Re: cable guy "vapor"-izes DSL 3X cable TV troll!

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#431

From: John Navas
Date: Sat, 03 Feb 2001 20:49

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

In <3A7C4718.CA70907F@ufl.edu>, "Randy S." <rswitt@ufl.edu> wrote:

>> The upstream channel can be anywhere from 160 Kbps (200 kHz QPSK) to 10
>> Mbps (3.2 MHz QAM 16).  My understanding is that 800 Khz QPSK is most
>> common in practice (due to signal quality and error rate issues), giving
>> an upstream channel capacity of 640 Kbps.  With 4 upstream channels per
>> downstream channel, the aggregate upstream capacity would be about 2.5
>> Mbps, as compared to downstream capacity of 27 Mbps.  Note that if the
>> upstream saturates, the downstream rate will drop to about the same
>> speed, a dramatic slowdown of about 90% (2.5 Mbps as compared to 27
>> Mbps)!
>
>Ahh, these look like much more realistic numbers than the ones I saw, thank
>you.  The document I saw was quoting maximum datarates per channel, which as
>you show here is 27 Mbps down and 10 Mbps up.  The up rate sounded too high
>to me, and the above statement points out why. Do most cable systems use a
>4:1 upstream to downstream channel ratio,

I don't know about "most" -- that's just my own experience, confirmed by
some references I've seen in the literature.

>and only allocate one channel for
>download?

What matters is the ratio of upstream to downstream, not the absolute
number of channels.  Simply scale my numbers for more channels.

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

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From: John Navas <spamfilter@navasgrp.dublin.ca.us>
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Subject: Re: cable guy "vapor"-izes DSL 3X cable TV troll!
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