CSMA - Carrier Sense Multiple Access

It is difficult to detect collision in the wireless environment.
The wireless nodes must detect actively.


Therefore there are two methods to prevent collisions:

1. Before sending data, wait a period of time to listen medium state if no one exploit and then wait a random time to send data (still no one use). Because different random time, it will decrease collision possibility.

2. Before sending data, send RTS(request to send) to target and wait for CTS(clear to send). If receiving CTS, start to send data. (RTS and CTS will clear the channel)
RTS and CTS are small frame and won't cause high transfer cost. (ex. IEEE 802.11)

In other word, RTS/CTS exploit the signals to notify other nodes that there are someone sending data, so others need to wait a second.