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

Quick crash-course on 802.11:

802.11 is a essentially a giant system of timers. The most common implementations of 802.11 utilize the distributed coordination function, DCF. The DCF allows for nodes to come in and out of the range of a radio channel being used for 802.11 and coordinate in a distributed fashion who should be sending and receiving data (ignoring hidden and exposed node problems for this discussion). Before any node can begin sending data on the channel they all must wait a period of DIFS, in which the channel is determined to be idle, if it is idle during a DIFS period the first node to grab the channel begins transmitting. In standard 802.11, i.e. non-802.11e implementations and non 802.11n, every single data packet that gets transmitted needs to be acknowledged by a physical layer, PHY, acknowledgment packet, irregardless of the upper layer protocol being used. After a data packet gets sent a SIFS time period needs to expire, after SIFS expires control frames destined for the node that has "taken" control of the channel may be sent, in this instance and acknowledgment frame is transmitted. SIFS allows the node that sent the data packet to switch from transmitting to receiving mode. If a packet does get lost and no ACK is received after SIFS/ACK timeout occurs, then exponential back-off is invoked. Exponential back-off, a.k.a contention window (CW), begins at a value CWmin, in some linux implementation this is 15 slot times, where a slot time varies depending on the 802.11 protocol that is being used. The CW value is then chosen from 1 to whatever the upper limit that has been calculated for CW. If the current packet was lost, then the CW is incremented from 15 to 30, and then a random value is chosen between 1 and 30. Every-time there is a consecutive lose the CW doubles up to 1023, at which point it hits a limit. Once a packet is received successfully the CW is reset back to CWmin.

In regards to 802.11n / 802.11e: Every data packet still needs to be acknowledged, but when using 802.11e (implemented into 802.11n) multiple data packets can be aggregated together in two different ways A-MSDU or A-MPDU. A-MSDU is a jumbo-frame that has one checksum for the entire aggregated packet being sent, within it are many sub-frames that contain each of the data frames that needed to be sent. If there is any error in the A-MSDU frame and it needs to be retransmitted, then every sub-frame is required to be resent. However, when using A-MPDU, each sub-frame has a small header and checksum that allow for any sub-frame that has an error in it to be retransmitted by itself/within another aggregated frame the next time the sending nodes gains the channel. With these aggregated packet sending schemes there is the notion of the block-ack. The block-ack contains a bitmap of the frames from a starting sequence number that were just sent in the aggregated packet and received correctly or incorrectly. Usage of aggregated frame sending greatly improves throughput performance as more data can be sent per channel acquisition by a sending node, also allowing out-of-order packet sending. However, out-order packet sending greatly complicates the 802.11 MAC layer.