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More commonly called attenuation, insertion loss is the loss of signal power between two points. Items that lead to signal loss are excessive cable length, temperature, humidity, and excess return loss.

All devices (such as splitters, amps, etc) that you add to a cable line have insertion loss.

In a cable modem environment, you want to have as little insertion loss as possible all throughout the house. Here's why:

Let's assume your cable modem is broadcasting at 49db. Remember that all cables lose signal over distance. We'll assume there is a 2 port splitter and an amplifier in your home.

At 150 feet, we'll assume that we've already lost 3db of the 49db your modem is broadcasting at. The splitter is going to suck an additional 5db, and then your one port amplifier will suck out an additional 7db. Now we have 34db left.

Let's not forget the input line...and the drop. Another 7 or 8db easily lost. The tap itself will probably suck another 9 db. Down to 17db. The rest of that db is used to send your cable modem signal to the neareast upstream amplifier on your street. In this case, 17db should be plenty.

Now, change the splitter to a 4 way, with RG59 cabling and do the math. You'd probably be left over with closer to 2 or 3 db...NOT good.

Of course, your cable modem would just boost the power in this case, but remember, it can only broadcast up to 62db, and most cable systems consider anything over 56 too high.

Feedback received on this FAQ entry:
  • this is very sketchy and inaccurate math. first, at 150 feet your only going to lose(or gain, which ever way you want to look at it) 1.5 or 2 db on the return at the most unless the line/drop/whatever piece of cable your talking about isn't good. second, if your splitters have been configured right than your modem is set up off the first 2 way which is 3db loss, not 5db. third, most amps these days are return passive, meaning they have very little effect on the return number (usually 1 db). fourth, if your modem is set up off of an amp, you've got a problem. Drop amps tend to do not good things to modems (ie speeds are all over the place). next, "the tap itself will probably suck another 9db" makes the assumption that your coming off a 9db tap, something that I've never seen or heard off. Actual tap value is usually irrelevant as typically there are more insertion losses to hit before getting to the next active (ie other taps, hard line dc or splitters). The cable company balances line levels so that if everything is running right return values at the tap are sufficient to easily accommodate drop plus 2 way plus outlet. lastly your looking at the way return is used backwards. Think of it as you and I being in a crowed house and trying to talk. In order for me to talk to you I have to talk loudly enough for you to hear me over the random noise that is always present but I can only talk so loud. A wall would represent a splitter. If I were to go into the next room I would have to talk louder in order for you to hear me. CMTS should receive at 0 and everything it hits there after adds to this number, One more thought, most modems have a max transmit of 57 but the CMTS (device similar to a dslam, where every cable modem talks back to) will usually accept a level within plus or minus 2 db of what it intends). so your modem is transmitting at 57 but it needs to be transmitting at 59 that works until the weather changes a little bit making the physical characteristics of the cable plant change and now the cable modem needs to transmit at 58 but it can't. occasionally the CMTS will accept the 3db difference but upload speeds won't be correct (ie packet loss) but more than likely your gonna have an intermittent connection. I could never recommend a transmit level above 53. PS, more commonly called insertion loss, not attenuation.

    2010-03-06 02:36:58

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by Raydr See Profile