Describe CSMA/CA or Carrier Sense Multiple Access with Collision Avoidance.

Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA):

This is a wireless network multiple access method defined by IEEE 802.11 standard. CSMA/CA is a modification to CSMA/CD. Collision detection as in CSMA/CD is not applicable for several reason, one reason is Hidden Terminal Problem.

To understand this problem, considering that we have three stations 1, 2 and 3. Station 1 and 3 are sending packets to station 2 at the same time and it may that these two stations can’t hear each other and thus, collision will occur. To prevent this, collisions must be avoided as follows,


  1. The sending station after sensing the station idle sends a special small frame called Request to Send (RTS) in which the station defines the total time it needs the medium.
  2. The receiver after receiving the RTS sends a small packet Clear to Send (CTS) to all the stations meaning that it has received the RTS and the medium is clear.
  3. Then the sender sends the data frame.
  4. The receiver acknowledges the receipt of data.

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