why do all compatible 802.11 standards use the same value of SIFS/DIFS?

802.11 standards define S开发者_StackOverflowIFS/DIFS value. Why do they use the same values? and if they use different values in the same network, what would happen?

The reason they are the same between different standards is for backward compatibility between 802.11g and 802.11b. If they are different, then simply backward compatibility is forfeited.

According to the standard, SIFS is:

The nominal time (in microseconds) that the MAC and PHY require in order to receive the last symbol of a frame at the air interface, process the frame, and respond with the first symbol on the air interface of the earliest possible response frame. See 9.3.7.

In the section 9.3.7, the standard states that:

aSIFSTime is: aRxRFDelay + aRxPLCPDelay + aMACProcessingDelay + aRxTxTurnaroundTime.

Therefore, SIFS is calculated by summing up the time that it takes the physical layer to receive the frame, the time that that it takes the physical layer to deliver the frame to the MAC layer, the time that the MAC needs to process the frame, and the time that the physical layer needs in order to switch from receiving the frame to sending and transmitting the first symbol of the next frame.

But the standard also states that this time is determined per PHY, meaning that the specific physical characteristics of the physical layer will determine this value. For example, in a OFDM (Orthogonal Frequency Division Multiplexing) PHY specification, depending on whether the channel spacing is 20 MHz, 10 MHz, or 5 MHz, the SIFS value are 16, 32 or 64 micro-seconds, respectively (see page 1623 of the standard).

So as you can see, SIFS (and other values like DIFS) strongly depends on the physical layer and on the interaction between the link layer and the physical layer, independently of the standard flavor (b, g, n...). If different values for SIFS were used, it's likely that more collisions would occur and the general performance of the network would drop.