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[POSTED TO comp.dcom.modems.cable; PLEASE REPLY THERE]

In <3A7B76D4.AA1FFC9F@ufl.edu>, "Randy S." <rswitt@ufl.edu> wrote:

>> 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.

A higher ratio of downlink to uplink does of course increase the risk of
uplink saturation (since there is less uplink capacity available).

>I.e. your download speed is fast enough that the return ACKs will overload
>the bandwidth of the much smaller uplink.

There's usually enough uplink capacity to handles return ACK's
(notwithstanding actual uplink to downlink ratios of 1:7 on popular
websites, as seen in tests I've run).  The problem is generally
triggered not by return ACK's for downstream traffic, but by upstream
traffic (uploading).

>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.

With FTP, the ratio of returning ACK's to downstream traffic is usually
well over 50:1 in bytes (not packets).  The ratio is higher for popular
websites (HTTP), but such traffic tends to be bursty, which tends to
mitigate the adverse effects.

>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.

The main danger is uploading (e.g., sending out a large multimedia
file).

>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.

I believe that the most common ratio is about 1500/128 or 12:1 (e.g.,
SBC companies).

>Cable, as you point out, can saturate
>the upstream link in other ways.

The ways are the same.  What's different from DSL is that uplink
saturation affects *all* subscribers on the same segment.

>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 .

I think that's right.

-- 
Best regards,
John Navas     <http://navasgrp.home.att.net/>
     CABLE MODEM/DSL GUIDE:  <http://Cable-DSL.home.att.net/>