[e2e] VJCC vs. Keshav
Jon.Crowcroft at cl.cam.ac.uk
Thu Apr 18 05:53:08 PDT 2013
In missive <516FE1DA.9050708 at web.de>, Detlef Bosau typed:
>>Yes. And being short and frankly: When it comes to mobile networks, I
>>don't believe a word of the packet-pair and packet-train stuff.
>>Even for the radio interface, it simply doesn't make sense to talk about
>>an "average bit rate" in this case, when this is associated with both, a
>>reasonable hight throughput and a reasonably low block corruption rate.
this depends on the model and simulator - if you model the signal
propagation and do it with fine grain enough discrete event rate, then
you can work out the fate of packets based o nthe fate of the signal,
and the bits in the packet - some simulation models let you do this,
but usually you simplify to make sure the simulations can run in human
lifetimes, if you are interested in packet level results, so then you
reduce the physical and link layer details to averages/aggreagtes and
run a black box sim for them that produces the overal right stats...
>>However, you can well implement packet pair and packet train stuff and
>>observe some funny numbers in real networks. That's nice, other people
>>collect stamps..... .
and some people build simulations that aren't wrong.
>>But when we put those numbers in c-code and derive "serialization
>>delays" from them, the result is simply fictional.
and some people put models in matlab (or other toolkits) and generate
code that produces the right aggregate results and use that code as a
module in a higher level simulator ...as above..
>>It took me years to learn this lessen and perhaps, it was one of my most
>>difficult lessons ever.
>>So, the question is: What is the reality, which really corresponds to
>>When I started my work in a project called COMCAR 13 years ago, and I=20
>>talked about QoS, the EE colleagues ridiculed at me "You dealt with=20
>>wired networks up to now, right?" and a colleague from CE simply=20
>>answered my question concerning bit error rates simply and harsh: "There=20
>>is nothing like bit error rates."
>>What I learnt meanwhile: Both were right.
>>And as there is no such thing like a "bit error rate", there is no such=20
>>thing like a (more or less or at least quasi more or less stationary)=20
>>And therefore, all this packet pair stuff and packet train stuff is
>>interesting stuff inspired by the behaviour of wired networks, but with
>>respect to mobile networks, we talk about pure fiction and artefacts here.
you could use packet pair to investigate whether the delay in a MAC
Layer (RTS/CTS or CPOD) was giving you indication of contention for the
media - this would be geniune measure for some wireless nets - or you
could map collisions into contention (and threfore L2 congestion )
also for a scheduled mac (TDMA) , you could just measure the delay
caused by the number of session demands made to the scheduler...
gosh, i think we did...
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