static asymmetry

Ingo Flaschberger if at
Sat Sep 3 23:32:42 UTC 2011

>> In my opinion. Home networking (including personal clouds) have to change 
>> the brain damaged model of asymmetric tail technologies. Giving back the 
>> original peer-to-peer nature of networking the asymmetricity of the access 
>> technologies will not be tolerable in such a level (1:10) we have today. 
>> Maybe 1:2 should be more acceptable.
> I think a more fundamental question is why in 2011 we're stuck with 
> statically
> shaped asymmetric up and down. You can pretty dynamically shape *within* a 
> given
> direction to do just about anything you want to the traffic, but I don't know 
> of
> last mile access technologies that do that *across* the up and downstream. If 
> it
> were more like ethernet that doesn't have those artificial distinctions, this
> conversation would be moot.
> I recall the reason that DOCSIS is asymmetric is had a lot to do with how 
> they
> carved out spectrum of the analog channels -- and relegating upstream to 
> slots
> that weren't very good for those analog channels. That's been about 15 years
> ago though and in the mean time the internet has sort of become important.

With dsl technologies like vdsl (flexible) or adsl (fixed 1/8 or 1/24) the 
total bandwidth (up+down) is not linear.

adsl: 1mbit up, 24mbit down - total 25mbit
 	can not be used with 12.5mbit up/down.

at the co the noise is very high, as there are many lines in a bundle and 
the dslams "cry" with high signal levels into the lines.
Also the crosstalk is high.

 	co-side: dslams send signal with high level + high level noise.
 	cpe-side: signal arrives damped, noise arrives damped -> signal to
 		noise (snr) is acceptable.
 	high bandwiths can be achieved.
 	cpe-side: cpe send signal with high level, low level noise
 	co-side: high level noise produce crosstalk to damped signal
 		that arrives from cpe -> signal to noise (snr) is low
 	only low bandwiths can be achieved.

so dsl technologies, that use old, unshielded cables operate now at the 
maximum what the cable can do (up to 30MHz with vdsl2).
Higher speeds can only be achieved with better cables; like fiber or coax.

coax technologies use in oposite to dsl technologies no point to point 
links but bus technology to connect several customers to one head-end.
asymmetric bandwith -> more clients per head-end.

high-speed symmetric services can only be offered with new network types 
like fiber.

Kind regards,
 	Ingo Flaschberger

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