🚀 go-pugleaf

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

In <3A7B357B.3C2C1C79@ufl.edu>, "Randy S." <rswitt@ufl.edu> wrote:

>> >We're getting closer to the key metric to compare cable with DSL:
>> >   *Upstream oversubscription rate*
>>
>> I don't think so.  Upstream oversubscription is really an issue only on
>> cable, since (1) DSL has dedicated upstream to the DSLAM, and (2)
>> backhaul links are symmetric (as compared to asymmetric subscriber
>> connections).
>
>Hmm, I think I see what you're saying.  Since the head end of a DSL link is
>symmetrical, and only the individual links are asymmetrical, the upstream
>backbone links will nearly always be considerably more lightly loaded than the
>downstream links.

Correct.

>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.

>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.

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