Why wait SIFS time before sending ACK?
Question about the MAC-protocol of 802.11 Wifi.
We have learned that when a station has received the data it waits for SIFS time. Then it sends the packet. When searching online the reason that is always mentioned is to give ACK packets a higher priority. This is understandable since a station first has to wait DIFS time when it wants to send normal data (and DIFS is larger than SIFS).
But why wait at all? Why not immediately send the ACK? The station knows the data has arrived and the CRC is correct, so why wait?
Answer
It is theoretically possible to know that the CRC is correct at the exact end of the received data from the wire, but in practice, you need to accumulate all the samples in the last block in order to run the IFFT, deconvolution, FEC, and then, finally, at the very end, after finally getting the input data out of the waveform, do you know that the CRC is correct. Also, you sometimes need to turn on transmit circuitry to send the ACK, which can hamper receive performance. If each step in the processing chain were instantaneous, and if the transmit circuitry definitely didn't interfere with the receive circuitry, and if there were no lead-time necessary for building the waveform for the ACK, it'd be possible to send the ACK immediately after getting the last bit of the wave-form. But, while each element in this chain takes some deterministic time, it is not instantaneous. SIFS gives the receiver time to get the data from the PHY, verify it, and send the response.
Disclaimer: I'm more familiar with Homeplug than 802.11.