Chapter 05 -- Local Area Networks

  1. Local Area nets are (normall cheap) networks
    1. that connect computers over a short distance.
    2. That all transmit on a common medium (as opposed to point-to-point).
      1. Always have problem of sharing transmission medium.
    3. Various examples include Ethernet, token ring, token bus, appletalk.
  2. ALOHA
    1. Build by UofHawaii in early 70s.
    2. They used transmission over radio waves at 9,600 bps
    3. Central node acks all correct packets and forwards them on a different frequency.
    4. If two stations stransmit at same time, packets become garbeled.  They are said to collide.
    5. Central station cannot ack garbeled packets.  Senders timeout and retransmit after random delay.
      1. Why RANDOM delay????
    6. Three types of ALOHA, slotted and oure.
      1. Pure can transmit at any time.
        1. Effeciency is 18%.
      2. Slotted can transmit only on slot boundries.
        1. Effeciency is 36%.
        2. Must have good clock synchonization.
      3. Spade (like aloha with reservations)
      4. Calculations Assume there are MANY nodes transmitting at random intervals.
        1. Fewer nodes improve effeciency.
        2. "Dialogs" improve effeciency.