In the example below, the AP sends sequence number 1991 – 2000 to a client, but the client did not receive frames 1992 & 1999.Is there a way to programatically turn off Downtime on a child device? The Block Ack has the capability to point out individual sequence numbers that were not received, and only those individual frames have to be retransmitted. The frames are not Ack’d individually as in “No Frame Aggregation” – the frames are being Ack’d via the “Block Ack” frame.
Again, the transmitter only has to win one transmit opportunity for the whole A-MPDU.
Every 802.3 frame gets its own 802.11 MAC header and they are aggregated into a single PPDU transmission. The big difference here is that the transmitter does not merge multiple Ethernet frames into a single 802.11 frame. In addition to A-MSDU, 802.11n also introduced a different form of frame aggregation: A-MPDU. Therefore, if you have a noisy environment with lots of retries, the cost of the retries might be bigger than the benefit of the aggregated Ethernet frames. However, if only a small portion of an A-MSDU is corrupted and not properly received, the transmitter has to gain another transmit-opportunity to retransmit the whole A-MSDU again. It aggregates multiple 802.3 frames into a single 802.11 frame, therefore the transmitter has to win one transmit opportunity to transmit multiple (in this case 2) 802.3 frames. The benefit of using A-MSDU is that you reduce overhead. However, the frame with sequence number 103 had to be retransmitted 3 times until it got Ack’d (and even shifted down from a transmission rate of 162mbps to 81mbps). In the example below, the 802.11 frames with sequence numbers 99 – 102 were transmitted and got Ack’d immediately. Those “data frames” are usually Ethernet frames that are transformed from 802.3 to 802.11 (and vice versa) by the AP and clients in order to send them over the wireless medium. Wi-Fi in its initial form transmitted single data frames that were Ack’d immediately in case of a successful transmission. To understand frame aggregation, we need to understand where it all came from. That’s why I want to show the idea of A-MSDU and A-MPDU in some real world examples in Wireshark, so you can easily spot them in future troubleshooting sessions. Whether you’re studying for exams like CWNA or CWAP, or you’re just in the deep ends of your Wi-Fi configuration, you probably come across the topic of frame aggregation – A-MSDU and A-MPDU in particular.Įven though the study guides for above exams and some blogs ( mrn-cciew, dot11ap) explain the concepts of frame aggregation in depth, they were very theoretical and hard for me to grasp when I first learned them.
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