dslreports logo
    All Forums Hot Topics Gallery


how-to block ads

Search Topic:
share rss forum feed


Tyler, TX

Upstream Power explained

Here's the short and skinny on Upstream power, the most important thing you can see without a meter.

Upstream power is basically how hard the modem has to push your signal to get back to the headend with a value of zero. So if it says 45 dBmV that means your modem is kicking out 45 dB's per millivolt of signal to get it back to the headend so it can hit the switches and all that whatnot.

Here's why this number is important. Every cable plant has noise. if you have a loose connector on your television, you will have ingress (noise insertion) into our cable system. Noise comes from everything, cell phones, toasters, your xbox, everthing! Here is an example, set your cell phone on top of your computer speaker, then make it ring, you will hear that buzz and tick right before it rings, that is interference (noise, ingress).

Okay now that we have noise figured out here is a scenario.

Your modem says 30 dBmV, that means your modem does not need to transmit very hard to get the signal out there. Here's what it also means, your signal coming out of the modem is not strong enough to overcome the noise in the system. Another problem in this scenario, when its cold it will be worse. This sounds stupid, but in this particular issue we need to "pad" your modem line, create resistance. When we pad it 10dB, we are actually forcing the modem to work harder with a little thing that looks just like a threaded connector (no you cannot buy these). This makes the modem push harder without dirtying (is that a word?) the signal quality.

Opposite side of the spectrum. You have a modem that is reading 58 dBmV. you have a bad modem or wiring that is not optimal. Your wiring can be good, but misconfigured at the splitter. SL installers are supposed to put the modem no farther than one 2 way splitter away from the groundblock. a 2 way has a value of -3.5db or 4 on each side. This knocks down your signal 3.5 db's obviously. Walmart and radio shack are not the place to get a splitter if you need one. Okay back to the 58dBmV, you have no headroom to go up and your modem is close to maxxing out. Motorola and Ambit modems start at 30 and max at 60. You have no headroom for temperature and your modem is just working to hard. Look at the wire, try to trace it back to the splitter, see if its on the lowest value side of the splitter, 3 ways ussually have a -3.5 leg and two -7's, 4 ways are most generally all 4 -7's. 6 and 8 ways are ussually -11 and require a house amplifier.

Your target value for Upstream power level is 50dBmV. Your modem can fluctuate up and down by 5 and it will push signal over the "noise floor" of the cable system easily.

Need for Speed
Winterville, NC
Thanks for the post...

My upstream power always seems to be like 41 or 42


Tyler, TX
41 and 42 are fine, that is actually a real world number. 50 is a perfect target, you have a fluctuation of at most 5 db's with temperature which would put you at 36-37 when its cold and 46-47 when its hot outside. when you get below 40 you may see alittle variance, under 35 or so, you are pushing it and may have sync problems, lights go solid for a second when they should blink and so forth, glitchy stuff.

Poway, CA
reply to SuddenLTech
Just curious here: If the modem is transmitting at 30 dBmv, and this isn't strong enough to overcome the system noise, I fail to see how inserting a 10dB attenuator will help things. True, the modem will now transmit at 40 dBmv, but the signal coming out of the attenuator will still be 30. If 30 dBmv coming directly out of the modem isn't sufficient to over-ride the system noise, how can 30 dBmv coming out of an attenuator be any more sufficient?

Saint Marys, WV

2 edits
reply to SuddenLTech
said by SuddenLTech :
Here's what it also means, your signal coming out of the modem is not strong enough to overcome the noise in the system.
Do you mean that the upstream snr is too low, and not the upstream power level?

If you're still talking about upstream power levels, it seems that if modem the were having trouble overcoming noise,
it would be transmitting at its maximum capacity (a theoretical 58+ dBmV).

said by SuddenLTech :
When we pad it 10dB, we are actually forcing the modem to work harder with a little thing that looks just like a threaded connector (no you cannot buy these).
You don't mean these, do you?



Tyler, TX
A pad is not an attenuator, it drops the incoming level so the modem see's a clean but low level, and it will up its own power to compensate resulting in a +10dBmV reading. A pad does nothing on the return path.

Upstream SNR is explained here »Cable Modems and Wiring Issues »Upstream SNR / Signal to Noise Ratio

Lookout, KY

1 edit
reply to SuddenLTech
You may also want to note that the requirements are differnt in every system.

Ambit modems Transmit at a max level of 61dbMv
Arris Modems Transmit at a max level of 58dbMv

You do not want to pad your modem lines as that may cause issues as in return levels and foward levels. Any passive equipment that your modem has to transmit through will require more power for your modem to return to the headend.

The same go's on the foward path lets say your drop comes off a 23 value tap (you should have around 14-16db hitting the ground block with that depending on what your plant is putting out) and hits a ground block with a attenuator or pad for 10db this will knock your signal down to 4DB of signal then lets say you add a three way your modem will be at 0 giving you no room for foward level changes your modem needs around -10/+10 anything over that on the modem channel is considerd over drive and your modem cannot get the information it needs For example you have +25 RX on your modem will have a harder time on getting information back and fourth. Over Drive also creates noise issues on the plant for example when the tempature drops the equipment requires less power to get signal to you and thus the equipment if not propperly setup will transmit hot for example AGC's not set correctly or a trunk amp set in manual.

You also need to look at the noise floor Anything below a noise floor of 17db is bad and causes issues on your return path and foward paths for modem. Modems will typically start to drop off around 10DB Signal to Noise levels. If the noise level on the cmts ports drop below a certin level we will have to go fix them Most systems you want to keep them above 20DB anything below that gives no margin for problems on the system

Three way spliters typically have a 3.5DB leg and two 7DB legs


reply to SuddenLTech
I really enjoyed your explination. Very nice! The only thing is that if you have a modem that reads a TX value of 58 dBmv it's not always the case of faulty wiring. Another cause could be a return power issue on the plant/network with possibly a return module in an amp or gain maker that has been set up wrong or failed and would make the modem(s) push harder to make it back to the head end at the correct configured value.


reply to SuddenLTech
Now I am curious- How would padding the forward have any affect on the return power? The forward and return systems are two different systems. On the forward system the CMTS/Routers are configured to Transmit at a certain level to reach cable modems at certain power values. Then on the return system the CMTS router is configured to have the cable modems in the field talk back at a certain power levels.