Alright, here is the deal....WARNING, this may get long and boring
A two-way DOCSIS cable modem (what 90% of cable operators use) operates on two separate Radio Frequencies within the cable lines. Think of these like "channels" on the TV side of cable - there just isn't any video on these channels, only data. The first is the forward or downstream frequency, this is information being sent to your modem (downloading). The second is the return or upstream frequency , this is the data you send out (upload).
Generally the Downstream frequency is a much higher frequency that the return. The nature of RF is that higher frequencies loose "strength" faster than lower frequencies. This is rated in how many dB is lost in every foot of cable. So in the drop from the tap to your home (say 100 feet) your forward signal may lose 5 dB, while your return only loses 1 dB.
You also loose signal strength in every "passive" device, for example a splitter. A two-way splitter will generally drop 3.5 dB off each leg (this is true across the entire frequency band, the loss is equal - not like loss in the cable). So if you had 10 dB going in, each line coming out would have 6.5 dB coming out. Splitters can be had in many varieties, 2-1, 3-1, 4-1, 6-1, etc...obviously the more splits, the higher your loss will be.
The ideal Forward signal level would be from -5 dB to 10 dB (the modems will work outside of this range, but may have problems). Return is best from 40 to 55 dB, this is how much power your modem has to inject in order to make it back to the CMTS (Cable Modem Termination System).
So let's say at the tap (where the drop to your home comes from) signal levels are 15 dB forward and 40 dB return (the return signal is how much signal is needed to get back to the headend). These figures do NOT take into account analog to digital conversion for meters, etc. I'm speaking in term of what the modem will see and need to inject.
So 15/40
Figure a 100 ft drop so you lose 5/1 that takes us to 10/41
Figure with splitters you lose 7 dB (guess) 3/48
So at the modem you will see 3 dB forward and need to inject 48 dB return......this is a good situation.
Now let's look at a bad one.
Tap 8/42
150 ft drop 7.5/1.5 = 1.5/43.5
Two-way splitter 3.5 = -2/47.5
Unbalanced Three-way splitter (one leg loses 4, the other two lose 7, we'll assume you are one of the 7's) = -9/55.5
So your return is still ok, but the forward is questionable. In this case the would do best to put your modem on one leg off the two-way (-2/47.5), but it might be enough to move your modem to the 4 dB loss leg of the three-way (-6/51.5), not great but it may be enough. In extreme cases, if the levels at the tap are too far off a "hard-line" tech may need to come and re-balance an amp, etc (to complex to discuss here).....
Yes, there are splitters that won't pass high enough frequencies, but if the splitter can from the cable company this generally isn't the issue. Also things like fittings can go bad and cause loss or leakage - leakage will allow ingress. Ingress is other RF getting on the cable line. For example HAM radios run in the same band that many modems run - if this gets in the line it can distort the modems signal. Etc, etc, etc.
That is my spiel for now, if anyone want more info let me know. I'll be glad to go into more detail, explain, whatever.
The long and the short of your problem could be very simple, or very difficult. What type of modem do you have? Depending on type you could likely look at some internal webpages and see the power levels.....
Nicholaus
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