Units of Measurement

• Units
  • As we discussed before our basic unit of measurement for storage is a bit.
    • bytes
      • But like a penny, this is very small.
      • So we group things in terms of bytes or 8 bits.
    • For network speeds, we tend to measure in bits.
      • Or bits per second.
    • For disk and other storage we measure in terms of bytes.
    A second measurement is a hertz or Hz
    • This is a measurement of events per second.
    • Remember, we talked about a pulse to drive the computer.
    • This is measured in Hz.
  • Unfortunately, the things we measure are either quite large so we need a way to extend these.
    • We use the si metric prefix system.
      • kilo (k), 1,000 units or $10^3$
      • mega (M), 1,000,000 units or $10^6$
      • giga (G), 1,000,000,000 units or $10^9$
      • tera (T), $10^{12}$ units
      • peta (P), $10^{15}$ units
      • exa (E), $10^{18}$ units
    • So a gigahertz (GH) is $10^9$ hertz or 1 million cycles per second.
      • Today clocks are in the 1-4 GHz rating
      • Or can do up to 4 million instructions per second.
    • I just purchased a 1TB drive
      • In theory this should be 1 billion bytes.
      • But we measure memory in blocks of 1024
      • So this is really $1024^4$ bytes
      • 1,099,511,627,776 bytes
      • This is officially a tebibyte
    • My home network is a 100 gigabit (Gb) network.
      • The network can transport 100 million bits per second.
      • Note the lower case b.
  • Sometimes we want to talk about very small units.
    • Mostly fractions of a second.
    • If a clock beats at 1KHz,
      • Each beat takes 1/1000 of a second.
      • This is one millisecond (ms).
      • And a very old computer.
      • Or $10^{-3}$ seconds.
    • Other units
      • A microsecond (μs) is $10^{-6}$ and is conventional hard drive speed.
      • A nanosecond (ns) is $10^{-9}$ and is memory speed.
      • A picosecond (ps) is $10^{-12}$ and is processor.