Re: Monitoring Downlink Traffic from one AP to multiple Stations using 802.11n and OnOff Application

[Rediet]

Hello Ahmed,

The application data rate that you have defined is too high for the channel capacity. Since all the corresponding generated packets will end up in the same BE queue, you'll end up having overflow.

A rule of thumb I usually employ is to consider what ideal application data rate a single STA could support, and divide that by the number of STAs.

BR,

Rediet

Le samedi 12 août 2017 02:14:38 UTC+2, Ahmed Ibrahim a écrit :

Hi there,

I have been struggling for a couple of days with a problem that I am facing. I have a very simple script (attached here) that has a network consisting of one Home AP and 6 Home Devices. I want to monitor the capacity of the downlink traffic when the home AP is not the bottleneck (i.e., the home AP runs its installed applications with very high rates).

I had a problem before that one device dominated the channel and all other devices were not able to access the channel. I checked the forum in here and found out that this is related to bug 187 and it should be bypassed by having the traffic generating applications (OnOff Application in my case) start at different times.

Now, I can see traffic going from the home AP to all stations (Devices). My concern is that the traffic does not make sense at all. Here is what I see

Position of Home AP: (2, 5, 0)

Traffic from Home AP to Home Stations: 500 Mbps

Position of Home Device 1: (11.4, 0.5, 0) - Distance to Home AP is 10.4216 - Throughput 4.59113 Mbit/s

Position of Home Device 2: (1.2, 9, 0) - Distance to Home AP is 4.07922 - Throughput 13.2058 Mbit/s

Position of Home Device 3: (9.6, 8, 0) - Distance to Home AP is 8.17068 - Throughput 2.97747 Mbit/s

Position of Home Device 4: (3.6, 1, 0) - Distance to Home AP is 4.30813 - Throughput 2.21447 Mbit/s

Position of Home Device 5: (7.2, 9, 0) - Distance to Home AP is 6.56049 - Throughput 13.4205 Mbit/s

Position of Home Device 6: (5.4, 5, 0) - Distance to Home AP is 3.4 - Throughput 13.7799 Mbit/s

As you see, Device 4 is very close to the home AP (4.3 meters), however it sees much lower rate than Device 1 which is much further to the home AP. I thought that the traffic should correlate to the distance from the home AP. 

I have three questions:

1- Is my understanding correct or I am missing something (either in my script or in my logic)?

2- If my understanding is correct, why this happens? If I only have two or three devices, the results matches my understanding that I explained earlier.

3- This is a side problem but when I run the script with the PyVix using --vis, it crashes when I have more than 4 stations but it works fine with four stations or less?!

I would appreciate it if someone can help me out.

Thank you.

Ahmed

[Ahmed Ibrahim]

Thank you Rediet for your reply.

Actually, based on my understanding, it does not matter how much rate I am defining at the AP. The maximum achievable rate is the rate that the channel can afford which should be around 40 Mbps.

I am trying to simulate a situation where the AP is not the bottleneck of the traffic. This is why I am assuming very high rate at the AP.

Anyway, the rate should make some sense. It does not make sense to me that further stations receive higher rates!

What do you think?

Ahmed

[Rediet]

Hi Ahmed,

Maybe they are getting higher rates because their packets have gone through and not those of closer STAs. Since you have set up one 500 Mbps application per STA, the AP's buffer drops packets like crazy. Try lowering this data rate and check if it's OK.

Rediet

PS: Another way to check would be to install the application sources in the STAs; you'll have only one app so no starvation within the same device.

[Hassam Mughal]

Hi Ahmed,

Could you please share the strategy you followed to monitor the data rate? I am trying to do it on multiple wireless interfaces on each mobile device.