Collision probability

[Yalda Edalat]

Hi,

I am going to calculate collision probability in Wifi. As far as I know,  the definition of collision is different in NS3. So, how can I calculate the actual collision in network? 

[Tommaso Pecorella]

The collision probability is pretty much accurate in ns-3. Please study the manual.

Thanks,

T.

[Yalda Edalat]

Thanks for the reply. As far as I know, Collision happens when two stations reach a backoff counter equal to zero simultaneously, or when there is a hidden node situation. However, it seems that the concept of collision is somewhat different in ns-3. The collision happens when the backoff count has reached zero and that the medium is busy. 

In one of the conversations some suggested that:

Collision probability = 2*RxError/(2*RxError+RxOk/2) 

Do you mean there is a better way to calculate it?

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[Tommaso Pecorella]

Please, let's not mix pears and apples.

One thing is the collision probability (you have to measure this quantity over a huge number of events). Another thing is the collision detection - i.e., how a node detects that a collision happened.

A collision happens if and only if two nodes sense the medium as free and they transmit at the same time (same time means a lot of things, and depends on the packet length and propagation delay). IF the receiving node can not decode either packet, THEN there is a collision, which can be less than expected due to capture effects and so on.

In other terms: it's not as easy as you could think.

T.

[Yalda Edalat]

Thanks. I really need your help.

I am interested in actual collisions (When two nodes sense the channel as idle and send at the same time) not the collision probability. Could you please give me some hints? Is there any straight forward method to calculate that?

[Tommaso Pecorella]

Hi,

that event is almost impossible in real systems, as it happens only if two nodes decide to transmit a packet with a maximum event difference equal to the transmission delay (about 1 microsecond if they are 300 m apart).

In a simulation it is possible to have this effect if one forgets to randomize the application start. This is because in a simulation time is exactly the same for all the nodes, while in real systems there are a lot of random effect that will de-sync the transmission.

Hope this helps,

T.

[Yalda Edalat]

So, how can we calculate that real collision? Is there any specific attribute? 

[Yalda Edalat]

I am confused from this link:

https://groups.google.com/forum/#!topic/ns-3-users/7byjPHGIcmo

Please let me know.

Thanks again

[Tommaso Pecorella]

Those are the collision in saturated case, i.e., when all the users have always something to transmit.

About how to calculate, use the same technique shown in the thread you mentioned.

T.