[e2e] Congestion control as a hot topic in IETF

Jon Crowcroft Jon.Crowcroft at cl.cam.ac.uk
Fri Mar 8 07:54:53 PST 2013

In missive <513A04BE.4060505 at web.de>, Detlef Bosau typed:

 >>> btw, path loss is very real (and not at the antennae)

 >>Or can you tell me where the waves have gone? Where the energy has gone? 

this is physics #101 - if you transmit with some power level, and spread that over a distance, guess what, at a
distance with a signal that is spreading out over a surface, the power level is lower

eventually the signal (to background noise) is too low level to carry any info in any way distinguishable

 >>Data corruption is a phenomenon which occurs at the receiver. The 
 >>problem is that the receiver cannot successfully rebuild a packet from 
 >>what he received. The air interface has no idea of which waves are 
 >>travelling along and whether they make any sense at all.

you're confusing interference with other sources and misreading the honorably Dave Reed

secondly, you are ignoring absorption (e.g. by water vapour which gets a little bit hotter) 
and also _self_ interferance (aka Ricean fading) and scattering (rayleigh fading)
 >>> - in freespace, with omnidorectional antennae its
 >>> a feature of inverse square law of spreading the signal over a speherical surface ...plus
 >>> there's atenuuation from signal energy being absorped (e.g. by water vapour or concrete) - etc etc

 >>that are wonderful formulae for the received power. They don't tell you 
 >>whether a packet will be successfully read. And that's why I said 
 >>formulae, which describe a "bandwidth" depending on the distance 
 >>base-station/mobile, I referred to "Modelling Computer Networks for 
 >>Emulation" by Rothermel, Herrscher, Leonhardi from 2002, are pleasant to 
 >>read, however the model is completely nonsense.

there are simple formulae for path loss - however you need to run them in immensely complex physical environments,
which is why most people rely on measurement...
 >>And I wonder, why no communication engineer and no signalling theorist 
 >>has made objections here so far, sometimes I think these guys simply 
 >>ignore us CS guys because we would continue telling nonsense here and no 
 >>one wants to spend his time in useless arguments. I thin, CE and EE guys 
 >>simply don't take us seriously.

well, i kind of have a physics degree as well as a CS masters and PhD so I beg to differ...
 >>> your mixing it up with interference with concurrent senders o nthe receive antennae (which is fixable using mimo and
 >>> smart processing - viz
 >>> http://conferences.sigcomm.org/sigcomm/2012/paper/sigcomm/p235.pdf

 >>I don't think so. In that paper, one discusses and uses an "effictive 
 >>SNR algorithm", is this correct?

 >>And where can you derive a throughput from the SNR? Do you mix up signal 
 >>power and data rate? If so, you simply misunderstood the Shannon-Hartley 

 >>You may wonder why I get a bit upset here. However, a whole research 
 >>project of mine yielded no results and it took years to understand, that 
 >>people simply pulled my leg here. I wasted 4 years of my life for this 
 >>research completely and years afterwards for indirect consequences.
 >>So, please let's spare discussions on elementary signalling theory.

this isn't elemntary signalling theory, its elementary physics...

once you know the power level at a receiver, then you can do shannon...
(and with multiple antennae, the mondodb clever stuff to extract max signal...)

but you still have to worry about propagation

if this wasn't the case, we'd all be toast because of all the incipient power on all of us from all the sources
around us adding to infinity 
(oh, ok there's quantum effects to limit that too;)


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