Packet Transport Mechanisms

Each station connects to the ether with a transceiver. 'The design of the transceiver must be an exercise in paranoia'. In particular, failures of the transceiver must not pollute the ether, power failure must not 'cloud' the ether and disconnection must not be noticed by other stations.

802.3 uses five significant mechanisms to reduce the probability and cost of losing a packet.

• carrier detection.
  Ethernet uses the carrier sense mechanism of phase encoding which guarantees that there is at least one phase transition on the ether during each bit time.

  The Aloha scheme does not use carrier detection and subsequently suffers a higher collision frequency.

  (Use of deference and acquisition).

• packet error detection (initially 2-byte CCITT-16, now 4-byte CRC-32).