physical

physical media

pictures of various physical media: a cable carrying red light; an a cable with four twisted-pairs and a plastic plug; a box with several lenses sitting atop a pole; a cable with a thick metalwire surrounded by a plastic jacket surrounded by a metal jacket surrounded by a rubber jacket; a Netgear-branded Wifi router; and a final box with ellipses.

same picture with labels: fiber carrying light; bundle of wires; infrared through air; single wire; radio

transmitting a signal

  • can vary…

    • voltage
    • radio/light intensity
    • radio/light frequency
    • …

some simplifying assumptions

  • signal low/high — no in between
  • only one ‘channel’
    • won’t have multiple wires/antennas/frequencies/etc.
    • won’t modulate different things same time
  • want to send receive bits (0 or 1)

clocking

keeping a clock

self-synchronizing?

  • can resynchronize clock by looking for transitions
  • but doesn’t work if lots of consecutive symbols
  • also, need to know where low and high point is
  • important to have transitions to low/high to calibrate this

self-sync and ‘start-message’

  • one idea: set start-message to have lots of 0/1/0/1/0/1/0/1/etc.
    • something 10Mbit Ethernet does
  • but probably not enough to ensure things stay in sync on big message <hr class=“vspace/>
  • alternate strategy — use encoding that adds extra transitions
  • example: Manchester encoding for 1-bit

other better encoding options

  • vary more than just one thing
    • example: pulse amplitude and duration
  • use more than just low/high
    • so more than just 0/1
    • gives space for dedicated “start” symbol
  • probably covered in ECE Signals course?
  • used by almost all “fast” encodings

Backup slides

Manchester encoding

Figure 25 from Section 2.2 of Computer Networks: A Systems Approach (6th ed) (Peterson and Davie)

problem with Manchester

  • fixed the problem of too few transitions .5cm

  • but now most transitions don’t send information

  • means we aren’t making good use of wire/etc. capacity .5cm

  • there are more clever compromises

    • (example: 4B5B encoding)