[ns] Re: transmission range distCST_

Saikat Ray raysaikat at lycos.com
Thu Jun 24 13:49:37 PDT 2004


Well, it is actually the other way. RXthreshold and CSthreshold are inherent properties of the card (that depends on modulation/demodulation, among other things). So for usual commercial cards, CSThreshold is considered to be about -90dBm (exact numbers are usually not released by the manufacturers). RXThreshold depends on the packet size. In my experience, -65 dBm is sufficient to decode very large packets (about 2000 byte) with very low probability of error; for very small packets, even -80 dBm may be enough. Also remember that these are not parameters of the card; meaning that you cannot control them. It just so happens that the card can reliably detect carrier only when the power is more than -90dBm; same with the packet.

threshold.cc actually calculates what should the RXThreshold be if you "want" the receive range to be 250m (or whatever is the distance). The same program will tell you what the CSThreshold should be if you want the carrier sense range to be 550m (which should be around -90 dBm). So it is simply provides a way to "cheat" in the simulation. In other words, if you want to simulate a situation where the RX-range is 400m and CSrange is 450m, using threshold.cc you can set the RXthresh_ and CSThresh_ appropriately, but that does not mean that there exists a physical card which will have those values of RX and CS thresholds.

----- Original Message -----
From: Leon Martin <leon70m at yahoo.com>
Date: Thu, 24 Jun 2004 11:10:10 -0700 (PDT)
To: raysaikat at lycos.com
Subject: Re: transmission range distCST_

> Hi, Saikat,
> 
> I found this message from ns mailing list. I would
> like to know what formula is used to calculate the
> carrier sense threshold.
> 
> The is a formula in ~ns/indep-utils/threshold.cc for
> calculating the receiving threshold RXThresh_. 
> 
> Using that code, I got the -65 dBm for the RXThresh_.
> But how do you get the -90 dBm for the carrier sense
> threshold?
> 
> Thanks!
> 
> Leon
> 
> 
> [ns] transmission range distCST_
> Saikat Ray raysaikat at lycos.com
> Thu Jun 17 09:05:53 PDT 2004
> 
> The carrier sense range should not change.
> Theoretically, carrier sensing is equivalent to
> decoding 1 bit (or less) of information that does not
> depend on the datarate one uses for sending packets.
> It does depend up on the time a card spends for doing
> carrier sensing (Clear channel assessment time, that
> determines the "Energy per bit" for the carrier
> sensing operation), and depending on implementation,
> there could be some weak dependence on the datarate.
> 
> However, it is quite true that tx range will be a
> decreasing function of datarate, and also a decreasing
> function of packet length. But I never found a
> definitive document in literature that relate
> datarate, tx range and packet length quantitatively.
> Maybe you can carry out some experiments with real
> cards, like Cisco aironet 350, and let us know your
> findings.
> 
> 
> ----------- Original Message ---------
> 
> DATE: Thu, 17 Jun 2004 16:45:50
> From: ns user <ns2user2000 at yahoo.co.uk>
> To: raysaikat at lycos.com
> Cc: ns-users at isi.edu
> 
> >Hi Saikat,
> >
> >Agreed with what you said and extending the same
> >question.
> >
> >We keep Tx range = 250 and Carrier sensing range =
> 550
> >
> >for granted for all datarates 1,2,5,11 Mbps.  How
> >correct is that ???
> >
> >
> >According to Shannon's law, the distance decreases as
> >rate increases...right??
> >
> >
> >
> >
> >
> >--- Saikat Ray <raysaikat at lycos.com> wrote: > 
> >> That is carrier sense threshold. Any real card can
> >> sense carrier at a much lower SNR level than what
> is
> >> required to correctly decode a packet. NS makes a
> >> conservative (but definitely not unrealistic)
> >> estimate: -65 dBm receive power is required to
> >> decode a packet, and (about) -90 dBm receive power
> >> is required for carrier sensing. Given the tx power
> >> of 100 mW (20dBm), and the default pathloss model
> >> used, the ranges comes around as 250m and 550m. So
> >> these numbers are not unrealistic, and the "waste"
> >> of "bandwidth" is the cost of using CSMA.
> >> 
> >> ----------- Original Message ---------
> >> 
> >> DATE: Wed, 16 Jun 2004 11:02:29
> >> From: "ronan de renesse"
> >> <ronan.de_renesse at kcl.ac.uk>
> >> To: <ns-users at ISI.EDU>
> >> Cc: 
> >> 
> >> >
> >> > 
> >> > 
> >> >-----Original Message-----
> >> >From: ronan de renesse
> >> [mailto:ronan.de_renesse at kcl.ac.uk] 
> >> >Sent: 16 June 2004 10:44
> >> >To: 'manet at ietf.org'
> >> >Subject: transmission range distCST_
> >> > 
> >> >Hi everybody,
> >> > 
> >> >I'm currently working on bandwidth calculation
> >> under ns for QoS Routing.
> >> >In wireless mode, the default transmission range
> >> for a node is supposed
> >> >to be 250 meters.
> >> >So why distCST_ is set to 550 meters by default?
> >> >It seems to me that every node hears whatever
> >> happens 550 meters around
> >> >it, but doesn't accept anything coming from a node
> >> outside 250 meters
> >> >range.
> >> >The problem is that data traffic will be heard 550
> >> meters away and
> >> >therefore will use bandwidth within 550 meters
> >> range instead of 250
> >> >meters.
> >> >This decreases performances for an illogical
> >> reason.
> >> > 
> >> >Does anyone could clarify why distCST_ is set to
> >> 550?
> >> >Would it be possible to set it to 250 instead? Is
> >> it realistic?
> >> > 
> >> >Thanks,
> >> >Ronan
> 
> 
> 		
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