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bradyr
Columbia College IT
Premium
join:2008-10-27
Sonora, CA

[Signals] signal level question (theories/etc)

Click for full size
my comcast tap
So I'm not having any issues with my service, so this is more me just trying to educate myself on how signals levels work on the plant/in the home.

Ever since I've had my service active (just comcast HSI, no other services) I keep an eye on my signal levels and they have pretty much been rock solid since day one (as reported on a Motorola sb-6120):

-Upstream channels (two, bonded) are 47-48dBmV
-Downstream channels (4 bonded) were 0 to +2, up until recently they are now all +3 to +5 depending on time of day/temperature (assuming there was an adjustment on the plant to cause the "hotter" downstream/forward signal)

SNR is always 34-36dB downstream and I was told my upstream SNR was 34dB when I called in on another provisioning related issue.

my wiring is as follows:

tap--> ~50-75 feet of "TFC-T10" --> ground block --> 50 feet of RG6-QS --> modem. No splitters, "TFC-T10" cable was installed June 2011 and the last 50 feet is also brand new, too.

I'll attach a picture of my tap, which is directly off of what i believe is a "trunk" amp.

My questions are: Even though my signals are all green, is it safe to assume i'm on a high value tap, which is what is causing the (in my opinion) "higher" upstream power levels? I used to think my signal was "weak" since i was ~0 to +1 on downstream with zero splitters and a short run to the modem, If i were to add a single splitter to add tv services or voice, i would instantly be "borderline" on upstream and a little negative on downstream (factoring the 3.5 dBmV loss on a splitter)

With my limited knowledge and self education, it's easy for me to speculate that comcast should replace the tap with a different value tap, or do some sort of adjustment on the plant, but hey, nothing is wrong with my service so if it's not broke, don't fix it. right?

Based on the design in my neighborhood, the only other person who could potentially be fed off my tap is just one of my neighbors. I suppose if they ordered triple play, it would force comcast to look at the design because my neighbor (in theory) would have a weak signal, too...

Anyways, just trying to educate myself. Looking for input/opinions/advice from anyone who knows how the plant works..

i'm guessing based on other info i've read, i can calculate my upstream loss by doing like 18-20 loss on the plant, plus the value of the tap (picture shows maybe a 23v tap?) plus loss over distance (100-125') gets me right at around 45 or something.

i also welcome any criticism if i'm dead wrong about anything i've typed...



bradyr
Columbia College IT
Premium
join:2008-10-27
Sonora, CA
Click for full size
comcast amp & tap
oh just for fun, here's another pic of that amp/tap, for those who like to look at this sorta thing... that lonely cable coming off that tap is my hook-up.. my neighbor was hooked up but must've discontinued the service, hence the other open but terminated port on the tap (or whatever you want to call it, in the business)

cooperaaaron

join:2004-04-10
Joliet, IL
That open port is supposed to be capped....maybe they will get to it in 2012, lol


gar187er
I do this for a living

join:2006-06-24
Dover, DE
kudos:4
reply to bradyr
tap right off an amp = higher upstream....

and no they wont drop the tap value...its designed to a certain level....changing that tap would raise your downstream as well....not alwasy a good thing....
--
I'm better than you!


bradyr
Columbia College IT
Premium
join:2008-10-27
Sonora, CA
at what point would there need to be a design change on the plant at this tap? like if I subscribed to triple play and needed to split the signal multiple times? or would there be a different solution at the customer premises level?


bradyr
Columbia College IT
Premium
join:2008-10-27
Sonora, CA
oh and can anyone tell me what exact value tap that is in the picture i posted? assuming you can tell from the picture. it says "32F / 23R" I dont think my camera will zoom in any further from the ground to read any other numbers on it..

just curious...

ExoticFish

join:2008-08-31
Stuarts Draft, VA
reply to bradyr
Why would there need to be a design change to add triple play ? All you need to do is call and add it.
--
»www.TheExoticFish.com
»www.ImTan.info


bradyr
Columbia College IT
Premium
join:2008-10-27
Sonora, CA
what i mean is if your signal power levels are like 3-5 dBmV, downstream and 47-48 dBmV upstream, at the grounding block, say you wanted to get triple play and needed to split the signal 3 or 4 ways. what i'm saying is after splitting the signal that many times if now you're out of spec (possibly), the design change question comes into play because the signal you're being pushed from the tap is now possibly out of spec (or no wiggle room left at all...talking about upstream) after splitting it for the service you ordered.

i just remember reading about this on a old thread around here somewhere. line techs are getting involved to help tune the plant due to there not being any wiggle room to add extra services (sometimes).

mrschultz02

join:2007-09-10
Media, PA
You would just put a 2 way splitter first so the modem is on one side of the first split, you should have a good enough signal for that, then use the other line and split it how ever many times you need for the TVs.


gar187er
I do this for a living

join:2006-06-24
Dover, DE
kudos:4
reply to bradyr
32f/23r means its a split value......

32 on the fwd...23 on the rtn.....

if you have at the GB, there wouldnt need to be a design fix, they would need to fix your drop, or balance levels out.....

system design is based off approx number of outlets in a house....around here they used 3.....more rural area, not too rich.....some newer areas are 5-6 outlets per house......thats not to say you cant make one work and not the other.....first split to modem, then house amp, and splitter for the rest of the outlets....
--
I'm better than you!


bradyr
Columbia College IT
Premium
join:2008-10-27
Sonora, CA
learn something new every day... so 32 on forward.. is that just because it's right off a amp? i take it coming right out of the amp, the forward signal is really hot? pardon my ignorance. 32 out of what? assuming that question even makes sense to you..


gar187er
I do this for a living

join:2006-06-24
Dover, DE
kudos:4
reply to bradyr
yes its cause its right off the amp......return levels arent affect by long spans cable as much as forward.......thus needing to lower the forward signal being closer to the amp...

the 32 signifies the loss that the tap has on the output to your drop....so if that amp out puts 38(ch2)/48(ch119) then out of that tap you would have 6(ch2)/16(ch119)....just an example....all areas run amps at different levels.....
--
I'm better than you!

JPnATL

join:2011-11-16
Bethlehem, GA
Reviews:
·Comcast
reply to bradyr
I think as long as it's working u may want to leave it at that because your transmit signal can be high from noise and the list goes on and on.when they say the cleaner the signal the lower the transmit power generally is would be clean meaning low noise.The CMTS sends a signal to the modem and then the modem sends one back and if the signal is not received the modem will transmit a higher signal until the CMTS gets a signal back at least that's how its suppose to work and all kinds of things can influence that from noise to the splitting of the signal to many times and the list goes on and on.My thinking is unless u plan on working for a cable provider then let them worry about that stuff because you have amps that can be out of balance man its their job to keep it maintained.


bradyr
Columbia College IT
Premium
join:2008-10-27
Sonora, CA
yeah i'm just trying to learn all that i can learn about how the system works, and how my own connection works and why it is as it is.

it's not broke and i'm not gonna try to fix it. by trade, i am in the tech/networking arena, so i do ...fixate over my line quality and signals as a lot of other people in this forum do, which is cool. It helps to keep an eye on things i did the same thing when I had DSL. it can help tremendously when it comes to troubleshooting issues.


Pay atten

@comcast.net
Start with this modem specs. Downstream +8 to -8 upstream 35 to 50. Your upstream is close to threshold but modems are designed to transmit up to 59 dbmv. This is the amount of power the modem has to produce to communicate with the CMTS in the headend. Now you have a 32/23 tap as explained earlier in previous post 32 is downstream and 23 is upstream, this is what is being attenuated at the tap. So a good rule of thumb for upstream is 20 dbmv plus tap value is what you should expect out of the tap. That starts you at around 43, then you have around 125' of coax add another 3 to 3 1/2 db. Around 46-47 pretty close to what you have. If tv's were installed you would split at ground block with a 2way one leg to modem other to additional split with amp, or use dc to additional split. And as far as fluctuating signal levels there are five factors to attenuation.
Size of the center conductor
Density of the dialectric material
Length of the cable
Frequency
TEMPATURE
So yes heat does cause signal fluctuations. Go luck with learning cable networks.


CableTool
Poorly Representing MYSELF.
Premium
join:2004-11-12
reply to gar187er
said by gar187er:

the 32 signifies the loss that the tap has on the output to your drop....so if that amp out puts 38(ch2)/48(ch119) then out of that tap you would have 6(ch2)/16(ch119)....just an example....all areas run amps at different levels.....

So the next tap value is generally cutting 2 or 3db less then the previous as the signal loss is greater. ( not familiar with how split values work)

So the inputs to the next tap would be say 46/36. Just 2db of attenuation/loss. It would be a 30 value forward cut. Levels out of tap? Still 16/6. So on and so on.

So "theoretically" the same levels would be out of every single tap down that run. Reality however though is the distance and attenuation between taps varies greatly. If there was a longer distance you would skip that 30 and go with a 28.

We do not use split value taps in my area so Im guessing at their steps/values.

But then you attenuate MORE on the high end so at the end of the run your higher freq levels are a bit lower and you are generally looking at minimal tilt between high and low pilot channels.

Generally we have 26,23,20,17 etc dropping in increments of 3. Same loss on forward and return in my area.

A quick word on Upstream levels while we are at it-
There definitely is a comfortable spot to be in but technically speaking, and this could be debated or argued, higher is better. ( within tolerance) 47 is perfect to me and I wouldnt mess with it.
The higher your uspstream the more "padding" you have between your traffic and the general noise in the system.
In theory though.
Shitty signals and various issues can affect a load.
--
CableTechs.org/"Horrible People with Integrity"

ctggzg
Premium
join:2005-02-11
USA
kudos:2
reply to bradyr
said by bradyr:

what i mean is if your signal power levels are like 3-5 dBmV, downstream and 47-48 dBmV upstream, at the grounding block, say you wanted to get triple play and needed to split the signal 3 or 4 ways. what i'm saying is after splitting the signal that many times if now you're out of spec (possibly), ...

A 4-way split is -7 dB. Even if your level was 0 now, -7 or -8 wouldn't be a problem. I wouldn't call it "borderline" until it was lower than -12. My modem has gone as low as -20 in the past before showing any symptoms.


gar187er
I do this for a living

join:2006-06-24
Dover, DE
kudos:4
reply to CableTool
we run straight values, but every now and then our contractors cut in 26/23.......our system would cripple itself if we used 46/36.....hell we only run 45/35fwd and 20rtn....
--
I'm better than you!