为什么要在发送ACK之前等待SIFS时间? [英] Why wait SIFS time before sending ACK?

问题描述

有关802.11 Wifi的MAC协议的问题.

Question about the MAC-protocol of 802.11 Wifi.

我们了解到，当站接收到数据时，它将等待SIFS时间.然后，它发送数据包.在线搜索时，总是提到的原因是给ACK数据包更高的优先级.这是可以理解的，因为站点要发送正常数据时首先必须等待DIFS时间(并且DIFS大于SIFS).

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).

但是为什么要等待呢?为什么不立即发送ACK?该站知道数据已到达并且CRC正确，那么为什么要等待?

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?

推荐答案

从理论上讲，有可能知道CRC在从线路接收到的数据的确切末尾是正确的，但实际上，您需要累加所有为了运行IFFT，反卷积，FEC，然后最后在最后，最后从波形中获取输入数据之后，在最后一个块中采样，您知道CRC是正确的.另外，有时您需要打开发送电路以发送ACK，这可能会影响接收性能.如果处理链中的每个步骤都是即时的，并且如果发送电路确实没有干扰接收电路，并且如果没有必要为构建ACK波形建立提前期，则可以发送获得波形的最后一位后立即进行ACK.但是，尽管此链中的每个元素都需要花费确定性的时间，但它不是瞬时的. SIFS使接收器有时间从PHY获取数据，进行验证并发送响应.

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.

免责声明:我比802.11对Homeplug更为熟悉.

Disclaimer: I'm more familiar with Homeplug than 802.11.

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