On a wired Ethernet, it is perfectly understandable that when a frame is transmitted, we assume that it has been received accurately. Radio links vary, especially so when the frequency utilized is the unlicensed industrial scientific and medical (ISM) band.
Narrowband devices are subject to noise and interference. However, it is essential that unlicensed devices should carry on functioning despite this interference that is bound to exist.
802.11 incorporates positive acknowledgment (ACK). It is essential that every frame must be acknowledged. This is illustrated in Figure 11.3. In case of failure in any part of the transfer, it is considered that the frame is lost.
The sequence illustrated in Figure 11.3 is an 'atomic' operation. 802.11 requires stations to restrain contention during atomic operations so that atomic sequences are not disturbed by other stations attempting to utilize the transmission medium.
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