Topic 'Signal Levels' in forum 'Cable users' - dslreports.com http://www.dslreports.com/forum/Signal-Levels-23054189 en Fri, 10 Feb 2012 17:49:18 EDT Fri, 10 Feb 2012 17:49:18 EDT Re: Signal Levels http://www.dslreports.com/forum/Re-Signal-Levels-23538051 To clarify this response, the headend is just a termination spot. the system design determines the injection level of the return plant. Most manufacturers have a noise figure of around 15dbmv on the return hybrid within the amplifier, therefore the injection to set up the return is determined by this noise figure. most cable companies choose to go a little above the noise figure, typically around 17dbmv to make the amp more efficient. so to correct your mistake, yes the headend requires a 0 dbmv input to the cmts, but system design determines the amount of output a modem would have to get there. ]]> http://www.dslreports.com/forum/Re-Signal-Levels-23538051 Wed, 23 Dec 2009 21:30:48 EDT Re: Signal Levels http://www.dslreports.com/forum/Re-Signal-Levels-23533156 said by cbt:

i think where you are misunderstanding is that there are more amps in between that modem and the cmts that just one in the modem.I am not misunderstanding anything. Of course, you are right, there can be multiple line extenders between the modem and the CMTS.

said by cbt:

and the headend does not tell the modem where to transmit, system design determines this
false. the headend does not necessarily say what absolute power to transmit because it doesn't know definitively how much gain there is or isn't in the reverse path. It CAN, and DOES say, increase/decrease gain.

Why do you think different modems transmit at different levels? Cable modems generally have 60dB of power adjustment. (I think the DOCSIS spec requires at least 56dB of range. How do you think they know how much power to transmit? Do you really think they GUESS?? Or do you think they all transmit the same power and the guys that write the spec put ridiculous amounts of gain control in just to make it tricky?

This thread is getting old... And I'm outta here. Good luck figuring it out guys...]]> http://www.dslreports.com/forum/Re-Signal-Levels-23533156 Tue, 22 Dec 2009 21:43:59 EDT Re: Signal Levels http://www.dslreports.com/forum/Re-Signal-Levels-23532981 http://www.dslreports.com/forum/Re-Signal-Levels-23532981 Tue, 22 Dec 2009 20:57:54 EDT Re: Signal Levels http://www.dslreports.com/forum/Re-Signal-Levels-23376911 http://www.dslreports.com/forum/Re-Signal-Levels-23376911 Sat, 21 Nov 2009 21:28:25 EDT Re: Signal Levels http://www.dslreports.com/forum/Re-Signal-Levels-23376904
Nearly every modern communication system in the world has this feedback going for it today. Including your cell phone.. Did you know that the cell tower tells your cell phone how "loud" to transmit? Did you know this occurs many multiple times per second? Did you know the whole CDMA cell network would fall apart if this weren't possible?

Do some research. It's actually not that hard to understand.]]> http://www.dslreports.com/forum/Re-Signal-Levels-23376904 Sat, 21 Nov 2009 21:26:36 EDT Re: Signal Levels http://www.dslreports.com/forum/Re-Signal-Levels-23376718 said by mjr:

Well, we can agree to disagree. As I stated before, the headend sets the modems transmit power. If it were too low, the headend would simply request more power, and the reverse amp would gain up, and provide it. I don't believe you understand how the reverse path works.
I think he has a better understanding than those who think the headend controls a reverse amp in the field. An amp does not adjust itself to accommodate one individual modem. If it is adjusted, it adjusts for the whole leg of that part of the system after it and affects every single modem in that line. And it is definitely not remote controlled by the CMTS or anything in the headend. I am sure there a lot of maintenance techs who only wish this was true.

I will give you one thing, though: I do like the concept of your idea. If it really did work the way you say it does, it would mean a lot less work for those techs who maintain the plant and a whole lot less trouble calls for those modems which would automatically adjust their upstream levels to compensate for a rising noise floor. If you could market that, it would be revolutionary. ]]> http://www.dslreports.com/forum/Re-Signal-Levels-23376718 Sat, 21 Nov 2009 20:32:29 EDT Re: Signal Levels http://www.dslreports.com/forum/Re-Signal-Levels-23252182 said by cypherstream:

If the upstream is too low, wouldn't the CMTS just request the modem to Tx a little bit higher? If it's working stable, then the CMTS most likely hears the signal loud and clear. If not it should say "Hey modem, can you talk a little bit louder, I can't hear you over all this noise!"

That's not really how it works. The modem transmits at whatever level is needed to overcome the "resistance" or loss on the way back to the CMTS. In other words, the level drops as it goes, until it is zero when it hits the CMTS. No matter what you put in it's way plant-wise, the CMTS will request that the modem increase it's power level until it reaches the equilibrium state where it will "use up" all the power before it hits the CMTS. If you put too many splitters between your modem and the CMTS, it will request a level higher than your modem can transmit and you will lose connection. So in that way, the CMTS *does* set the transmit level, but it's not a set value. It's a whatever it takes value.

Noise, on the other hand, doesn't care about anything. It is what it is. And when what it is become a value too high in relation to your transmit signal level, you have problems. The CMTS can't request more signal, because it doesn't really set the request, the plant does. So if the Tx level is 29, and you'd like to see 43 db Carrier to Noise or better, noise in the -15 range is going to be a big problem. Given the Funnel Effect and cumulative nature of the return path, that is easily reached.]]> http://www.dslreports.com/forum/Re-Signal-Levels-23252182 Tue, 27 Oct 2009 23:17:34 EDT Re: Signal Levels http://www.dslreports.com/forum/Re-Signal-Levels-23204988
In my experience I've seen most modems in the 40's, but at a buddies house his is in the mid 30's, but get this, he gets 7mbps upload on Comcast at 16QAM, whereas in my house with a 0 downstream /49 upstream level I get about 3.5 mbps.]]> http://www.dslreports.com/forum/Re-Signal-Levels-23204988 Sun, 18 Oct 2009 23:05:15 EDT Re: Signal Levels http://www.dslreports.com/forum/Re-Signal-Levels-23201663 http://www.dslreports.com/forum/Re-Signal-Levels-23201663 Sun, 18 Oct 2009 02:38:44 EDT Re: Signal Levels http://www.dslreports.com/forum/Re-Signal-Levels-23160616
Having "the lower the better" attitude isn't always better. It is way too low to the noise floor. The CMTS will of course tell the modem to range even higher output if it cant "hear" it, but that low to the noise floor, how many times does it not hear it? Several times a minute?

We call that a modem that is "flapping", which is continuously ranging for proper output, and will cause slow and intermittent connections.]]> http://www.dslreports.com/forum/Re-Signal-Levels-23160616 Fri, 09 Oct 2009 21:39:23 EDT Re: Signal Levels http://www.dslreports.com/forum/Re-Signal-Levels-23086184
"The easiest way to improve the carrier-to-ingress ratio is to make certain that all upstream transmitters in modems and set-top boxes are operating in the upper end of their transmit power ranges."

"Forcing transmitters into the upper end of their power ranges maximizes carrier-to-ingress ratio at the point where most ingress gets into the plant (i.e., at the subscriber drop).

»www.cedmagazine.com/Article-reve···n-1.aspx]]> http://www.dslreports.com/forum/Re-Signal-Levels-23086184 Sat, 26 Sep 2009 10:22:12 EDT Re: Signal Levels http://www.dslreports.com/forum/Re-Signal-Levels-23085935
To the original poster, if you're having a problem, please let us know what the problem is. If you're just inquiring about whether the levels look ok, you have a few responses from a few different people to make your own determination. My opinion is expressed in my previous post.

Good luck. ]]> http://www.dslreports.com/forum/Re-Signal-Levels-23085935 Sat, 26 Sep 2009 08:27:46 EDT Re: Signal Levels http://www.dslreports.com/forum/Re-Signal-Levels-23085009
Running at a low TX will make your connection susceptible to any fluctuation in noise on the line between your modem and the cmts. Keeping it above a 38+ on the return will generally keep you away from this situation and allow you a more trouble free experience. When you are down around 30, it won't take much in the way of noise to knock your modem offline. You are very close to the noise floor on the node, if you went any lower or the noise came up on the line, your modem would be drowned out / not "heard" by the headend/cmts any longer. (borrowed from another post I made)
--
Tech at the Beach.
I speak for myself, not my employer.]]> http://www.dslreports.com/forum/Re-Signal-Levels-23085009 Fri, 25 Sep 2009 22:44:35 EDT Re: Signal Levels http://www.dslreports.com/forum/Re-Signal-Levels-23084938
I don't think there's anything to fix, at least from what was provided by the OP. (The OP also didn't indicate whether he is having a problem.)]]> http://www.dslreports.com/forum/Re-Signal-Levels-23084938 Fri, 25 Sep 2009 22:29:26 EDT Re: Signal Levels http://www.dslreports.com/forum/Re-Signal-Levels-23072973 said by Chip:

said by beachintech:

SNR is fine, Upstream TX is below spec. You will be susceptible to any noise issues on the node at that low of a level. I would have a tech come out and fix it.
A low upstream tx could indicate a very clean upstream path. There's no way to tell without the upstream SNR
I never said it wasn't. I said it was below where a tech would want to leave it. If any noise were to come into the system, modems running at a low TX level will be the first to see issues and lost connections.
--
Tech at the Beach.
I speak for myself, not my employer.]]> http://www.dslreports.com/forum/Re-Signal-Levels-23072973 Wed, 23 Sep 2009 22:29:39 EDT Re: Signal Levels http://www.dslreports.com/forum/Re-Signal-Levels-23072214 said by beachintech:

SNR is fine, Upstream TX is below spec. You will be susceptible to any noise issues on the node at that low of a level. I would have a tech come out and fix it.
A low upstream tx could indicate a very clean upstream path. There's no way to tell without the upstream SNR
--
The three great strategies for obscuring an issue are to introduce irrelevancies, to arouse prejudice, and to excite ridicule--Bergen Evans]]> http://www.dslreports.com/forum/Re-Signal-Levels-23072214 Wed, 23 Sep 2009 19:53:20 EDT Re: Signal Levels http://www.dslreports.com/forum/Re-Signal-Levels-23059343 --
Tech at the Beach.
I speak for myself, not my employer.]]> http://www.dslreports.com/forum/Re-Signal-Levels-23059343 Mon, 21 Sep 2009 15:35:12 EDT Re: Signal Levels http://www.dslreports.com/forum/Re-Signal-Levels-23056469 homepage.ntlworld.com/robin.d.h.···goodlevs

Both look good
--
I tried to remain child-like, all I achieved was childish.]]> http://www.dslreports.com/forum/Re-Signal-Levels-23056469 Mon, 21 Sep 2009 00:44:40 EDT Signal Levels http://www.dslreports.com/forum/Signal-Levels-23054189
Is upstream Pwer too low

Downstream
Freq/Power: 699.000 MHz 4 dBmV
Signal to Noise Ratio: 36 dB
Modulation: QAM256

----------------------------------------------------------
Upstream
Freq/Power: 29.000 MHz 29 dBmV
Channel Type: DOCSIS 2.0 (ATDMA)

Symbol Rate: 5120 kSym/sec

Modulation: QAM32
--
- Bill]]> http://www.dslreports.com/forum/Signal-Levels-23054189 Sun, 20 Sep 2009 13:59:41 EDT