Physical: Basis for data communication

information can be transmitted by varying voltage on a wire

a link has properties:

• bit rate: bits per second, depends on channel’s bandwidth
• delay: how long it takes a bit to get to the end of the link
• error rate: probability of bit flipping
• bandwidth: analog (cutoff frequency in Hz) or digital (max data rate in bits per sec)

there are some limits to how data can be transmitted:

• bandwidth-delay product: bits have a physical size on the channel
  • a channel can store (bit rate × delay) bits
• signal has a finite propagation speed, noise increases, and the signal is attenuated by km
• lower bandwidth limits the harmonics that can be transmitted (and decreases quality, as seen in Fourier analysis)
• a channel has a maximum data rate
  • Nyquist (perfect noiseless channel):
    • 2B log2V bits/sec
    • B bandwidth
    • V number of signal levels)
  • Shannon (more realistic):
    • B log2(1 + S/N) bits/sec
    • B bandwidth
    • S/N signal strength to noise ratio