I am trying to understand the behavior of the Duration/ID field in 802.11 specs. Its role and behavior are outlined in the standard. This field tells other stations how long the transmitter is going to be using the network (including, e.g., ACKs sent by the receiver).

My question: can I expect the Duration Field to depend on signal strength?

My reasoning is that it is possible that noisier connections will require longer transmission time, due to errors incurred to the packet/frame.

  • Errors are not anticpated but a weaker connection causes a lower bitrate which causes longer transmission. – Zac67 Mar 24 '20 at 12:57
  • @Zac67 So one can expect the Duration Field to be independent of SNR? Low SNR will not cause frames to be longer, it will just force the station to send more frames? – Yair Daon Mar 24 '20 at 14:32
  • Low SNR causes a lower link rate, causing longer transmission periods = longer frames in time. It will not cause more frames directly. – Zac67 Mar 24 '20 at 15:04

This is actually a specific question, within the broader topic of 'rate adaptation algorithms'. The duration field is decided prior to the transmission of the frame, based on the modulation type, code rate, symbol duration etc. (collectively given by the modulation and coding scheme). In other words, 802.11 operates with a range of MCS levels, each level describing a unique communication scheme and hence a unique duration.

A rate adaptation algorithm (RAA) decides which MCS is to be used. As you mentioned, if the algorithm takes into account the SNR, then naturally, higher the SNR, shorter the duration (because the rate is higher) and vice-versa. However, if your algorithm is more naive and simply works with a constant rate (a fixed MCS) for all frames, then the duration field will remain the same as well.

In short, the answer to your question is - it depends on the RAA. Note that the duration field simply describes the duration of that particular frame, it is independent of whether the frame will be delivered successfully or not.

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