Hello Imad The 802.11 Standard uses CSMA/CA ( Carrier Sense Multiple Access with Collision Avoidance ). It does not use collision detection. And 802.11.ccshould be coded according to the standard, so I guess you won't be able to capture the collided packets inside 802.11.cc. I think you can only capture the packets that did not pass the CRC check.
Best regards, -- Waleed Tuffaha Date: Thu, 13 Nov 2008 10:23:05 -0500 > From: "Imad Abdeljaouad" <[EMAIL PROTECTED]> > Subject: Re: [ns] collisions in 802.11 or interference > To: "Basim Javed" <[EMAIL PROTECTED]> > Cc: "ns-users@isi.edu" <ns-users@ISI.EDU> > Message-ID: > <[EMAIL PROTECTED]> > Content-Type: text/plain; charset=ISO-8859-1 > > Well, my idea was to simulate interference. You are right absolutely right, > interference happens in the air so the receiver has no idea about it. But > this is in real life. Since there are no signals in ns2, my idea was to > calculate interference at the MAC layer which will know what are the > packets > (signals) that were sent at the same time on the air. > So when the MAC receives more than one packet, I want to record these > packets somewhere (and this will give me access to where they are coming > from and going to, plus their powers), then I will apply a formula that > will > calculate interference between all these packets (in real life: signals). > You are absolutely right, in case of one channel, this will be considered > as > collision IF the received packets are destined to the same receiver/. But > there are two other cases, if the packet is not destined to that receiver, > then I think this will be strong interference. Also, if the interfering > packet belongs to a different channel (that I am willing to add after I > figure out the interference) then this is will be interference depending on > the correlation between the channels. > Do you see my point? > The first step I believe would be to capture all packets that come at the > same time while I am receiving another packet. This way I can process them > and decide if it is collision or strong or weak interference. I have the > formula, all I need are the packets along with their powers and channels > they belong to (don't care about the channels now, coz that's coming > after). > I am still wondering how and where in the 802.11.cc code I can capture all > colliding/interfering packets and put them somewhere for processing. > cheers, > > On Thu, Nov 13, 2008 at 5:00 AM, Basim Javed <[EMAIL PROTECTED]> wrote: > > > hello Imad > > > > I can see ur point. > > Lets talk on the issues one by one: > > > > The interference of one packet to an already sent packet creates > collision. > > right? so interference and collisions happen in the air. I dont see why > it > > is important that the packets should be sent to one receiver and they if > > they interfere, then collision happens. I think even those nodes who are > > just listening to the channel, know that collision has happened in the > air, > > even they were not recipients of the packet. > > > > Now, how does the node knows that a collision has happened.. through the > > received signal power, which is lower than a threshold level (i forgot > the > > name). As far as I remember, the power level can be so low, that a node > can > > not receive a packet, or it is bit high but still not good enough to > decrypt > > the bits. > > > > What I am talking above is related to the situation of same channel > (single > > hop). I think multiple channels are used in neighbouring BSSs. > > > > About ur formula: how a receiving node can know about the power of the > > signal when the packet was sent, in real situation? > > > > regards > > basim > > > > > > > > On Wed, Nov 12, 2008 at 4:16 PM, Imad Abdeljaouad <[EMAIL PROTECTED] > >wrote: > > > >> Thank you so much, I really need help! > >> I would like to add interference support in 802.11. If my understanding > is > >> right, interference happens when signals send on the same channel (or > >> adjacent channels, like 1 and 2 or 3 ... to 6) overlap in the medium > (the > >> air) and cause attenuations to each other. I think this will not be > possible > >> in ns2 since there are no "real" signals. So I was thinking that what I > >> should do is collect the signals that arrive at the same time at the > >> receiver MAC (and this is where I got collisions) and this means that > these > >> signals already overlapped in the air since they got to the receiver MAC > at > >> "the same time". > >> Do you see my point? > >> Once I know which siganls overlapped, I can use a formula to calculate > >> interference (the fornula uses Power of the signal sent + which channel > does > >> it belong to, to calculate the effect of one sigan on another ). > >> > >> So the point is, I need to capture the packet*s* that collide while I am > >> receiving another packet and for how long they collided. > >> I don't know if you get my point, but please do not hesitate to ask me > >> more questions! > >> thank you again !!! > >> > >> On Wed, Nov 12, 2008 at 4:36 AM, Basim Javed <[EMAIL PROTECTED] > >wrote: > >> > >>> hello Imad > >>> > >>> I want to help you, but I dont understand your definition of collision > >>> and interference. Can you clarify it plz? > >>> basim > >>> > >>> On Wed, Nov 12, 2008 at 2:55 AM, Imad Abdeljaouad < > [EMAIL PROTECTED] > >>> > wrote: > >>> > >>>> > >>>> Hi Helber, > >>>> Thanks for the prompt response. Actually, I am looking for the code in > >>>> ns2 > >>>> (source code) where we can detect collisions. I want to simulate > >>>> interference, so I need to get the packets that were received at the > >>>> same > >>>> time on some node. I know interference happens on the physical layer, > >>>> but I > >>>> think it would be much more easier to implement it on the MAC in ns2. > So > >>>> I > >>>> am actually looking the source code part that says here is another > >>>> packet > >>>> being transmitted while receiving another packet ( which is collision > if > >>>> both packets are meant for the same receiver, and interference if both > >>>> packets belong to the same (or an adjacent) channel. > >>>> Can you help with that ? > >>>> cheers, > >>>> > >>>> On Tue, Nov 11, 2008 at 8:35 PM, Helber Wagner < > [EMAIL PROTECTED] > >>>> >wrote: > >>>> > >>>> > Hi, Imad. > >>>> > > >>>> > As far as I understood your question, you should analyze the trace > >>>> file > >>>> > generated > >>>> > by your simulations. > >>>> > > >>>> > If the MAC trace is enabled in your .tcl file, it will be a row > like > >>>> "D > >>>> > ... COL ..." > >>>> > indicating the time, the node, the packet and other information > about > >>>> a > >>>> > collision > >>>> > that has been happened. > >>>> > > >>>> > Finally, a simple perl/awk script can retrieve this information > from > >>>> the > >>>> > trace file. > >>>> > > >>>> > You can find more about mac trace in NS2 over the Internet. > >>>> > > >>>> > Good luck. > >>>> > > >>>> > --- > >>>> > Helber Wagner da Silva > >>>> > Federal University of Cear? - Brazil > >>>> > URL: www.great.ufc.br/~helberhws< > http://www.great.ufc.br/%7Ehelberhws>< > >>>> http://www.great.ufc.br/%7Ehelberhws> > >>>> > > >>>> > > >>>> > > >>>> > ------------------------------ > >>>> > *De:* Imad Abdeljaouad <[EMAIL PROTECTED]> > >>>> > *Para:* "ns-users@isi.edu" <ns-users@ISI.EDU> > >>>> > *Enviadas:* Ter?a-feira, 11 de Novembro de 2008 23:17:46 > >>>> > *Assunto:* [ns] collisions in 802.11 or interference > >>>> > > >>>> > > >>>> > Hello there, > >>>> > I am looking for the code where collisions in 802.11 occur so that I > >>>> know > >>>> > which packet(s) collided and for how long they collided? this is a > >>>> step in > >>>> > trying to implement interference! > >>>> > thank you! > >>>> > > >>>> > -- > >>>> > best regards, > >>>> > ________________________ > >>>> > Imad Abdeljaouad > >>>> > > >>>> > ------------------------------ > >>>> > Novos endere?os, o Yahoo! que voc? conhece. Crie um email novo< > >>>> > http://br.rd.yahoo.com/mail/taglines/mail/*http://br.new.mail.yahoo.com/addresses > >com > >>>> a sua cara @ > >>>> > ymail.com ou @rocketmail.com. > >>>> > > >>>> > >>>> > >>>> > >>>> -- > >>>> best regards, > >>>> ________________________ > >>>> Imad Abdeljaouad > >>>> > >>> > >>> > >> > >> > >> -- > >> best regards, > >> ________________________ > >> Imad Abdeljaouad > >> > >
