Chapter 3 -- Physical Layer
  1. Signals
    1. Almost all signals are electromagnetic waves
    2. Voice/sound is almost never used.
    3. Light and microwaves are type of electromagnetic wave, but very different electrical.
    4. Signal travels at the speed of light in the material, which is 200,000km/second for copper or optic cable.
  2. Electrical Cable
    1. Is cheap and easy and won't kill or blind.
    2. distortions occur because
      1. Wires have impedence (makes sharp edges look rounded).
        1. slow down  or use good receiveres are only solutions
        2. Sine waves get thru better than square waves!
      2. Wired act like antenas (pick up crosstalk).
        1. Why radios in cars can whine.
        2. Shielded coax very good at stopping this.
        3. twisted pair not bad at stopping this.
      3. Signals can be reflected of end and come back
        1. Proper termination can fix this.
        2. Just a resistor (get value from EE, or just ask cable maker)
    3. Coding can help
      1. Use square waves only short slow communication over short wires
      2. Use frequency shift keying, amplitude shift keying, or phase shift keying
  3. Receiveing
    1. Getting clocking right.
      1. Using short bit sequences
        1. All bit sequences are N bits long, start with a '1', and end with a long '0'.
        2. Has lots of idle time
        3. Lets clocks drift and still is O.K.
      2. Manchester Encoding
        1. A '0' is indicated by a downward voltage sitft in the middle of the bit
        2. A '1' is indicated by an upward voltage shift in the middle of the bit.
        3. At the begining of each bit, the voltage is set so that there IS a shift in the middle.
        4. Easy to keep track of the clock, cannot screw it up.
        5. cannot even get 1/2 off, since sometimes there is no transmission at the begining of bits, but there is a transision at every bit-middle.
        6. Is self-synchonizing.