Data Transmission & Error Detection

Data Transmission

Data transmission is the movement of data (bits) between at least two digital devices. It can be over a short distance (e.g. from your computer to a peripheral) or over longer distances (e.g. networks & the internet).

Factors to be considered and agreed between sender and receiver:

Media of transmission (physically how bits travel between the devices)
Method of transmission (how many bits are sent at the same time) – Serial or Parallel
Direction of transmission (i.e. one direction or both) – Simplex, Half Duplex or Full Duplex
Synchronisation method between the two devices – Asynchronous and Synchronous
Error Detection what methods of detecting and correcting errors will be used

Before we can talk about anything else, we need to decide how to connect the digital devices. All three main types of connections (wires, light, radio) use electromagnetic signals.

Intro: Code.org: The Internet: Wires, Cables & Wifi

Wires (often copper): Signal is transmitted by changing the voltages in the wire & travels at the speed of light in that medium. Examples include all sorts of cabling, but commonly
- Coaxial & Twisted Pair are configurations that reduce interference and loss, commonly used for aerial cable and phone cables respectively
- Ethernet usually uses CAT4-CAT6 - fast communication (twisted pair inside)
Light (Optical Communication): Signal is encoded in light intensity. Examples:
- Optical Cable - rays of light are bounced down a glass thread using total internal reflection - the internet backbone is based on this technology
- Free Space Communication a laser through space or air, e.g., potential inter-satellite communication
- Li-Fi: Similar to Wi-Fi but uses light for higher bandwidth but usually a smaller range
Radio (also in the Microwave range)
- Wi-Fi - Technology mainly used for Local Area Networks. Works un the SHF to UHF radio bands to microwave.
- Bluetooth - short-range radio technology for linking peripherals
- Mobile networks: from GSM to 5G
- Satellite communication (radio dish)

Serial vs Parallel

Serial transmits data down a single wire one bit at a time. Parallel transmits data multiple bits at a time. (This is separate from whether it is uni- or bi-directional)

Serial: Traditionally used for external mice and keyboards. Lots of different plugs now standardised in Universal Serial Bus. faster and the hardware is cheaper, so the standardized USB is everywhere!

Parallel: Traditionally used for external printers and scanners. Still used inside computers as data and address busses.

Advantages of parallel:

Can send many bits at once --> potentially faster
Can directly tie into a computer bus --> less hardware required to (de)serialize

Disadvantages of parallel:

Data skewing can lead to errors and limit speed and distance
Not universal

Parallel has fallen out of favour and now almost everything talks using a USB standard.

What was used before USB? Serial and Parallel ports

USB 3.2 Is not really Serial... & is annoyingly named!

Intel advert featuring USB co-inventor

Simplex vs Half Duplex vs Duplex

Simplex is like a single-lane road where data can only flow in one direction. It is also like a AM/FM radio station - people can listen but not talk back.

Half-Duplex is like a road with road-works on one side and a someone letting cars go down the other side in one direction then the other. It is also the way a walkie-talkie works - you can only transmit or listen at any one time.

(Full)-Duplex is like a two-lane road - data can flow in both directions simultaneously. This is like a telephone call where you can talk and listen at the same time (technically speaking)

Synchronisation

Synchronous vs Asynchronous

Error Detection

parity checks, check digits, checksums and Automatic Repeat reQuests

Extension: Hash, CRC, etc...

Exam Questions

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