Why wait SIFS time before sending ACK?

Question

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 searchin

Answer 1

It is like that because Distributed Coordination Function (DCF) and Point Coordination Function (PCF) mode can coexist within one cell. That is a base station may use polling while at the same time the cell can use disitributed coordination using CSMA/CA.

So during SIFS, control frames or next fragment may be sent. During PIFS, PCF frames may be sent and during DIFS DCF frames may be sent. During SIFS and PIFS, PCF can work its magic.

Even though not all base stations support PCF all stations must wait since some may support it.

Update:

The way I understand this now is that during SIFS the station may send RTS,CTS or ACK and have enough time to switch back to receiving mode before the sender starts to transmit. If that's correct, it will send ACK during SIFS. Then it will change to receive mode and wait until SIFS elapses. When SIFS has elapsed the transmitter will start sending.

Also, PCF is controlled by PIFS which comes after SIFS and before DIFS and is therefor not relevant for this discussion (my mistake). That is, SIFS < PIFS < DIFS < EIFS.

Sources: This PDF (page 8) and Computer Networks by Andrew S. Tanenbaum