🚀 go-pugleaf

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

In <3A7C3C37.3A68AD2C@ufl.edu>, "Randy S." <rswitt@ufl.edu> wrote:

>I think John is saying that this is MUCH more likely to happen
>on Cable due to the shared ASYMMETRICAL upstream channel.  At least by the time
>the DSL upstream channel is shared, it's symmetrical.

Correct -- that's the big difference.

>Yes, this is the fallout of our MUCH earlier TCP asymmetry discussion (which
>predated this thread).  The consequences of this are STILL raising new issues,
>such as the one John pointed out.  I don't think the Internet tech community
>had much considered the effect of asymmetry on Internet traffic before just
>recently.

I think that's right -- the earliest reference I've seen is:

   H. Balakrishnan, V. N. Padmanabhan, R. H. Katz, "The Effects 
   of Asymmetry on TCP Performance", Proc. ACM/IEEE Mobicom, Budapest, 
   Hungary, September 1997 

>> I suspect that few considered the impacts of upstream saturation on ADSL
>> throughput...or ADSL downstream traffic saturating the reverse channel
>> of their T1 customers...hmmmmm.
>
>I wonder if any Network engineers responsible for backbone maintenance are
>reading this.  I bet their blood pressure just went up! ;-)

I don't think so -- symmetric links don't have the asymmetry problem.
Simple pipe congestion is well understood and relatively easy to manage.

Let's assume a DS3 pipe of 45 Mbps in both directions, and further
assume that (say) webhosting customers are using 40 Mbps uplink and 5
Mbps downlink.  Now let's marry that to ADSL with an expected profile of
5 Mbps uplink and 40 Mbps downlink, thereby running the pipe at 100% in
both directions.

What happens if uplink demand from ADSL users exceeds 5 Mbps, say 10
Mbps (bringing total uplink demand to 50 Mbps)?  Probably nothing to
webhosting, because a competent ISP should have reserved 40 Mbps for the
webhosting customers, in which case the ADSL customers would see a
slowdown of 50% on the uplink.  That might well degrade ADSL downlink,
but the degradation would not exceed roughly the same percentage (and
would probably be less), since the DS3 pipe is symmetric.

Even if the ISP did not reserve capacity for webhosting, and webhosting
competed with ADSL for uplink, the degradation would be no more than
about 10% (50 / 45), as compared to the 80-90% degradation that occurs
on asymmetric consumer service.

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