What is CSMA/CA?
It stands for Carrier Sense Multiple Access with Collision Detection. CSMA/CA is a network contention protocol that listens to a network in order to avoid collisions, unlike CSMA/CD that deals with network transmissions once collisions have been detected. CSMA/CA acts to prevent collisions before they happen. CSMA/CA protocol is used in wireless networks because they cannot detect the collision so the only solution is collision avoidance.
It stands for Carrier Sense Multiple Access with Collision Detection. CSMA/CA is a network conflict protocol which listens to a system network to be able to prevent the collision, not like CSMA/CD that addresses network transmissions once the collision is detected. CSMA/CA functions to stop collisions before they occur. CSMA/CA protocol is utilized in wireless networks as they can’t detect the collision so that the only resolution is collision avoidance.
Why we need the CSMA/CA protocol?
CSMA/CD (carrier sense multiple access) methods are often prone to face the “hidden terminal problem”, and suffer from the “exposed terminal problem” at times. The CSMA/CA (carrier sense multiple access with collision avoidance) protocols are used to avoid the collision in case of the “hidden terminal problem” and offers a simplistic approach towards the “exposed Terminal problem”.
What is the Hidden Terminal Problem?
Let’s first imagine a situation where the “hidden terminal problem” can occur. Suppose that in a network with device A, B and C, B can communicate with both A and C, but A and C cannot communicate with each other. In such a situation, if both A and C attempt to transmit data to B at the same time, they would find the channel free, but B would have received both the transmission at the same time, which would result in a collision.
“Hidden terminal problem” occurs when a node is visible from a wireless access point (AP), but not from other nodes communicating with said AP. This lead to difficulties in media access control.
What is Exposed Terminal Problem?
The “Exposed Terminal Problem” is the opposite of the “hidden terminal problem” and occurs when a device capable of listening to another far away device defers nearby communication unnecessarily. For example, in the above diagram, device c, which is capable of listening to a communication between A and B, defers its local communication with D due to the far away communication it detects. In such a situation, excessive traffic in the remote devices can render another device idle for a prolonged time.
How CSMA /CA work?
The CSMA/CA protocol improves on the collision detection part of the CSMA/CD protocol, reducing dependence on carrier sense. CSMA/CA usually works by sending a short Request To Send (RTS) packet to the destination device, which in turn respond with a Clear To Send (CTS) packet. Any Device that listens to any such request on the network stops its own transmission for a certain amount of time to avoid a collision.
CSMA/CA extends the collision Avoidance feature by including a time frame for the proposed data transfer. For example, in the above diagram, if device B can listen to both A and C, but A and C cannot detect each another, collision is avoided this way: When A sends RTS to B, it includes the expected time for transfer of the actual data packet. B, when it sends the CTS signal to A, includes this information in the signal. While C is unable to listen to A directly, it can listen to B and can learn how long it needs to wait to keep the network clear for communication between A and B. Thus the “hidden terminal problem” is avoided.
In case of the other example, device c, which is in range of device B, but not device A, can hear B’s RTS to A but Cannot hear A’s CTS signal. As such, it is free to transmit without delaying, thereby avoiding the “exposed terminal problem”.