A-MPDU limit violated

[Vineet]

Hello,

I am simulating a dumbbell network topology as shown below. 

Network settings:

1. The STA and AP nodes are connected via 802.11ac protocol and the APs are connected through a p2p link of capacity 1000 Gbps.

2. Maximum size of A-MPDU at the STA nodes is set to 5000 bytes and A-MSDU is disabled.

3. The total size of an MPDU is 676 bytes, which means that there can be at most 7 MPDUs in an A-MPDU.

4. The application layer is generating periodic messages with an interval of 1 ms i.e at rate of 1000 messages/sec. This amounts to an overall sending rate of ~6Mbps per source node.

5. The source and receiver nodes are always located at the opposite ends of the network.

Scenario 1 - Unidirectional traffic:

Initially, I simulated a scenario where nodes at the left side of the network send data to nodes at the right side. I varied the number of sources from 1 to 10. The pcap trace indeed shows a maximum of 7 MPDUs arriving at the same time for all the case. 

So far so good!

Scenario 2 - Bidirectional traffic:

Here, I simulated a scenario where each node in the network is a sender as well as a receiver. For up to 4 nodes at each end, the MPDU aggregation works as expected. However, as the number of sources at each end increases further, the aggregation limit seems to be violated as I can see a higher number of MPDUs (even up to 25) arriving together. In all of these cases, it is ensured that there is no congestion as the end-to-end delay is steady and well below its corresponding maximum.

I observed similar behavior when I switched the wireless network to 802.11n.

Is there a justification for this behaviour?

Thanks in advance.

[Vineet]

Attached is the code that can be run to see this behavior.