As I work for a cable company and am a Network Engineer, I can attest that
sub-optimal modem levels impact speed.  However, this is dependant on the
modem.  (Some seem to handle a less clean plant.)  Essentially you are
looking for approximately 40 to 50 dB on the Reverse and between 5 to -5 dB
on the Forward.  The further you are from those ranges, the more likely you
are to accumulate errors, dropped packets, retransmits and other networking
headaches which chew up your bandwidth.

Amazingly, that's why those little numbers on splitters and pads are
important.  They actually add up to something.

Except for a directional coupler, every split in a splitter accounts for
approximately 3.5 dB.

2 way = (2)3.5dB
3 way = (1)3.5dB, (2)7dB
4 way = (4)7dB

As you can see to make a 3 way, you need to split one that is already split.

These help pad and adjust the levels in addition to thelevels coming into
the house. (Incoming levels could vary depending on equipment at the
street.)

Now, if you have a dirty plant where signal to noise is down at 20dB, a 35dB
reverse could be impacted by the noise even though DOCSIS modems supposedly
can handle a reverse down at 8dB to 58dB.  That's the difference between a
test bench controlled environment and the real world with snow and cold
weather, cars hitting polls and peds, humidity, and a billion other
variables.

I think you'll now see, cable is NOT either ON or OFF.

============================
Daniel Rysztak, CCNA

-----Original Message-----
From: tclug-list-bounces at mn-linux.org
[mailto:tclug-list-bounces at mn-linux.org]On Behalf Of Jack Ungerleider
Sent: Saturday, February 07, 2004 10:49 PM
To: TCLUG Mailing List
Subject: Re: [TCLUG] New Comcast speeds


On Saturday 07 February 2004 02:56 pm, David Phillips wrote:
> Jima writes:
> >  My immediate guess is that maybe the coax feed to your cable modem is
> > sub-optimal.  I'm not a cable expert, though.
>
> Cable is either on or off.  A bad signal (often caused by too many
> splitters) will cause failure, not lower speeds.

The way I understood it from the guy who did the cable check for my cable
modem in Duluth was that there are optimal signal voltage levels for the
data
channels. If the signal is outside that level the modem can have trouble. To
me that implies the possibility of sub-optimal performance.

But like Jima I'm no cable expert.

--
Jack Ungerleider
jack at jacku.com


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