Why SIFS is 10µs in 2.4GHz and 16µs in 5GHz?. SIFS is the small time gap required by a receiving station to process (Demodulation + RX→TX turnaround) a received frame before sending an ACK or response. It may look like SIFS differs between frequency bands but in reality, the difference comes from the PHY type, not the band.
For legacy 802.11b, which uses DSSS/CCK modulation, SIFS is defined as 10µs. DSSS is relatively simple, with straightforward modulation and signaling, so the receiver does not need much processing time before it can reply. Whereas in OFDM-based PHYs require more complex processing, such as FFT operations, which take additional time. To accommodate this, the standard defines SIFS as 16 µs for OFDM PHYs.
The interesting case here is 802.11g. It brought OFDM into the 2.4GHz, but instead of using the 16µs SIFS like 11a, it kept 10µs. The reason is backward compatibility: 802.11g needed to coexist with 802.11b devices, which only understand 10µs SIFS. But 11g requires 16µs to process the received OFDM frame and reply back. To resolve this mismatch, 802.11g introduced a clever mechanism called Signal Extension. Every OFDM frame in 11g carries an additional 6µs of signal (dead air) after the payload which helps 11g receiver additional required time for processing received OFDM frame. This ensures that, although the nominal SIFS is still 10µs, the effective processing time available to an 11g receiver is 10µs + 6µs = 16µs, exactly what OFDM requires.
From the perspective of an 802.11b station, this 6µs signal extension is simply ignored it just looks like the medium is busy.
SE = Signal Extension.
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