CSMA/CA

• CSMA/CA (carrier-sense multiple access with collision avoidance) is a protocol used in wireless networking. It ensures that network collisions (when two devices attempt to communicate at the same time) are avoided.
• Carrier-sense


  The first step is to listen to the channel to check for the presence of any other signals. It is at this point that a decision is made whether to send the message or to try again at a later time.
• Collision avoidance

  Upon the detection of an existing transmission on a channel, the node — an end device, which is either sending or receiving data across the network — that wanted to send will wait for a period of time before trying again. Typically, this is a random time within a range.
• Transmission

  Once the channel is free, the data is sent. Then, the sender needs to receive an acknowledgement message from the receiver to verify that the data was received successfully. Otherwise, the sender will attempt to resend the message.
• In wireless networks, there may be an issue known as the hidden node problem, which interferes with the data transmission. When a sending device checks whether the channel is idle, it can only check within its broadcast range. However, a device can be out of range from the broadcast of another device, but still be within the range of the WAP. Therefore, they could both be sending data to the WAP at the same time, but would be unaware of the presence of the other device and, as a result, the WAP would not be able to receive any data due to the data collisions.
• Request-to-send/Clear-to-send (RTS/CTS) is an optional method that alleviates the problem of hidden nodes, although it does create an additional overhead that can slow down network communication. Once the channel is determined to be idle, a signal is sent called Request to Send (RTS). The answering device then sends a Clear to Send (CTS) reply. This way it knows which device asked to send first and can ensure that only that device will send its message, by not responding to further RTS signals until the original sender has finished.