Carrier sense multiple access with collision detection

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Carrier sense multiple access with collision detection (CSMA/CD) is a computer networking access method in which:

  • a carrier sensing scheme is used.
  • a transmitting data station that detects another signal while transmitting a frame, stops transmitting that frame, transmits a jam signal, and then waits for a random time interval before trying to send that frame again.

CSMA/CD is a modification of pure carrier sense multiple access (CSMA). CSMA/CD is used to improve CSMA performance by terminating transmission as soon as a collision is detected, thus reducing the probability of a second collision on retry.

Contents

Details

CSMA/CD is a layer 2 access method, not a protocol of the OSI model.[citation needed]

When a station wants to send some information, it uses the following algorithm:[1]

Main procedure

Collision detected procedure

This can be likened to what happens at a dinner party, where all the guests talk to each other through a common medium (the air). Before speaking, each guest politely waits for the current speaker to finish. If two guests start speaking at the same time, both stop and wait for short, random periods of time (in Ethernet, this time is measured in microseconds). The hope is that by each choosing a random period of time, both guests will not choose the same time to try to speak again, thus avoiding another collision.

Methods for collision detection are media dependent, but on an electrical bus such as 10BASE-5 or 10BASE-2, collisions can be detected by comparing transmitted data with received data or by recognizing a higher than normal signal amplitude on the bus.

Applications

CSMA/CD was used in bus topology Ethernet variants and in early versions of twisted-pair Ethernet. Modern Ethernet networks built with switches and/or full-duplex connections no longer utilize CSMA/CD. IEEE Std 802.3, which defines all Ethernet variants, for historical reasons still bares the title "Carrier sense multiple access with collision detection (CSMA/CD) access method and physical layer specifications".

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