Contention-Based Protocols

Overview

• Contention-based protocols operate on a first-come, first-served principle.
• They manage how workstations compete for access to the LAN medium.

Carrier Sense Multiple Access with Collision Detection (CSMA/CD)

CSMA/CD Flowchart

• CSMA/CD is the most common example of a contention-based protocol.
• Operation:
  • A workstation can transmit data if the medium is clear (i.e., no other workstation is transmitting).
  • If the medium is busy, the workstation backs off and waits.

Collision Handling

• Collision: Occurs if two workstations transmit simultaneously.
• When a collision is detected:
  • Both workstations stop transmitting immediately.
  • Each workstation then backs off for a random amount of time before attempting to retransmit.
• The random backoff helps to prevent repeated collisions by ensuring that both workstations do not retransmit at exactly the same time.

Characteristics

• CSMA/CD is an example of a nondeterministic protocol, as it does not guarantee when a particular workstation will get access to the medium.