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Cable Modem Signal Levels
Questions and Answers
ALL INFORMATION ON THIS PAGE IS COPYRIGHTED CABLETVAMPS, 2002-2007

How do you know whether you need an active return amplifier? Is there a way to be reasonably sure that active return is the way to go? There is no way to be absolutely sure, but in most cases you can figure it out simply by looking at your cable modem. The cable modem can tell you a lot about the condition of the cabling or the signal coming into the house. The cable modem can display all of the signal levels at any point where it is connected.

Q. How do I access my cable modem to get diagnostics, power levels, and Signal to Noise (S/N) Ratio information?
A.
Most cable modems have a built-in diagnostics page at a fixed IP address. You can generally access this page by browsing to http://192.168.100.1. For our example here, we are using a Motorola Surfboard SB5100 cable modem. The Motorola Surfboard is one of the most common cable modems in use. When you browse to the cable modem diagnostics page, you should see something very similar to the picture below.

electroline amplifier

If you click on the Signal link at the top of the page, it will take you to the page with the important information, similar to the one below.

electroline amplifier

What is this page showing?

For diagnostic purposes, the cable modem measures and reports key information. Some of these are the Downstream Received Power and the Downstream Signal-to-Noise Ratio (SNR), and Upstream Power Level. The Signal-to-Noise Ratio is an indicator of how "clean" your signal is when the cable modem receives it. A high Signal-to-Noise Ratio indicates that the signal can be readily decoded, and information can be extracted without excessive errors. Even if the power level is high enough, your cable modem may have problems receiving data if the S/N Ratio is too low.

Q. What are acceptable Downstream power levels?
A. The Signal-to-Noise Ratio should be 30 dB or higher - the higher the better. As the Signal-to-Noise Ratio decreases below 30 dB, performance will steadily decrease and errors will increase. The cable modem may stop working properly if the Signal-to-Noise Ratio drops into the low 20 dBs.

Received Power: DOCSIS specifications require cable modems to function correctly with downstream power levels in the range of -15 dBmV to +15 dBmV. Power readings at or close to those extremes could be problematic and cause poor cable modem performance. However, when connected to a real-life cable TV network, a cable modem might only function over a narrower range. Each cable provider may specify a narrower range based on their own network design. So, even if your downstream received power may be within DOCSIS specification, your cable provider may have a tighter range required for proper operation. One thing to note here - different brands of cable modems may not work as well (or at all) at the power level extremes.

Generally if the downstream Signal-to-Noise Ratio is high enough, the downstream power level should be satisfactory. In the picture above, our cable modem is connected to an EDA-FT08100 amplifier, and it could not be happier.


Q. What are acceptable Upstream power levels?
A.
Upstream Power Level is an indicator of how hard the cable modem has to work in order to get the signal back to the cable company's headend. The cable modem's Upstream Power Level is actually set by commands from the headend so that the headend hears relatively the same signal level from all cable modems on that particular upstream channel ID. Since every cable modem is at a different distance from the headend, they will each have different return path signal characteristics - mainly the signal loss due to the cable length. In addition, the wiring inside every home may be different, and the point of installation for the cable modem makes a difference in the return path losses. That is why so often the cable company will install a 2-way splitter at the cable drop point, and run a dedicated connection for the cable modem. When they do this, they are assuring that the return path signal loss will be minimized for the cable modem. The better the return path is, the lower the upstream transmit power will be.

The Upstream Power Level will be within the range of +8 to +58 dBmV. During installation most cable providers will try to target a value below 55dBmv. Values in the 40+ dBmV range are the most common - and totally normal. According to DOCSIS speciifcations, cable modems cannot transmit at levels higher than +58 dBmV. Generally if the Upstream Power Level is pegged in the mid to high 50 dBmV range the cable modem is probably not providing satisfactory performance. If other problem symptoms are also present, an upstream transmit power of +58 dBmV would be one more valid supporting reason for requesting technical support from a cable ISP.

The Upstream Signal-to-Noise Ratio can be sensed only at the headend, so don't bother looking for this information. A cable technician would have to find it out.

The outdoor cable infrastructure is also affected by weather conditions. Often the temperature fluctuations between day and night will influence the cabling characteristics. You should expect to see small fluctuations in the power levels and Signal-to-Noise Ratios at different times of the day. If your power levels and Signal-to-Noise Ratios are marginal, they might fluctuate enough to cause the cable modem to stop working in certain weather conditions, and recover in others.

PRINT THIS PAGE AND FILL IN YOUR CABLE MODEM NUMBERS FOR YOUR REFERENCE.

 

My Cable Modem Signal Levels

Operating Range Desired Range

Problem Indicators

Actions

Downstream Signal to Noise (S/N) Ratio
50-1000MHz
Forward Path
  Above 25 Above 30

Signal to Noise Ratios below 30 can indicate signal strength problems. Anything above 30 should not give you any problems.

In the range of 30 or below, you need a forward path boost. Look at the return path signals to determine if you need an active return amplifier. All of the EDA 2000/8000 Series amplifiers will boost the forward path
Downstream Power Level
50-1000MHz
Forward Path
  -15dBmV to +15dBmV -10dBmV to ~+14dBmV Signal levels at either end of the range may be problematic. Generally you will not find signal levels that are too high unless you already have an amplifier installed. Signal levels lower than -10dBmV may indicate the need for an amplifier. If higher than -10dBmV, it should generally be OK. If the signal is at or near +15dBmV you probably need to remove or bypass your amplifier. the signal is too strong.

If the signal is below -10dBmV, installing an amplifier will likely help.

Upstream Power Level
5-42 MHz Return Path
  +8 to +58dBmV   Below 50, the cable modem is operating at a normal level. Above 50, the cable modem is operating at the higher end of it's power range. At 58dBmV the cable modem is "screaming" and not operating at an optimal level. Levels above 50 indicate that an Active Return amplifier may help.

Levels above 55 - an Active Return amplifier is highly recommended.

Q. OK, I have all the numbers from the cable modem, now what do I do?
A.
Compare your cable modem signal levels to what we have listed here.

If the Downstream Power Level or Signal-to-Noise Ratio are too low, you may simply need an amplifier which boosts the Forward Path (50-1000MHz), with passive return. this would generally be an EDA-2100 or EDA-2400, placed at a point in front of the cable modem so that the signal level to the cable modem is increased.

If the Upstream Power Level is 55dBmV or higher, your cable modem may be having difficulties maintaining a reliable connection back to the cable company's headend. If the cable company is unwilling or unable to fix the problem for you, then an active return amplifier (EDA-FT08100) is generally recommended. Connect your cable modem directly to the EDA-FT08100

Special considerations: It is possible that you might have a very high Downstream Signal-to-Noise Ratio, while having a high Upstream Power Level. This can occur when a passive return amplifier is already installed and the return path has too much loss to propagate the return path signals reliably. In this case one of two actions can be taken.
1) The existing passive return path amplifier can be replaced by an active return path amplifier. This effectively keeps the forward path at a boosted level while boosting the return path simultaneously.
2) An Electroline EDA-ERA4100 amplifier can be installed at a point in the wiring between the cable modem and the initial connection point from the cable company. The ERA4100 boosts the Return Path only without affecting the forward path.

Q. How can I use my cable modem as a general diagnostic tool, to check my signal levels in my home?
A.
Your cable modem can be used as a generic signal meter, by connecting it at different points in your home and observing the Downstream Power Level. The FCC mandates that the cable company provide a signal level of 0dBmV at the point of entry, which should be enough to connect 4 TVs via a splitter without seriously degrading the picture. If you connect your cable modem directly to the incoming cable from the cable company (without any splitters in the way) the cable modem will give you a fairly accurate reading of your incoming signal level. If the Downstream Power Level is a negative number, your cable company may not be providing enough signal.

After observing the signal level at your home's entry point, you can then connect the cable modem at various points in your home to observe the changing signal levels and strength. The downside is that if the signal levels are too low for the cable modem to "lock", you might not be able to get reliable readings from the cable modem diagnostic page. One other small problem - in order to get to the web page we showed you above, the cable modem has to be connected to your home network or perhaps a laptop computer.

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ALL INFORMATION ON THIS PAGE IS COPYRIGHTED CABLETVAMPS, 2002-2007