I am searching for this answer for quite some time now. Why there is CSMA/CD in LAN but CSMA/CA in WLAN?
The best explanation I could find is "due to extreme ratio in transmitting and receiving power, it is very impractical to transmit data on the same channel. Hence collision avoidance is used". Couldn't get the meaning. Even if you're using two separate channels for transmit and receive, CSMA is used to decide which node will use channel, so no point in dropping CD in place of CA. So this explanation somehow doesn't seem fit.
The only reason I could think of is that if number of nodes are low, hence chances of collision is low, we should use CD, if chances of collision are high, we should use CA. But there isn't a difference of number of users between LAN and WLAN.
If someone could please explain.
Best Answer
In a wired CSMA/CD Ethernet environment, it is possible to detect a collision because there are separate TX and RX pairs (using the example of 10BaseT). If a half-duplex 10BaseT NIC sends a frame on the TX pair, but sees that frame is corrupted on the RX pair, the NIC detected a collision.
However, with an 802.11 wireless device, there are no "conductors," just antennas which do not simultaneously transmit and receive. When an 802.11 device is transmitting it cannot in practical terms listen for another signal transmitting at the same time on the same frequency. The reason for this is that RF signal strength drops off very quickly when transmitting.
Even if we built an imaginary WiFi device that could simultaneously receive and transmit, it will only be able to hear a downstream collision if the other device is using a much higher output power (either raw power or through passive/active gain of some sort). Normally its own TX signal will be too strong and "drown" out any other received signal.
So another process was required, resulting in the need for CSMA/CA.