10-13-2016 02:17 PM - last edited on 03-19-2017 02:46 PM by Julia_R
I keep seeing questions about this cropping up in various topics in and other sections of the forums, so I'd like to share my personal knowledge from an experienced employee perspective for common understanding. My hope is that this topic can help be a reference for other topics on these forums.
It's important to understand that while there may be other steps that can be done in troubleshooting, and that not every situation that matches what I write here may require a TC (troublecall; sending a technician out to repair), most situations that have critical signal levels do require on-site help from us. Determining & scheduling TCs are a case-by-case basis. Proper troubleshooting needs to be done. Nothing I say here ultimate supersedes that, nor should it be interpreted that way. I have reviewed this with my colleagues for added accuracy.
I am sharing this with purely the interest of helping others, I'm not on the clock or being otherwise incentivized to write any of this here.
(these mean the same thing, equivalent terminology)
Forward = Downstream = Receive = Rx
Return = Upstream = Transmit = Tx
Units of measure terminology:
dBmV = decibel millivolt = an exponentially-weighted measure of the signal's power in reference to 1 millivolt. The more out of range it gets, the exponentially more extreme that difference is! The decibel is used because it turns multiplication and division into addition and subtraction, making mental math far easier when doing estimations.
(Power)measured in dBmV = 20 * log 10 (RMS signal voltage in mV)measured in mV
Coaxial systems Charter/TWC uses (and same stuff Dish, DirectTV, Comcast, etc uses) is all "radio grade" (RG) wiring all have an impedance of 75 ohms.
The reference value for power of 0 dBmV is the power level for 1 mV (RMS) though 75 ohms of impedance.
MHz = mega hertz = millions of cycles per second, a measure of frequency and also a way of notating the centers upon which channels are using which frequency to communicate.
FORWARD SIGNAL GENERAL GUIDELINES:
(signal from the neighborhood’s node to the cable modem)
≥ +10 dBmV is critically high.
Although some models can handle up to +15 dBmV just fine. Most model modems and TV boxes handle up to +12 dBmV with no issues whatsoever.
That said, we don’t consider it “passing” if it’s much above +12 dBmV on the signal being received by the modem.
Thus, this is typically grounds for a TC (troublecall, a repair service call that you won't be charged for) unless the customer did something (like install an amplifier) that can be easily removed. This will likely require reconfiguring splitters or the amp in the home. It could also be a maintenance issue (a problem with the utility tap itself).
+10 dBmV to > +8 dBmV is borderline high.
This is almost always not service impacting, but it could also stand room for minor improvement. MERs/SNR may be impacted slightly, but service should still function.
It’s more of a risk at this range that if it fluctuates above 10 that there’s likely a risk of signal issues. A bit too much received signal is in fact better than too little signal, >0 (+) better than <0 (-), as it allows the communications to be better heard over any background noise on the line(s).
This is not valid grounds/reasoning for a troublecall/truck-roll.
+8 dBmV to -8 dBmV is ok
This is a good range, but the closer to the 0 dBmV balance point the better. Keep in mind the "perfect" balance point (which is 0 dBmV) is nearly impossible to realistically achieve. The "ideal" is -2 dBmV to +3 dBmV for receiving, but it's not realistic/sensible to expect a troublecall/truckroll if you're only a few decibel millivolts off from that..
And when it comes to a (+) or (-), it’s better to have a bit more signal (+) than a bit weaker (-). This is in terms of real-world performance, not lab conditions.
This is "normal"
< -8 dBmV to -10 dBmV is borderline low
This may or may not be "service impacting", but highly depends on what the customer reports with symptoms. Some people have no issues in this range, others do. Check for a splitter. If the signal is split too many times, it'll be too weak. This could be legit grounds for a troublecall/truck-roll, or it could not.
The main issue when people have service intermittency issues at this range isn’t really because the signal is too weak, but rather there’s usually some ingress-leak-related noise on the line. With a “quieter”/weaker signal, the noise is relatively a bigger issue.
<-10 dBmV is critically low, bad
This is typically grounds for a TC (troublecall, a repair service call that you won't be charged for) unless the customer did something like install a splitter than can be easily removed. Check for a splitter. If the signal is split too many times, it'll be too weak. If it's not an issue with equipment, then only an on-site TC will resolve this.
The signals below this are truly just too weak. It’s be like your modem is trying to listen to a whisper, and any background noise from an ingress leak from your home or your immediate neighbors is going to result in intermittent service issues.
Pretty much always a valid reason for a troublecall/truck-roll for received signal level(s) that are this low.
RETURN SIGNAL GENERAL GUIDELINES:
(signal from the modem, back up to the neighborhood’s HFC node)
> 52 dBmV is critically high
This is typically grounds for a TC (troublecall, a repair service call that you won't be charged for).
Many modem’s have sensors that cap out at 52.2 dBmV, so the actual reading could be even higher than what your modem can report!
Also, if you see some channels on a 4-channel DOCSIS 3.0 modem that are missing, and others that are at 52 or 53, then odds are the missing ones are well above the max threshold.
Occasionally this can be a modem issue (when the modulator is starting to fail), but most of the time it’s coupled with low forward signals and the real issue is that there’s just too many splitters in the pathway. If it's not an issue with equipment (replacing the modem), then a unity-gain (aka. No loss, no gain) bidirectional amp will likely be needed... regardless, a strong possibility of needing a TC or installation appointment.
This will only affect modem upstream (2-way) functionality (which includes On Demand functionality on HD STBs and HD DVRs), it will not affect DTAs (aka. miniboxes) if the return signals are too high.
52 dBmV to >50 dBmV is borderline high
Some people have no issues in this range, others do. Internet and phone connectivity are more sensitive than cable TV. Many times for transmitting/return issues, it's an equipment issue or an amplifier issue. If not, then an on-site TC might be the best solution to resolve this.
Occasionally this can be a modem issue (when the modulator is starting to fail), but most of the time it’s coupled with low forward signals and the real issue is that there’s just too many splitters in the pathway. If it's not an issue with equipment (replacing the modem), then a unity-gain (aka. No loss, no gain) bidirectional amp will likely be needed -- a strong possibility of needing a TC or installation appointment, frankly if it's this high your are likely to experience intermittent issues durring day time and especially during the warmer summer months.
This will only affect modems functionality (including On Demand), it will not affect DTAs if the return signals are too high.
50 dBmV to >30 dBmV is ok
This is a good range, but the lower the better (in theory), in practice anything around a low 40’s to upper 30’s is pretty much perfect. Keep in mind the perfect balance point is nearly impossible to realistically achieve.
What this in layman’s terms is measuring is how much power your modem’s modulator is having to “push” the signal uphill to the local HFC node. Each utility tap has an impedance value that is relative to where it is on the run. This is a long explanation but my point is that a 43 might be the very best possible at your utility tap, meaning you’ll never see lower than this no matter what you do and that’s ok just like it is.
Contrary to what some inexperienced people will tell you, 50 dBmV is actually just fine. It’s not 49 dBmV as the upper bound for this range of good return signal power levels.
≤ 30 dBmV is unusually low
Where can I see these levels on my internet modem?
Type in 192.168.100.1 directly to the URL bar of your internet browser (Chrome, Safari, IE, Edge, Firefox, etc) and you should be looking at a page that shows you the signal levels of your equipment. If it’s not immediately there, look for a “summary” page of some kind.
I see some signal frequencies higher or lower than others, is that ok?
Some deviation is expected, but you don’t want too much. How much is too much? For most purposes a deviation of more than Δ 5 or Δ 6 dBmV between each is too much in the DOCSIS frequencies, but it also depends on how dramatic of a spike is being seen.
If you see a slope in the frequencies, that’s actually normal as long as it’s a nice gradual linear slope.
What is being transmitted on the coax wire anyways?
The signals being transmitted and received are actually radio frequencies. These frequencies are typically measured in MHz (millions of cycles per second) units.
Charter/TWC uses signals that range from between 15 to 40 MHz for return/Tx (SHF radio frequencies),
55 to nearly 1000 MHz (aka. 1 GHz) for forward/Rx (UHF radio frequencies),
and 1150 to 1500 MHz for MoCA (which is often used for whole-home DVR box-to-box communications).
The reference of “0 dBmV” for forward/downstream signal strength is 1 mV exactly, which is roughly a mere 13 nW (nanowatts, billionth of a watt) of electromagnetic power coming from the tap into your home. It’s not dangerous, but it is very precise & sensitive; thus it's vital to not be leaking in (called ingress) or leaking out (called egress).
Are some types services (TV, HSD, Phone) more sensitive to this than other services?
Simple TV equipment such as DTAs are the most tolerant to out-of-range signals. Some equipment like DTAs just “listen” to the forward signal and don’t even need the return signals. Boxless TVs (coax direct into the TV’s built-in QAM tuner) are just like DTAs in this way, listen-only, receive-only.
DVRs and STBs really only use the return signal when trying do something interactive like ordering on-demand.
Internet and phone on the other hand are the most sensitive to these signal levels being out of range. It can result in dropped calls, intermittently going in & out internet (modem rebooting constantly trying to self-restore the lost channel frequencies), and packet loss.
Does temperature affect the signal levels?
In all electrical systems, cooler temperatures will mean less resistance and warmer temperature will mean more resistance. In this case we’re talking resistance in terms of impedance, but any resistor is temperature dependent. If you have your amp or splitters in an area that is getting direct sunlight or is baking in a hot attic, then you will see your signal levels fluctuate much more.
Who’s responsible/accountable for what when it comes to service-impacting repairs?