In a wireless LAN, when a station sends a frame, the frame may not reach the destination station for a variety of reasons. The main reason is due to high bit error rates of wireless channels. To deal with this problem, the 802.11 MAC protocol uses acknowledgments scheme.
In wired Ethernet, bit error rate is very negligible and transmission in wired Ethernet is very reliable. Therefore, acknowledgements are not required in wired Ethernet.
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