Bug 2932

Summary:	Stations are blocked in 802.11 networks with many stations
Product:	ns-3	Reporter:	Szymon Szott <szott>
Component:	wifi	Assignee:	sebastien.deronne
Status:	RESOLVED FIXED
Severity:	normal	CC:	ns-bugs, tomh
Priority:	P3
Version:	ns-3-dev
Hardware:	All
OS:	All
Attachments:	WiFi network scenario with stations transmitting to AP

Description Szymon Szott 2018-06-19 10:47:46 UTC

Created attachment 3115 [details]
WiFi network scenario with stations transmitting to AP

Overview: In a Wi-Fi scenario with a large number of 802.11 stations transmitting to an AP under saturation conditions, some stations become blocked and do not transmit. 

Steps to Reproduce:  Run the attached code with the following (default) parameters:
./waf --run "scratch/blocked-sta --simulationTime=30 --nSta=128 --dataRate=0.5Mbps --RngRun=1"

Actual Results: The flows from several stations (e.g., 1, 25, 35) to the AP have zero received packets.

Expected Results: All flows should have some received packets.

Build Date & Hardware: ns-3-dev (changeset 13656:f85ec657c4b0) on Ubuntu 16.04.3

Additional Builds and Platforms:  Doesn't occur on ns-3.28 on Ubuntu 16.04.3

Additional Information: The following methods to fix this problem were tried and did not work:

- increasing the simulation time (also, stopping the transmission from non-blocked stations in the middle of the simulation does not affect the blocked stations),
- changing RngRun (other stations become blocked),
- disabling aggregation (in relation to Bug 2470),  
- using lower MCS values,
- changing to 802.11a,
- starting flows at different times,
- populating ARP cache prior to simulation start.

Comment 1 Tom Henderson 2018-06-19 11:01:23 UTC

This may be a duplicate of bug 2470.  In saturation condition, certain STAs become blocked because block ack agreement handshake fails.

Comment 2 Szymon Szott 2018-06-19 11:10:26 UTC

It is possibly a duplicate of bug 2470, but the problem appears also when using 802.11a (which I believe shouldn't have Block Ack). Also, the resulting pcap files do not show any Block Acks being sent (for 802.11n, aggregation is turned off).

Comment 3 Szymon Szott 2018-07-06 05:19:46 UTC

The bug no longer appears since changeset 13663:ed75ddbe630d which was related to fixing bug 2926.