Networks & the Internet

• A network topology is the way a network is arranged and how the devices within it are connected
• A bus topology is where all devices are connected to a single main cable
• Bus topologies are cheap but have poor security, struggle under heavy traffic, and completely break if the main cable fails
• In a star topology all devices are connected to a central device
• Star topologies can be expensive to set up, and if the central node goes down, the entire network fails
• In star topologies, single wires can fail and the rest of the network will be fine. It is easy to add more devices, and there are no data collisions either
• A physical topology describes how devices are connected in physical space, while a logical topology describes the method of communication between them. A physical star topology could be a logical bus topology by utilising a switch
• In a client-server model, each individual device (client) connects to a central device (server), where all data can be stored and requested from
• In a peer-to-peer model such as the Internet, all devices have equal status and each can act as a client or server - there is no central device
• Peer-to-peer networks have enabled mass piracy over the Internet
• WiFi is a standard that allows for wireless networking - literally, networks that don't need wires
• WiFi is very convenient and often cheaper and easier to set up
• WiFi speeds will always be slower than wired connections. It depends on signal strength, and the signal can be obstructed easily. It is also less secure
• To connect to WiFi a device needs a Wireless Network Interface Card
• A device connects to WiFi by communicating with a Wireless Access Point (WAP)
• WiFi requires a modem and router to communicate to the Internet
• The modem, router and WAP are usually combined in homes
• We identify wireless networks using a Service Set Identifier (SSID) which must be used by all devices connecting to that network
• The SSID can broadcast publicly or be hidden meaning you must know the name to connect
• SSIDs can be protected by passwords to prevent unauthorized access
• WiFi signals are transmitted on channels across different frequencies (ranging across about 22 MHz). Similar frequencies may interfere, so different channels with less interference may lead to faster speeds
• Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) is a protocol for communicating across wireless networks
• CSMA/CA checks if the channel is idle, and if it isn't, waits a random period of time and then tries again
• RTS/CTS (request to send, clear to send) is an addition to CSMA/CA where devices make a request before sending data. Then the channel can respond clear to send, and the data can be transmitted
• CSMA/CA and RTS/CTS solve the hidden node problem: two nodes can see a WAP but not each other, so cannot prevent collisions. Thus the WAP must organise transmission
• The Internet is a massive Wide Area Network composed of countless fibreoptic wires across the globe connecting billions of devices
• In the Internet, computers have routing tables that map web addresses to IPs
• A router (at least its original meaning) receives packets and forwards them using an algorithm to determine the best route
• A gateway is a router that can handle routing between devices with different protocols, by stripping the header off a packet and adding a new one with the correct format
• To transmit data across a network, data is broken up into smaller parts called packets, which are then 'wrapped up' with additional information needed to send them
• A packet contains its sequence number, source and destination IP and MAC addresses, a checksum, and the data itself
• Packets are routed across the internet factoring in the latency and bandwidth of each connection. Packets in the same sequence may take different routes
• A Uniform Resource Locator (URL) is a unique address for a web page on the internet comprising of multiple different parts
• A domain name, like youtube.com, is the unique name of a website. When combined with a host, for example www, it becomes a fully qualified domain name
• A method like http or https comes at the start of a URL and is used to specify the protocol for transmitting data
• Following the domain name in the URL is the location within that site, and sometimes the resource - a specific file that has been loaded
• Internet registries are 5 major organisations responsible for allocating IP addresses. They aim to assign previously allocated addresses rather than giving brand new addresses. They also ensure all domain names are unique
• Domain name organisation - domain names are structured in a hierarchy separated by dots. For example bbc or amazon might be at the bottom, co or gov might be in the middle, and uk or com might be at the top
• The domain name system (DNS) is the internationally agreed structure for translating domain names into IP addresses
• An Internet Protocol (IP) Address is a unique address given to a network device