G4G0-1/Semester 2/Computer Systems Internals & Linux/Week 1/Week 2 Computer Systems Internals.md

Clock Drift

Recalibrate clocks when out of sync:

• Async Transmission
  • Send a small amount of data, then 1, then 0, then some more data.
• Manchester Encoding
  • Send each bit as 2 values
  • Original 10MBase-T uses this
• HDB3 encoding
  • 3 logic levels to ensure no static sequences
  • Used for long distance cables
• 4B5B encoding
  • Transmit 5 bits of signalling to represent 4 bits of data
  • 100MBase-T uses this.

Quantities in Digital Communication

Bit

• Short for "binary digit". Byte = 8 bits

Data Rate

• Bits per second

Baud Rate

• Times per second the transmitted signal changes.

Examples

• A modem used for dialup will encode at least six bits of data for each unit of signalling.
• Ethernet uses between 1.2 and 2 units of signalling per bit of data.

Text Encoding Communication

• The word "Up" -> ASCII is 85, 112
• Binary stream -> 010101010110000 This demonstrates the worst case scenario for 4B5B, and 4 zeros to demonstrate HDB3.