The Ethernet Protocol

• The set of rules that regulate access to the communication channel in the Ethernet is a refinement of the ALOHA protocol.
• The following modifications are introduced:
  • Time is not slotted, i.e., transmissions can commence at any time.
  • Before beginning to transmit, stations must listen if another station is already transmitting. If so, the transmission must be delayed until after the current transmission is complete.
  • While transmitting, the stations must listen to the channel and determine if a collision is occuring. If there is a collision they must cease to transmit immediately and attempt retransmission after a random delay.
• The official name for this protocol is CSMA/CD for Carrier Sensing Multiple Access with Collision Detection.
• The second rule above describes carrier sensing.
• Before transmitting, a station must listen to (sense) the channel for the presence of a signal (carrier).
• CSMA is sometimes called a ``polite'' form of ALOHA.
• Collision detection is employed to abort unsuccessful, collided transmissions as quickly as possible.
• The CSMA/CD protocol is very good at carrying bursty data traffic with very low delays.
• Question: How can collisions occur in the first place when carrier sensing is employed? Doesn't CSMA avoid collisions altogether?