[ns] Another CSThresh Question

Wed Jan 28 00:03:08 PST 2004

Well, I agree with you. I think I found out what the problem is. Its
the gridkeeper, I had set a radius which equalled the distance set for
RXthresh. Packets are not allowed to propagate beyond this radius. So
even if the cs range is more, it wouldnt matter.

Anyway thanks for your help

On Tue, 27 Jan 2004, Saikat Ray wrote:

>
> If packet power is greater than CSthrehold, but less than RxThreshold, then the packet is passed to the higher layer indicating that there is error. However, the higher layer advances nav for the packet duration before discarding an erroneous packet. So, this packet (with power less than RxThreshold but greater than CSThreshold) will make the receiving defer. This is the way NS implements carrier sensing. So, clearly CSthreshold has effects.
>
> My suggestion was to check the range with two node topology with the set of (CS,Rx) threshold values for which you expect to get 40m range. This check would eliminate the possibility that the program you are using to map range (in m) to threshold (in watt) was not used properly.
>
> >
> >Well, I just created a simple two node topology and used the default
> >values to check the distance as suggested by you.
> >
> >Anyway I just took a look at /mac/wireless-phy.cc where CSTHresh is used. In this code, the
> >power level Pr is compared to CSThresh, if Pr < CSThresh, no
> >packets are received. Even if Pr > CSThresh but Pr < RXThresh, the packets are
> >considered errored and dropped. So I guess thats the only purpose CSTHresh
> >is serving, it basically serves no purpose ( am I right ). There is no
> >carrier sense happening for higher CS range, so RXthresh determines the
> >carrier sense range.
> >
> >On Tue, 27 Jan 2004, Saikat Ray wrote:
> >
> >> It is not possible in a real network to have a smaller CS range! This is tautological, since if CS threshold is smaller, what you are claiming is that you can decode the packet but cannot sense it!!!!
> >>
> >> You initially said that nodes are 40m apart, but now you are saying that the minimum distance is 725m. If all the nodes are already within the transmit range, then there would not be any difference depending on the CS value.
> >>
> >> Next time if you attach a script of your scenario, as small as possible, I can take a look at it.
> >>
> >> ----------- Original Message ---------
> >>
> >>
> >> >
> >> >Minimum distance is 725m i.e RxThresh= 3.65e-10 and CSThresh=1.559e-11
> >> >(all default value), beyond which no packets are recevied.
> >> >For nodes less than 725 m apart, when CSThresh <= RXthresh
> >> >(i.e cs range is more), packets are received without
> >> >any drops. When CSthresh > RXthresh (cs range low) all packets are dropped
> >> >which makes sense.
> >> >Well so again it comes back to my question, in my previous topology why
> >> >are the rsults similar i.e why are there same number of collisions when
> >> >the CSThresh < RXThresh as compared to CSThresh=RXThresh
> >> >
> >> >On Tue, 27 Jan 2004, Saikat Ray wrote:
> >> >
> >> >>
> >> >> Typically NS scripts use Propagation/TwoRayGround. So, make sure that your calculations are fine.
> >> >>
> >> >> Did you check whether the range is set correctly, say, by simulating a two node scenario with a distance 'd' between them, and increasing the distance to find the minimum 'd' so that all packets are dropped? If your transmission range found this way matches the calculations, then very likely your CS-range will also match.
> >> >>
> >> >> ----------- Original Message ---------
> >> >>
> >> >> DATE: Tue, 27 Jan 2004 07:26:54
> >> >>
> >> >>
> >> >> >
> >> >> >Hi Saikat,
> >> >> >In your previous post, you have mentioned that you can
> >> >> >use the same program threshold.cc to calculate
> >> >> >RXThresh and CSThresh
> >> >> >http://mailman.isi.edu/pipermail/ns-users/2003-November/037646.html
> >> >> >
> >> >> >and I did just that. However if I calculated the
> >> >> >CSThresh for 40m, that doesn't mean the CS range is
> >> >> >40m right.
> >> >> >
> >> >> >I am using the FreeSpace Prop model, is there a way to
> >> >> >set the carrier sense range to 40m in the TCL code, if
> >> >> >not how do I go about doing so (what paramaeters do I
> >> >> >have to change) ?
> >> >> >
> >> >> >Thank You
> >> >> >
> >> >> >>
> >> >> >> What do you mean by "similar *results*"? If you are
> >> >> >> just looking at throughput, especially of some
> >> >> >> particular node, that may remain same, or may even
> >> >> >> decrease. However, if you count the number of
> >> >> >> collided frames, that should be greatly reduced when
> >> >> >> CS-range is larger.
> >> >> >>
> >> >> >> Secondly, how do you exactly adjust the "range" of
> >> >> >> carrier sensing? CSThreshold is a *power* parameter
> >> >> >> (receiver sensitivity). The actual distance is
> >> >> >> determined by the path-loss model in conjunction
> >> >> >> with CSThreshold.
> >> >> >>
> >> >> >> ----------- Original Message ---------
> >> >> >>
> >> >> >> DATE: Mon, 26 Jan 2004 19:57:28
> >> >> >>> >> >>
> >> >> >> >
> >> >> >> >
> >> >> >>
> >> >> >>
> >> >> >>
> >> >> >>
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