A router reads each packet's destination IP address and sends it to the next router on a route towards it. Hop by hop, the packet reaches the right network.

IPv6

• IPv4 gives about 4.3 billion addresses (2³²): too few for every phone, laptop and smart device.
• IPv6 was created to give far more. It is 128 bits, written as eight groups of four hexadecimal digits separated by colons, e.g. 2001:0db8:85a3:0000:0000:8a2e:0370:7334.
• IP addressing standards mean every device in the world uses the same formats, so any device can address any other.

Devices that build a LAN

• A switch connects the devices on a LAN. It reads where each packet is going and sends it only to that device.
• A wireless access point (WAP) lets wireless devices join a wired network.
• A modem converts the computer's digital data into signals that travel along the telephone or cable line to the internet, and back. A home "router" box usually combines a modem, router, switch and WAP.

Big ideas

• A network connects devices so they can share data and resources. It brings gains (sharing, central backup and management) and costs (hardware, maintenance, security risks).
• A LAN covers a small area and is owned by one organisation; a WAN covers a wide area and links LANs. The internet is the biggest WAN.
• Client-server networks centralise files and security on servers; peer-to-peer networks share them between equal computers.
• Speeds are measured in bits per second: time = file size in bits ÷ rate in bits per second.
• A star links every device to a central switch: a cut cable affects one device, but if the switch fails the whole network fails.
• A mesh gives devices many paths, so it self-heals, but it needs a lot of cable and is complex to set up.
• A bus shares one backbone and a ring passes data round a loop: both are cheap, but one break can stop the whole network.
• A protocol is a set of rules for transferring data. Both devices must follow the same one, and different jobs use different protocols.
• In the 4-layer TCP/IP model, HTTP, HTTPS and SMTP work at the application layer, TCP at the transport layer and IP at the network layer.
• Data is split into packets; each has a header (destination address, source address, packet number) and a payload.
• In packet switching, routers send each packet separately, so packets may take different routes and arrive out of order; the receiver reorders them using the packet numbers.
• A URL holds a domain name; a DNS server converts it into the web server's IP address.
• The browser requests the page with HTTP or HTTPS (encrypted) and renders the HTML that comes back.
• An IP address identifies a device on a network. Routers read each packet's destination IP address and forward it hop by hop.
• IPv4 is 32 bits (four numbers, 0–255); IPv6 is 128 bits in hex, giving far more addresses.
• Switches connect devices on a LAN, a WAP adds wireless devices, and a modem links the network to the internet line.