Carrier sense multiple access with collision avoidance

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Carrier sense multiple access with collision avoidance (CSMA/CA), in computer networking, is a wireless network multiple access method in which:

  • a carrier sensing scheme is used.
  • a node wishing to transmit data has to first listen to the channel for a predetermined amount of time to determine whether or not another node is transmitting on the channel within the wireless range. If the channel is sensed "idle," then the node is permitted to begin the transmission process. If the channel is sensed as "busy," the node defers its transmission for a random period of time. Once the transmission process begins, it is still possible for the actual transmission of application data to not occur.[1][2]

CSMA/CA is a modification of carrier sense multiple access.

Collision avoidance is used to improve CSMA performance by not allowing wireless transmission of a node if another node is transmitting, thus reducing the probability of collision due to the use of a random truncated binary exponential backoff time.

Optionally, but almost always implemented, an IEEE 802.11 RTS/CTS exchange can be required to better handle situations such as the hidden node problem in wireless networking.[3]

CSMA/CA is a layer 2 access method, not a protocol of the OSI model.[4]



The use of collision avoidance is used to improve the performance of CSMA by attempting to divide the wireless channel up somewhat equally among all transmitting nodes within the collision domain. CSMA/CA differs from CSMA/CD due to the nature of the medium, the radio frequency spectrum. Collisions cannot be detected while occurring at the sending node, thus it is vital for CSMA/CA or another access method to be implemented. CSMA/CA is used in 802.11 based wireless LANs and other wired and wireless communication systems. One of the problems of wireless data communications is that it is not possible to listen while sending, therefore collision detection is not possible. Another reason is the hidden terminal problem, whereby a node A, in range of the receiver R, is not in range of the sender S, and therefore cannot know that S is transmitting to R.[5]

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