Subject: [manet] How to cause and measure the hidden terminal (or collisions)?

From: Claudio Lavecchia <[email-masked]>
Date: Tue, 01 Feb 2005 10:56:01 +0100

Hello you MANET guys,

I have a challenging question for you: I am running some tests on a 
MANET testbed composed of 4 nodes equipped of Dell Truemobile 1150 WLAN 
cards and routed with OLSR.
I would like to do some performance measurements in presence of the 
hidden terminal problem. Now I believe that causing the hidden terminal 
problem with 4 nodes is almost impossible.
Is there anyone that knows how I could provoke the hidden terminal 
problem in such conditions and measure its effects?  Is there a way to 
measure collisions that happen during a 802.11 communication?

I hope my question is clear enough. Any contribution would be greatly 
appreciated.

Thx

Claudio

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From: "Sherif M. ElRakabawy" <[email-masked]>
Date: Tue, 1 Feb 2005 11:20:38 +0100

Hi,

If you disable RTS/CTS you can get hidden terminals with only 3 nodes ..
You can Measure the collisions on the MAC layer by counting the number
of
unsuccessful data packet transmissions, i.e. 4 transmissions without
receiving a
MAC ACK. You may compare the values you get with a case where there are
no 
hidden terminals, i.e. RTS/CTS is enabled.

Regards

Sherif

--------------------------------------------
Sherif M. ElRakabawy
University of Dortmund
Department of Computer Science			   Tel. +49 231 755 2690
Mobile Computing Systems Group			   Fax. +49 231 755 2417
August-Schmidt-Str. 12
44227 Dortmund / Germany
WWW: http://mobicom.cs.uni-dortmund.de/~moustafa/

> -----Ursprüngliche Nachricht-----
> Von: [email-masked] [mailto:[email-masked]] 
> Im Auftrag von Claudio Lavecchia
> Gesendet: Tuesday, February 01, 2005 10:56 AM
> An: [email-masked]
> Betreff: [manet] How to cause and measure the hidden terminal 
> (orcollisions)?
> 
> 
> Hello you MANET guys,
>

---

From: Imad Aad <[email-masked]>
Date: Tue, 1 Feb 2005 11:26:45 +0100 (CET)

On Tue, 1 Feb 2005, Claudio Lavecchia wrote:

> Hello you MANET guys,
>
> I have a challenging question for you: I am running some tests on a MANET 
> testbed composed of 4 nodes equipped of Dell Truemobile 1150 WLAN cards and 
> routed with OLSR.
> I would like to do some performance measurements in presence of the hidden 
> terminal problem. Now I believe that causing the hidden terminal problem with 
> 4 nodes is almost impossible.

Why?!
3 stations far enough from each other would make a good hidden node 
scenario, am I missing something? (don't forget to take the carrier sense 
threshold/range into consideration)

> Is there anyone that knows how I could provoke the hidden terminal problem in 
> such conditions and measure its effects?  Is there a way to measure 
> collisions that happen during a 802.11 communication?

On a high level: measure the throughput drop with respect to a scenario 
with no hidden stations (but the same number of stations and connections)

On a low level: use a station in monitor mode (if your card and card 
driver permit this), you'll be able to see which packets got ACKed and 
which ones did not.

I'm sure there's plenty of other ways to monitor what's happening on 
the channel.

imad

---

From: Claudio Lavecchia <[email-masked]>
Date: Tue, 01 Feb 2005 15:35:42 +0100

Thx Sherif,

Your idea of disabling the RTS is interesting and leads me further in my 
reasoning.
Let me try to explain you what I would like to achieve:

Let's imagine that I have three nodes in a row, say A,B and C. Node B 
acts as a relay for traffic between A and C. Then we have a fourth node, 
say node D.
Node D is disturbing the communication between A and C by trying to jam 
node B.
What I would like to have is node B that from time to time fails in 
forwarding packets that are directed from A to C. I tought that it would 
be possible to achieve this using node D.
If node A sends a packet to node B, but node B does not receive it 
because node D starts talking to node B at the same time, that would be 
enough for my purposes.
In this sense I do not strictly need to mount an "hidden terminal" 
scenario in the sense that for me physical separation of devices is not 
mandatory.

Now, if I understand correctly, if I apply your suggestion to the 
scenario I just described, I could have node A talking to node C through 
the relay of node B. Node D has disabled the RTS and hence it starts 
talking to node B without sensing the media.
What do I have to do to make sure that node B privileges reception of 
packets from node D (the interfering node)? Do I have to play with the 
transmission power?
How do I measure the number of MAC ACKs received? is there any statistic 
tool provided with WLAN cards?
How do I disable RTS? I guess that this is an option of iwconfig?

If you (or anyone else) could help me to advance on those questions that 
would be of great help.

Ciao

Claudio