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| | | | FAQ Revisions | Editors: Raydr  , Axilla
Last modified on 2007-09-20 18:13:49
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9. Terminology·Tap
·Pedestal / Ped
·Junction
·Input Line
·Star / Home Run system
·Loop System
·Downstream Power
·Downstream SNR / Signal to Noise Ratio
·Upstream SNR / Signal to Noise Ratio
·Upstream Power
·SNMP
·Trap
·Node
·Drop
·Insertion Loss
·Terminators
·DOCSIS
| | | Tap - Usually the connection point at the utility pole or ped where your cable is hooked up.
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by Raydr
last modified: 2002-02-27 16:30:10 | | | Pedestal / Ped: Some sort of enclosure which contains a Tap. Usually mounted on the side of an apartment building or a stub in someone's yard.
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by Raydr | | | Junction:
Please note that not all houses have a junction, and some houses have more than one junction.
The cable junction is where all of your cable lines meet and hook into a splitter.
In an ideal situation, you will have an input line running from your ground block to your junction. Then, all of the outlets in the house will have a line running to the junction. This is referred to as a star or home run system.
This junction may be located behind one of the outlets, in a closet, in your attic, or at the ground block. If you are having cable problems and need to call the cable company, please have your junction located.
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by Raydr | | | Generally, this is the line running from your ground block to your junction or first splitter in a loop system.
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by Raydr | | | This is a CATV wiring scheme where all of the cable lines in the house meet at one central location, the junction.
Not all houses are wired this way. Some are wired as a loop system.
In the above picture, you can see the benefit of having a junction. Not only is the signal distributed more evenly among the house, but now no matter where you put the cable modem, you have great signal, and only one splitter to pass through.
This is obviously not a representation of the size and configuration of every house. Some houses only have on cable outlet, and I've been to one that had 17.
In cases like that where you have a large number of outlets, generally a reconfiguring of the splitters is in order. (More about this in my splitters section.)
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by Raydr
last modified: 2002-02-27 17:37:49 | | | This is a CATV wiring scheme where the coax runs from outlet to outlet. The input line runs to one outlet, which has a splitter, which runs to another outlet with a splitter, which runs to another outlet, and so on.
These schemes are a pain to work with, as if one line goes bad, every line after it won't have signal as well. Also, signal distribution is very uneven, as each outlet after the first one gets less and less signal. Usually in these cases, only the first 2 or 3 outlets will be able to run digital cable or a cable modem. The last outlets have too little signal or too much insertion loss to run these digital devices correctly.
This type of wiring is done rarely anymore, and anyone who does it should be shot (IMO).
Using DC Taps instead of splitters can usually make a loop system efficient!
In the above picture (which we'll all imagine as a beautiful, professionally done diagram, OBVIOUSLY not done with MSPaint), we see a very small 2 bedroom house with 4 outlets.
The numbers in green are the signal levels at that point. The numbers in RED are the signal loss from passing through the 2 way splitter (3.5db loss per leg).
I used 700Mhz because most cable operators run their cable modems in the 500Mhz to 900Mhz range. The higher frequency a signal is, the faster it is attenuated (lost).
Negative signal readings are NOT necessarily bad. Most TVs expect a signal between 7db and -5 db. After -5, the picture starts to get a little grainy. At -10, it is very grainy, and at -17 (depending on your TV), you have barely any picture at all.
Most cable modems run fine without a performance decrease at -8. In fact, most cable modems will probably receive their signal at -5 or so.
In this diagram, the cable modem would be receiving plenty of signal, even at the last outlet, but it still would not run (within specification) at the last 1 or 2 outlets.
Why?
Remember, cable modems have to talk BACK to your provider.
Please read my "provisioning" section about how cable modems sync with your provider.
In this case, the cable modem has too many obstacles to go through to get back to your provider. Your poor little device has to sit there and broadcast it's OWN signal up what is supposed to be a one way (downstream only) line. Of course, this depends on your area, but in my area, if I go through more than 2 or 3 splitters, the cable modem will be working too hard (upstream power) and is not acceptable.
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by Raydr
last modified: 2002-03-01 07:54:55 | | | This is how much signal your cable modem is receiving. Most cable modems like to have between 15db and -15db.
If you have more than 15db, it can cause interference and corrupted data, which will hurt your performance.
If you have less than -15db, you have a very weak signal. Make sure your splitters are configured properly for cable modems, or if you are on a loop system, consider running a new line for the cable modem or moving the cable modem to a better location.
(Thanks to soganta for a correction.)
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by Raydr
last modified: 2005-05-17 09:55:02 | | | This basically shows how much signal YOU are getting compared to noise. The higher this number is, the better. As this number goes down, it means there is more and more noise in the line.
Usually this is caused by bad shielding, R59 cabling, or bad connectors/wall plates. It's not easy to determine the location of interference.
Downstream interference means that you are getting a lot of noise around the frequency that your cable modem runs at. Troubleshoot this just like you would "fuzzy HIGH channels".
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by Raydr edited by ergibbs
last modified: 2003-07-10 06:28:26 | | | This basically shows how much signal the HEAD END is getting compared to noise. The higher this number is, the better. As this number goes down, it means there is more and more noise in the line.
Usually this is caused by bad shielding, R59 cabling, or bad connectors/wall plates. It's not easy to determine the location of interference.
Upstream interference means that the head end is getting a lot of noise around the frequency that your cable modem broadcasts at (15 to 50Mhz). Troubleshoot this just like you would "fuzzy LOW channels".
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by Raydr edited by ergibbs
last modified: 2003-07-10 06:29:10 | | | This shows how hard your cable modem is working to talk back to your provider. If this number is above 55, it means that your cable modem is not running efficiently.
Make sure that you are not making the cable modem go through unnecessary splitters, as this adds to the upstream power.
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by Raydr | | | SNMP stands for the Simple Network Management Protocol. At its heart, SNMP is a set of rules that allows a computer to get statistics from another computer across the Internet.
Computers keep track of various statistics that measure what they're doing. For example, routers can keep track of the number of bytes, packets, and errors that were transmitted and received on each interface (port). Web servers might keep a tally of the number of hits they have received. Other kinds of equipment has configuration information that's available through SNMP.
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by Raydr | | | There are several different types of traps, so I will not go into detail about them.
Basically, traps are designed to either block out a channel (or group of channels), or to block out the interference which scrambles a channel.
If your provider wants to prevent your cable modem from detecting a signal, they can place a trap on your line that blocks the range of frequencies that the cable modem runs on.
You can find more information about traps online elsewhere.
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by Raydr | | | The cable system is divided into nodes, which in my system are usually 2 to 5 blocks big.
In most cable systems, there is fiber optic cable running from the cable company to a large box somewhere in that node. Inside that box there is a media converter which converts the signal from optical (light)to RF (radio frequency).
Then, hardline (very thick 1 inch cable) runs up and down the street, with line extenders (big amplifiers) every couple of hundred of feet. Taps are installed where necessary, and from there it's regular coax cable running to your home.
As far as I know, there are two fiber lines run to each node, one for incoming signal, and one for outgoing.
This is how cable companies usually manage bandwidth and saturation. If a node becomes too busy, they will split it into separate nodes (with separate fiber lines running to each node).
Of course, they only do this when absolutely necessary, because laying fiber is extremely expensive.
Note by habu187 : .750, .860 and higher thickness cable is usually referred to as "trunk cable" which is amplified by "trunk amps" . LEs only amplify "feeder" or distribution cable, usually .500 .
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by Raydr
last modified: 2005-05-17 09:55:57 | | | The drop is the cable line running from the tap to your ground block. Any problems with this line is generally the cable provider's responsibility.
The line can be aerial (from a pole to your house) or underground (buried up to your house).
If there is more than one cable company in your area, you may have more than one drop running to your house. (Cable companies don't like touching each other's stuff. It's very illegal.)
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by Raydr | | | 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.
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by Raydr | | | Terminators are generally used at the tap for three reasons:
1.) To prevent ingress from entering the system through an open port at the tap.
2.) To prevent signal reflections (which really isn't too big of a problem).
3.) To prevent cable theft.
Sometimes, you can find little mini-terminators that come with splitters (they look like little caps that just screw onto unused splitter ports). You can also buy packs of them at Radio Shack (last I checked, a 2 pack was $1.99).
All terminators essentially do the same thing, but they come in many different shapes and sizes.
For example, the ones that come with some splitters you can just unscrew with your fingers, but the ones that cable companies can ONLY be unscrewed using a special tool. These are VERY hard to get off without the tool, and usually you end up breaking off the tap port before you get the terminator off (I've done this ;).
Picture of the ones you can get a Radio Shack:
Picture of the ones that we use:
(Ignore the bent lead inside of it. ;)
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by Raydr
last modified: 2002-03-18 21:50:14 | | | DOCSIS (Data Over Cable Service Interface Specification) is the Cable Modem standard that defines the interface requirements of high-speed data transmission over cable networks. You can also see the CableLabs® Certified logo on some modems, which also means the modem is DOCSIS compliant.
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by RedXII1234 edited by Raydr
last modified: 2002-03-15 09:13:52 |
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