🚀 go-pugleaf

"Randy S." <rswitt@ufl.edu> wrote in message news:3A7B76D4.AA1FFC9F@ufl.edu...
| > >So the only way a DSL line will be subject to an overloaded
| > >upstream channel is if the provider uses too high an asymmetry ratio (I think
| > >we calculated that the effect would be seen starting at around a 10:1 ratio,
| > >and be very serious at around 15:1).
| >
| > High ratios of downlink to uplink decrease (not increase) the risk of
| > saturation.  I presume you actually mean "oversubscription rate" (rather
| > than "asymmetry ratio").  Higher oversubscription rates would of course
| > increase the risk of uplink saturation, but downlink saturation would
| > probably occur first.  BTW, I understand that "oversubscription rates"
| > for consumer DSL are commonly on the order of 100:1.
|
| Oops, now I'm lost again.  From what I understand, the uplink saturation happens
| (or CAN happen) when a large imbalance between uplink and downlink bandwidth
| exist.  I.e. your download speed is fast enough that the return ACKs will overload
| the bandwidth of the much smaller uplink.  So when I say 10:1 asymmetry ratio, I
| mean that the download bandwidth is 10 times the rate of the upload bandwidth.  In
| back of the envelope calculations I seemed to see that when downloading at the
| maximum rate and with a normal TCP receive window (say 32 or 64K), a connection
| with an asymmetry ratio of 10:1 would start to have delayed downstream packets due
| to late ACKs because the upstream channel couldn't send them fast enough.  An
| asymmetry ratio of 15:1 would drag you to a standstill.  This is obviously not the
| ONLY thing that can saturate the upstream link, but it would seem to be the main
| danger for uplink saturation for DSL.  Most DSL asymmetry ratios would seem to not
| be high enough to see this problem (1.472 Mbps down/256 Kbps which is common is a
| 5.75 ratio), but I've seen offers of DSL at rates as asymmetrical as 640 Kbps down
| / 64 Kbps up which is in danger territory.  Cable, as you point out, can saturate
| the upstream link in other ways.  Also as you point out, upstream of the DSLAM, DSL
| would seem likely to saturate it's downstream link well before it ever saturated
| it's upstream link .
|
| > >Conversely, since cable aggregates the
| > >channels along each segment, it can overload the upstream channel even IF each
| > >individual asymmetry ratio is not too high.
| >
| > The shared nature of cable is the killer because (1) the aggregate
| > uplink capacity is typically much less then the aggregate downlink
| > capacity [unlike DSL, where the aggregate is usually symmetrical] and
| > (2) saturation of the shared uplink kills the downlink for everyone on
| > the segment.
|
| Yes, this was the point you made that really caught my attention, and made LOTS of
| sense.
|
| Randy S.

So, a couple of kids on a "segment" using Napster for instance, downloading a few tunes
and at the same time other users gettting tunes from them, could really slow or "kill"
that whole segment for all that are on it.  Right?

Michael
 mgbaker@bellsouth.net
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