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Computer Science SLR4 Revision - Coggle Diagram
Computer Science SLR4 Revision
The Internet
IP Addressing
Every internet-connected device is assigned a unique IP address so they can allow data packets to be routed to them (IP stands for Internet Protocol)
Most devices use a dynamic ip address
Dynamic IP Addresses
IP addresses that change every time a device connects or disconnects from a network
Static IP Addresses
IP addresses that never change
Routers and Web-servers have these
Domain Names
A human friendly IP address that uses words rather than numbers (e.g. gmail.com)
What Happens When a Web Page is Requested
When a URL is sent to the DNS via a web broswer, it asks for the corresponding IP Address in return in which the DNS delivers
Once it has received the IP Address, the Brower sends a HTTP page request to the web server - which holds the page data
Once recieved the browser renders
the page and displays it on screen
Packet Switching
A large file is broken up into data packets
Packets are routed across the network by internet protocol. Routers decide what the most efficient path is for each packet
Packets will need to take many different routes across a network and may not arrive in the correct order
The receiving computer reorganises the data by using information from packet headers
Data Packets
Header
Contains: IP Addresses of destination and source, Sequence ID, total number of packets + checksum
Payload
Part of the data for a web page, email, streamed movie, etc
Footer
End of packet flag
Receiving computers...
check that each of the packets is complete and uncorrupted
checks that all the data packets that were sent were received
if necessary, ask the source computer to resend missing or corrupted data packets
reassembles the packets in the correct order using the sequence ID
Routers
form connections over 2 or more networks and forward data packets across many different connections
Routing Table
when a router receives an incoming packet it finds the packet's destination address
routers use this to decide the most efficient journey the data packet can take
Keep each other informed of traffic conditions on their part of the internet
if route is congested, routers will send packets a different way
data packets from the same transmission may take different routes and may arrive at their destination out of order
Connectivity on a LAN
Installation
Difficult for wired
Every device needs its own dedicated cable
Easy for wireless
A WLAN does not take long to install and it's easy to add new devices. Only WAPs need to be connected by cable
Flexibility
Limited for wired
The number and location of cable connections is fixed. Making changes once the LAN is up is likely disruptive and time-consuming
Easy for wireless
Devices can be moved around without losing the connection, granted they don't move out of the connection range. New users can easily be added by just knowing the SSID and password
Range
Long for wired
Up to 100m. Range may be further enhanced if a signal booster is added
Short for wireless
Walls and objects may obstruct the signal
Bandwidth
High for wired
Up to 10 gigabits per second (Gbps) per connection
Low for wireless
Up to 3.2Gbps. All the active devices on the network must share the bandwidth
Latency
Low for wired
The cables have protected coverings making them less susceptible to interference
High for wireless
Susceptible to interference from other devices
Security
Good for wired
Impossible to tap into physical cables without being in the same location as the cable
Bad for wireless
Anyone within range can intercept transmissions
Routers assign an internal IP address to each device irrespective of its connection method - which enables them all to be apart of the same network
Wired versus Wireless
Wired
Makes physical connections to all computers in a network
Copper Wire
made of thin strands of copper and transmits data as electrical impulses
up to 100m in range
Up to 10Gbps in bandwidth
susceptible to electrical interference
used to connect devices in LANs
Fibre Optic
made of thin strands of glass and transmits data as pulses of light
Up to 80km in range
Up to 100Gbps in bandwidth
immune to electrical interference
used for long distance data traffic
expensive
difficult to handle
Wireless
uses radio waves to transmit data through the air
Wi-Fi
Up to 100m but physical objects can obstruct the signal
High power consumption
Used to network devices to LANs and connect devices to the internet
Bluetooth
Up to 10m in range
Low power consumption
Used to pair devices over short distances (e.g. headset to a phone
RFID
Up to 1m in range
Low power consumption
Used for security tags, passports and implants
NFC
Used in close proximity (around 10cm)
Very low power consumption
Used for contactless payment systems
Zigbee
Up to 100m in range
Low power consumption
Home automation and IoT (e.g. smart light bulbs)
Network Speeds
speed of a network is the data transfer rate - which is the number of bits that cam be transferred over a network in a given period of time
Measured in bits per second (bps)
The formula for data transfer (bps) is:
File size (bits) ÷ time (seconds)
Time (s) = File size (bits) ÷ Data transfer rate (bps)
File size (bits) = Data Transfer Rate (bps) x Time (s)
Network Protocols
Protocols
defines the rules and governs how data is formatted, transmitted, and received on a network
contains data formats so data is exchanged consistently and correctly
contains address formats to identify senders and recipients and to ensure data goes in the right places
contains routing to provide the right information so that data can flow through networks correctly
Protocol Layers
Protocols work in layers
Protocols in the top layer are the ones users can see
Protocols in the bottom layers handle the technicalities of converting binary into electrical, light or radio signals so they can be transmitted across a network
Application Layer Protocols
File Transmission Protocol
Provides the rules for file transfer between computers
Often used for files that are too large for attachment to email
Hypertext Transfer Protocol
Provides the rules to be followed by a web browser and web server when requesting and supplying information
It's used to send requests from a web client (browser), to a web server and returning web content from the server back to the client
HTTPS (Secure)
Ensures communications between a client and a server are secure by encrypting communications
Simple Mail Transfer Protocol
Provides the rules for sending email messages from client to server then server to server until it reaches its destination
Post Office Protocol (version 3)
Used by a client to retrieve emails from a mail server
All of the messages are downloaded when there is a connection between client and the server
Messages are deleted from the email server once they have been downloaded
Internet Message Access Protocol
does not download messages like POP
Emails can be read and stored on the server
Better for users with multiple different devices as they can be read from all devices rather than just being downloaded to one
The TCP/IP Model
A hierarchical stack of protocols that work together to enable devices to communicate with each other over the internet
Layers within the stack don't know how layers above and below them function only how to pass data to them
Application Layer
Sending: Provides the interfaces and protocols needed by the user
Receiving: Displays received info to the user
Protocols: FTP, HTTP, HTTPS, SMTP, POP3, IMAP
Transport Layer
Sending: Splits outgoing data into packets and numbers them. Adds a header containing a sequence ID and checksum to each packet
Receiving: Checks incoming packets and sends a resend request for any damaged or lost packets. Notifies sender when all packets have arrived. Reassembles data packets and passes data to the appropriate protocol in the application layer
Protocol: TCP
Internet Layer
Sending: Adds the source and destination IP Addresses to packet headers - which enables routers to guide each packet to its destination
Receiving: strips address information from incoming packet headers
Protocol: IP
Link Layer
Sending: Uses network-specific protocols to convert binary data into electrical, light or radio signals for network transmission
Receiving: Converts incoming signals into binary data
Protocols: Ethernet, Wi-Fi
Networks
Arrangement of 2 or more computing devices connected together in order to communicate with each other and share resources
Reasons for Connecting Computers on a Network
share data and software
share printers, hard drives and other hardware peripherals
share internet connections and services such as the web and web-based software
provide centralised support and backup services
enable rapid deployment of new software and updates
enable people to communicate with each other using services such as email and video conferencing
support collaborative working
Local Area Networks
Covers a small geographical area (e.g. home, school, university, etc)
Managed by a local manager or team at the site
Allows members of the home to access the internet using a wireless router
Wide Area Networks
Connect separate LANs together over a large geographical area to form a network of networks
Large companies can connect LANs at their different sites in order to share resources and data
Computers in a WAN can communicate with computers and users in other locations
Managed by several different people or parts of an organisation working together (collective ownership)
Alternatively each LAN could be managed independently (distributed ownership)
Network Topologies
How devices are arranged and connected to each other within a network
Star
Each computer/client is connected individually to a central point/node which can be a hub or a switch
Data is sent only to the intended computer
Network traffic is kept to a minimum
If one link falls the other devices will still be able to operate
Easy to add new devices w/o disrupting the network
If the central node falls then so will the entire network
Requires lots of cabling since each computer has to connect to the central node
Mesh
Each computer in the network is at least connected to one other computer
Commonly connected on devices where demand is high
Data can be transmitted from different devices simultaneously
If one component falls, there is always an alternate route for data
Can handle high volumes of data traffic
Adding more devices will not slow data transmission
Overall cost is high. More cable is required unless a wireless network is used
Difficult to manage and requires expert supervision
Bus
Each node is connected to a cable called a 'bus'
Easy to set up
Relatively cheap to install because only one cable is needed
Easy to add extra devices
Lots of data collisions occur when multiple devices transmit data at the same time. Devices detect collisions and resend data - which slows down the network
If the main cable fails or gets damaged the whole network will fail
The whole network will fail if a terminator is removed
