[e2e] Delivery Times .... was Re: Bandwidth, was: Re: Free Internet & IPv6

Detlef Bosau detlef.bosau at web.de
Thu Dec 27 01:05:36 PST 2012

Am 27.12.2012 00:01, schrieb dpreed at reed.com:
> In mobile networks (let's include wifi there) a packet is either
> reliably delivered - in unpredictable time.
> Or it is unreliably delivered - that is possible in predictable time.
> That is also true of wired networks.

Absolutely. However, in wired networks packet corruption is (hopefully) 
that rare, that the "unreliably delivered" packets are delivered 
"unreliably" with p_succ= 0.9999 or something similar. (Simply spoken: 
Corruption loss is typically ignored.)
> It is the fundamental constraint of packet networking.  At any level, 
> one can decide to choose either case.  Rather than "unpredictable", 
> one should say "unbounded stochastic".

E.g. in GPRS, the standard defines certain quantiles. E.g. for 1024 
bytes, we have certain delay classes:

                  mean                   0.95 quantil

1.:             < 2s                    < 7
2.:		< 15s			< 75s	
3.: 		< 75s			< 375s

I don't now whether "mean" refers to "average value" or "median".

Each class refers to some different sdu corruption probability. (I don't 
have the values in mind, may be 10^-3, 10^-5, 10^-9, however 10^-9 will 
ever be implemented.... ;-))

If you have a look at typical simulation systems, e.g. the NS-2, 
wireless interfaces are assigned a certain "rate". Which implicitly 
assumes a more or less stationary delay. Does that make sense?

Particularly, as the extremely high latency 375 s most likely refers to 
some "countless" repetition of packets (100 times, 1000 times....), 
which may indicate a situation where we should rather choose a different 
interface or a different route if possible.

Detlef Bosau
Galileistraße 30
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detlef.bosau at web.de                     http://www.detlef-bosau.de
The nonsense that passes for knowledge around wireless networking,
even taught by "professors of networking" is appalling.  It's the
blind leading the blind. (D.P. Reed, 2012/12/25)

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