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This FAQ text is copyright dslreports.com
Reproduction of all or part only with our permission..
This FAQ is edited by: Raydr See Profile, Axilla See Profile
It was last modified on 2009-01-21 22:34:38

1. Introduction

Why are you writing this FAQ?

Many people believe that if they have problems with their Cable Modem/TV service, that it is automatically the fault of their cable provider. This is not always true, as bad products, bad wiring, and even bad customers can cause problems as well.

This FAQ is designed to educate the reader about the most common culprits behind CATV problems. I have been unable to find a comprehensive FAQ regarding actual cabling and HSD issues.

This FAQ will is currently being DEVELOPED. It is nowhere near complete. I am adding onto it whenever I have time. If you have any suggestions or questions, please let me know and it will become a priority question for this FAQ.

What does this FAQ NOT cover?

This FAQ is meant to be as non-technical as possible. You will find VERY FEW references to computers at all in this FAQ. If you are having a computer problem, look at the other FAQs on the site, or ask around in a forum.

This FAQ will NOT deal with the following:
Computer Problems
Networking (this includes routers)
Cat5/5e/6 Wiring
Hacking/Uncapping Bandwidth

Is there anything I need to know before using any of your information?

Yes. You first need to determine what type of wiring scheme your house uses: Home Run, Loop, or a combination of both. You will then need to locate your Junction, and go from there.

You may need to adjust these techniques to your home, because I can't predict what you may run into. Apartments, Condos, Townhouses, etc are different stories, so you may not be able to use a lot of this information.

Almost everything that I put in bold has a definition in the "Terminology" section of this FAQ (at the very bottom).

Please submit any questions while LOGGED IN so that I may properly credit you for asking/answering the question.

If you find you have a unique situation, please feel free to let me know so that I may add it to the FAQ.

Where can I buy all the things you talk about here?

grcmptrnrd See Profile recently pointed out this site:


If you come across any good sites, please let me know!

How to deal with your cable modem installer (please read this)

You'd be surprised how many times I've heard of a customer not understanding that you need a CABLE OUTLET to run a CABLE MODEM. You NEED a cable outlet wherever you want the cable modem, preferably a dedicated line. You CAN share a line with a TV, but it's best for the modem to have it's very own line.

You decide you want to get a cable modem. You call up the provider and get the pricing information. At that point, you need to decide if you want them to install the modem for you, or if you want to do it yourself.

If you decide that the provider is going to install it, you have to choose between a standard install or a full install. With a standard install, they will usually not touch the computer. They'll check signal levels, identify (and replace) any bad lines and hook up the cable modem. Note: If it is in the afternoon, you cannot expect the technician to do any rewiring as it is too hot to safely rewire your house. Attics reach 150 to 160 degrees in the afternoon. The computer part is up to YOU. Please do not ask a technician to do a full install if you're only paying for a standard install.

With a full install, they'll usually install a network card (if needed), configure your computer for the modem, make sure it's working properly, and give you a quick demonstration on how to access the internet.

Now, here are some important things you need to know:

The installers are not computer repair technicians: If your computer is not working properly when the installer arrives, don't expect it to work properly after he leaves, EVEN IF YOU PAID FOR A FULL INSTALL. If there are problems with your computer that will not allow the technician to install the cable modem, please do not blame the technician or even the provider. The tech will notify you that there is a problem with your computer and you will need to get it fixed before we will continue the install.

Also, please note that just because you never knew that there was a problem before you were notified, that doesn't mean it didn't exist. Computer problems are usually not noticed until you attempt to do a specific task, such as installing a cable modem. Just because your AOL or dial-up accounts work, that doesn't mean that your computer is capable or ready for high speed internet.

Technicians are not furniture movers: Please move all furniture at least a foot away from the cable outlets BEFORE the installer arrives. They are not allowed to move your furniture for you, and most technicians will refuse to. All too often, when a tech decides to be "nice" and move something, it breaks. Also, technicians are generally on a schedule, and waiting 15 minutes for your furniture to be moved could easily be used for educating you or rerunning a wire.

Please stay off the phone or temporarily suspend any distracting activities during the install process: Again, technicians are on a schedule. We understand that you get important phone calls and have things to do, but if you find the technician standing around waiting for you to hang up just so he/she can ask you a question, it's a big waste of time and quite annoying. Your time is valuable, and so is theirs. It's not easy to explain to the next customer that you were late because the last customer decided to take a shower and you had to wait to get the work order signed.

Do not follow the technician around unless he/she asks you to: Most people cannot do their jobs properly if there is someone breathing down their neck. That includes your cable technician.

Be courteous to the technician: If he comes out of your attic sweating all over the place, offer him/her some water!! This one action will go a VERY long way with most techs.

Understand that the technicians are there to make YOU, the customer happy, but it's much easier to please the customers if they are not making the technicians unhappy. Treat your cable technicians in a warm, friendly manner, and you'll notice a definite difference in the way the installation process is completed.

2. CATV Wiring

What are the most common types of CATV wiring?

The most common grades of CATV cable used in homes today are RG59 and RG6 (aka RG56).

RG59 was the standard for home wiring many years ago, but is generally problematic today. RG59 is a lower grade of coaxial cable, consisting of a small center conductor, a small insulating dielectric, and typically, a single outer shield.

RG6 is the standard today and has a larger center conductor, a dual or quad shield, and a much larger insulating dielectric, thus ensuring more bandwidth and a lower frequency loss per foot.

RG11 is higher quality than RG6, but also much thicker and harder to work with. It is usually used for longer cable runs (over 150 feet) because it loses signal more slowly. I do not know of any stores that carry RG11/F11 cable.

There are several other grades, but they are uncommon and beyond the scope of this FAQ.

Anatomy of a CATV Wire.

All conductors lose a certain amount of signal over distance. The "bigger" a conductor is, the less signal is lost over distance (you'll see why this is important later).

Also, dual/quad shield refers to how many layers of "shielding", or layers of "silver metal", cover the dielectric.

The dielectric is next, which is the opaque, white, waxy part of the cable. This is insulation from shielding.

The shielding is the layer of braided wire. The purpose of the shielding is just that. It shields the conductor from stray "outside" signals, also known as ingress (more on ingress later). It also keeps the signal being carried along the conductor INSIDE the cable (prevents egress, which is signal leaking OUT). Most RG6 has 2 layers of shielding (dual), but you can also find quad-shield (4 layers) cable out there. It is more expensive, and harder to work with.

The last layer, of course, is the rubber. This is just more insulation and there's no point in trying to describe what IT does.

Regular Coax Cable:

Dual-Shield Coax Cable:

Quad-Shield Coax Cable:

These pictures and more information on different types of coax can be found at:

Does the quality of the line for digital cable have to be better than analog?

Not really. My first step when doing a digital cable install is to see how the TV channels look, and see if there is any ingress.

Just FYI, digital cable will look the same no matter how good or bad a signal you get. When you are watching a digital channel, it is almost as if you were downloading a movie from the internet and watching it on your TV. Even if the signal level is really low, if the digital box and recieve it and decode it, it will still look good on your TV.

Digital Cable does not generally require that the line be as conditioned as a cable modem line. You still want the signal to be good (between -8 and +12), and you do not want an excess amount of ingress or your digital cable box will not get an IP address.

If a cable modem will run on the line with no problems, so will digital cable.

Will old wiring in my house cause problems with the new wiring?

In other words, if a lot of noise is injected into an old wire, will the signal go through the splitter and get onto the new wire and cause problems?

This really depends on two things: The strength of the noise, and the isolation between ports in the splitter.

The isolation between ports means how much signal is blocked between ports. Most splitters will have between 15 to 25db isolation.

This means that if the noise on the wire is 15db, it won't really affect the other ports, but if it were like 70db or so, it probably would.

This is also why if you hook up a splitter wrong, the TV looks like crap.

Most people won't have this problem.

Thanks to Mike G. for this question.

Why does my CATV co. need to install new cable for internet?

If you have any older RG59 cable, see any cuts in the lines, or if they detect ingress (noise) on the line, they may require that the line be replaced before you can receive digital services on it.

The reason is that any problems on the line may cause the device to malfunction, work harder than it needs to, or just not work at all. It's probably more preventative maintenance than anything, but it will save you a headache or two down the road.

I want use RG11 from my modem to the wall, will this be better than RG6?

Unless your wall and modem are a really really really long distance from each other (250' or more), it would be an unnecessary waste of wire, room, and money (if it costs). RG11 wire is not very flexible, and a bit thicker than RG6, so it's harder to run as well.

Chart of Signal Loss per 100ft

Coax Cable Signal (Attenuation) Loss per 100ft
Loss at

What about connectors?

I've seen it all when it comes to connectors, from the push-ons, screw-ons, and crimp-ons.

Firstly, prepare your wire! I know that sounds elementary, but I have seen countless times where the shielding isn't completely stripped away and a small strand gets accidentally caught leaning against the stringer. Buy a cable stripping tool and use it! (Actually, the tool is pretty inexpensive, and is easier/safer than using wire strippers or a knife)

Stay away from the push-on style of connectors. They will fall off eventually, or get loose, and ground out your signal.

The crimp-on connectors can also be just as fatal, especially if not crimped using a high quality CABLE crimping tool. Don't use needle nose pliers, or a 3.99 tool, you'll get a 3.99 connection, and that just wont cut it with longer cable runs and digital cable. If you use a good quality connector, a high quality crimping tool, and decent technique in wire prep and crimping, the crimp-on connector is preferred, as it lasts the longest, will not come loose, and will maintain the best overall all connection in the long run.

Screw-on types of connectors were introduced about 10 years ago, but remember that you have to use the proper sized connector for the the cable that you are using. And many homes have multiple sizes in cable in them due to the fact that the industry is constantly changing in terms of what types of materials are currently used and available in stores. Your home probably will have different cable wire in it, especially if some cable wire has been added into it by your brother-in-law, or other friend or relative.

Screw-on connectors will work ok for most homeowner installations, if the wire is properly prepared, and the proper-sized connector is installed tightly on the wire.

So how do I know all about this??? Been there, done that, and learned from my mistakes!

Good luck with your home wiring! And don't forget to check out the FAQs on the types of splitters and amplifiers to use, as most discount and home improvement stores don't sell what you should be using at home.

Should flat coaxial designed for going through windows be avoided?

Said by Darkk See Profile HERE.

There is no way you're going to avoid an impedance bump with "flat" coax, and that will definitely introduce undesirable effects on the signal in the cable that are detrimental to TV as well as HSI signals. Not to mention that ingress and poor common-mode noise rejection are a problem with this sort of cable.

Mainly this sort of cable is very poorly shielded, and should really be used as a last resort.

Below is an example of flat coax.
Click for full size

3. Common Problems


In order to save yourself tons of time, there is a very quick way to see if a problem is inside your house or from the cable company.

WARNING: Disconnecting the ground block can be
dangerous if your electrical system is not grounded properly! If you suspect
your home wiring to be old or in very bad shape, do NOT remove the ground block
or you could potentially be shocked!

Take your smallest TV out to your ground block, and hook it up there. If you are still experiencing a problem at the ground block, call the cable company and have them fix it. If your problem goes away, then you have an internal wiring problem and you need to continue troubleshooting from the junction (if you have one).

You may be asking yourself how this applies to cable modems. Your cable modem requires a much cleaner signal than your TV does. If you have a crappy picture on your TV, your cable modem is going to receive a crappy signal as well, which can cause retransmits, slower performance, dropped connections, etc.


Unfortunately, most of the troubleshooting information cannot be applied to those who live in Apartment Complexes. Generally apartments do not have ground blocks, and you do not have access to the input line, lock box, or any other place to do troubleshooting.

Your troubleshooting is limited checking things from the outlet to the junction (usually in one of the closets). Unfortunately, it's sometimes impossible to run new lines in apartments, and you may just have to run a line on the floor from another room to make up for the bad outlet.

The cable company does NOT own the wiring throughout the building, and you do not either. The apartment complex does.

It is my experience that some complexes take responsibility for bad wiring and will pay to have someone replace or run new lines. Other complexes will tell YOU to contact someone and YOU to pay to have it fixed. Your results may vary.


If you live in a mobile home, then you are in a GREAT position. Mobile homes are very easy to rewire when it comes to CATV.

If you have any problems with cabling at all, just drill a hole into your floor, and run a new wire under the trailer to the ground block. You will usually find a splitter under the home near a corner.

Loop systems are rare in mobile homes, and even then, they are usually so small that it doesn't matter. If you have a loop system with more than 3 outlets, go put on some jeans and crawl under there. It won't take you more than an hour or so.

Who to call when the cableco is unwilling/unable to fix poor signals?

Keep this in mind: Most cable companies have a set of guidelines as far as their signal levels go. These guidelines also vary for each neighborhood. As long as their numbers at the tap are within range, they may not do a thing.

However, if you *know* there are signal issues at the tap, and they refuse to fix them, I'm honestly not sure what to tell you. What CAN we do when a big company refuses to do something for us?

Giggleberrie had this to say:
If the cable company isn't meeting it's obligation as agreed upon in the franchise agreement with the community, a call to the franchise authority almost always encourages them to comply.

I know some people here will just start driving over the peds and stuff. I don't condone this. :X

Good Luck.

I want to check my signal at the ground block, but I don't have video service!

Please read THIS MESSAGE before reading the information below or it may not make sense.

Some people have cable modems, but no video service.
For those of you in this situation, I wanted to point out something. Did you know that you are probably paying an extra $10 a month just because you aren't subscribing to video services? Did you know that most cable companies also have "limited" or "basic" cable for around $10 a month? Think about it. ;)

If you want to determine if your cable modem problem is an internal wiring issue or an outside problem, and you do not have video service, I hope you have a laptop or can rig something.

Instead of taking a small TV out to, why not just take your cable modem? If you have a laptop (or a really long network cable), you can hook up your cable modem at the ground block and get signal levels there using DOCDIAG, or, if your cable company doesn't mind giving you your signal levels, you can just call them and get the numbers.

If your cable modem problem or signal issue goes away at the ground block, then you know that you have an internal wiring problem.

List of hard to find problems that cause HUGE headaches! GOOD READ!

This is a list of little things that are very hard to find, yet cause really huge unexplainable problems:

1.) One tiny strand of shielding gets wrapped around the stinger by Raydr:
This one causes nightmares for digital cable and cable modems. Remember, the shielding is supposed to keep stray signal from leaking into the cable. HOWEVER, if one little strand gets wrapped around the stinger, you've just created a huge "noise antenna" that projects all of that noise onto the cable line. If you look at your connectors and see this, don't try and fix it. Just cut the connector off and put a new one on.

2.) CALAN Sweep Transmitters by Engineer88:
Acterna (Calan) low-level sweep transmitters will cause downstream errors if the guardbands aren't set properly. This will cause packet loss to many customers on any plant fed by the sweep. Some modems are more sensitive to this than others.

Sweep must be at least 14 dB below video levels and guardbands must be at least 3.2 MHz wide to avoid interference.

3.) by zedsdead: Here's one i see a lot, the plastic insulation in around the center pin is not completely stripped off when stripping the cable (caused by dull blades), causing significant signal reduction.
Editor's Note: I see this a bit too! It's annoying, but pull that little bit of dielectric off. It can really hurt stuff!

Grounding Issues

Things which cause excess electrical current to flow over the cable system are very, very, bad!!!

A few things which cause grounding problems:
1. Grounding cheater plugs. (Those handy little 3 to 2 prong electrical outlet adapters) Used on equipment connected to the cable system , this will almost guarantee the appliance will use the cable as a ground.
2. Improperly wired outlets. (i.e.: Hot-Neutral wires reversed) Will cause hot chassis (current on ground) condition on A/V and computer equipment.
3. Polarized plugs, inserted backwards. These plugs are designed to be plugged in one-way for a reason, don't force them or modify them to fit in backwards.
4. Replacing 2-prong outlets with 3-prong outlets and not connecting a ground wire. Occurs mostly in older homes with handyman electricians around, causes a false sense of security and a very dangerous electrical problem.
5. Disconnecting or not having the coax cable ground. Illegal situation per the National Electric Code. Severe damage can occur to both cable company equipment and customer equipment if electrical surges occur. (Lightening, electrical line crossing cable lines, dropping a TV in a bathtub, etc...) Coax cable systems are grounded to prevent excess current from the customers home reaching the cable plant and to prevent excess current from the cable plant reaching the customer's home.

There shouldn't be more than 1 volt measurable between the coax cable and a known good ground.

How to check for grounding problems:
1. Check for the problems listed above. 90% of the time the reasons listed above are the causes for excess voltage on the coax cable system.
2. Use a electrical outlet tester.
3. Use a volt-meter or multi-meter, if you have proper instruction.

How to fix grounding problems:
1. Call you cable company to verify your house coax cable system is properly grounded. This should be done at no cost as it is required by law.
2. Call a certified electrician to verify proper grounding of your home electrical wiring and repair any problems found.
3. Remove any offending electrical equipment which is causing grounding problems.

Grounding problems, at their least, will cause intermittent or no connection on cable modems and poor or no picture on televisions. At their worst, grounding problems can lead to destruction of property and loss of life.

See the following link for interesting information on grounding issues and the problems they cause:
»www.epanorama.net/documents/grou ··· undloop/

Thanks to MacLeech for this submission.

Note by habu187 See Profile: Improper grounding can also cause "humbars" in pix.

3.1 Ingress

What is Ingress?

Ingress is basically the leaking IN of a signal. Ingress is bad, because it can totally trash a signal.

If there are any cuts, strips, bad crimps, or bad shielding on the line, TV channel 5 leaks into our CATV channel 5, and completely distorts the picture and the sound is usually messed up too. This happens a LOT with houses wired with RG59, because of the lack of good shielding. Also, if the conductor is exposed to the air at ANY point, that is an entry point for stray signal.

I apologize for the bad pictures, but these were taken with a digital camera. Whenever I get around to getting my TV Tuner hooked back up, I'll provide some much better shots. But for now:

Channel with ingress:

Same channel without ingress a few secs later:

Ingress, as it pertains to cable modem access

Two-way digital data signals are more susceptible than one-way signals to stresses in the condition of the HFC network. Degradation in video signal quality might not be noticed, but when two-way digital signals share the network with video signals, digital signals might be hampered. More information can be found here. (Thanks to JTRockville for providing this information)

How do I fix Ingress?

If you are experiencing ingress on your line, the FIRST thing to check are your connections. Make sure that all of your lines, from the wallplate to your equipment are ALL tight and secure.

Whenever I'm on a service call for ingress, the first thing I do is shake the wires behind the TV and see if it comes and goes. Half of the time that is the problem.

Push on connecters are NOT your friend and are a very common cause of ingress. Screw on ones are very annoying, but they are much more secure.

If that is not your problem, then it's probably the wiring. If you have a long cable running around the room from the cable outlet, take a look at it and make sure it says RG6 on it. If not, I strongly suggest you pick it up, throw it away, and head out to Wal-Mart and buy a decent quality cable.

If you've done this, and you still are experiencing ingress, then it's time to get some wires replaced. If you aren't scared of running new lines, go to your nearest hardware store (I recommend GrayBar) and buy a thousand feet of RG6 and start running new lines.

If you don't like doing work like this, see if your cable company offers any kind of wiring maintenance plan. They are usually pretty cheap ($2 or $3/mth) and will get you new lines run for free.

I recently went to a house to rewire it because they were having tons of problems. I located their junction and found this horrible piece of work:

Welcome to ingress hell.

Note by wth See Profile: You also need to pull the wallplate off and check the cable & connector on the backside of the plate also.

Are terminators suggested/needed on jacks inside the home?

This is really a system by system question. In areas where there is potential for a lot of noise (metro areas, high population), they may be necessary. From what I've seen, generally no. I suggest that instead of leaving open jacks, that you just disconnect the line and use a smaller splitter. Not only will this prevent ingress from unused jacks, but it will also send more signal to the used jacks.

I also received this response from jkintner See Profile:

"I was just reading your cable modem wiring FAQ and bumped into this question. I'm a contractor in Las Vegas, NV. We actually require the termination of any open port or wall jack in the home because of leakage and ingress problems. Having an open port acts just like an antenna and ends up backfeeding signal back into the system.

One noted case was a customer having problems with their modem dropping off during the day time. We re-wired his house from tap to all outlets to try and fix the problem. After about 4 weeks of troubleshooting and getting an engineer involved, we found that a neighbor had an open port in a bedroom and was using a remote controlled car. The motor in the car was putting off just enough interference on the upstream carrier for the modem, that it was knocking our customer offline."

3.2 Fuzzy Picture on ALL Channels

Fuzzy Pictures on ALL Channels on ONE TV

If you have fuzzy picture on all channels only on ONE tv, the first obvious step is to check all the wires from the outlet to the TV. This is usually the problem.

Next, determine the wiring scheme of your home (Home Run or Loop, or a combination of both).

If you have a junction, determine which line at the splitter feeds the TV you're having problems with. Make sure that the connector on it is in good shape, is screwed onto the splitter properly, etc.

If everything checks out, then the line running from the junction or splitter to the cable outlet may need to be replaced.

If your scheme is Loop, see if any outlets AFTER this outlet has the same problem. If there are outlets AFTER this one, and they look good, then there is either a problem with your TV, a problem with the cables running from the outlet to the TV, or in very rare cases, one of the legs of the splitter may be bad.

If this is the very last outlet in a loop system, you may have just run out of signal. Please read my definition of Loop System and consider running a new line to the FIRST splitter to send more signal that way.

(Read my section on Splitters to understand this better.)

Fuzzy Pictures on ALL Channels on ALL TVs

Usually, this is either an input line problem or a drop problem, and even sometimes a tap problem.

Go read "READ THIS BEFORE ATTEMPTING TO FIX A PROBLEM!" and determine if it's your problem or the cable company's problem. If it's their problem, call them and have them fix it.

If it's YOUR problem, make sure that the input line is connected properly to the first splitter. MAKE SURE YOUR SPLITTER IS NOT HOOKED UP BACKWARDS! I see this more often than you'd think.

Make sure your splitter isn't in bad condition. Replace your splitter(s) if necessary.

If this still does not fix it, have your input line replaced.

3.3 Fuzzy Picture on LOW channels

Common cause for Fuzzy Pictures on Low Channels (2 through 7)

Follow my "READ THIS FIRST" instructions on determining if it's an inside or outside problem.

If these are the only channels you are having problems with, it is usually a bad connector somewhere. A bad wire can cause this as well.

First, make sure all of the cables from the outlet to the TV are secure. Then, replace the barrel (F-81) connector on the wallplate itself. These sometimes get worn out a lot in rental homes.

If that did not help, start replacing connectors from the outlet to the junction, then out to the ground block until the problem is fixed.

If this didn't fix it....you actually might have a bad barrel on the back of the TV.

If this is happening on ALL of your TVs, you may have a bad splitter, or it may be an outside problem. GROUND BLOCKS ARE BARRELS TOO.

Try bypassing the groundblock and see if it clears up then. If it does, replace the ground block or the barrel. DO NOT LEAVE THE GROUND BLOCK DISCONNECTED!

bobafett assisted in answering this question.

Note by habu187 See Profile: Bad tap plates, loose seizure screws, suck-outs and low signal from actives can be culprits, too.

Note by cableguyrye See Profile: Water is a big problem with low band problems, also push on jumpers can cause those problems on an analog picture. U/G areas are notorious for water in drops, black center conducters and such, these i find are more problematic with low band complaints.

Note by joetaxpayer See Profile: Too many AC wires crossing near the cable caused 7 to be unwatchable. Replacing the cheap coax line made the problem go away completely.

3.4 Fuzzy Picture on HIGH channels

Fuzzy picture on channels 37 and up.

In this case, I would attempt to track down all of the splitters and replace any splitters that are under 900MHz.

If you refer to a channel/frequency chart, you can see why a low rated splitter would cause fuzzy pictures.

For example, if you have a 300MHz splitter somewhere, channels 37 and up would not look too great.

A 500MHz splitter wouldn't cause any noticeable problems if your cable channels don't go above channel 70 (which describes most cities). HOWEVER, cable modems usually run from 500 to 900, so you can see that there would be a problem.

Oh, and before someone tries and prove me wrong, I do know that many cable companies will run some sort of programming on channels 95 through 99, and even with a 300Mhz splitter, they'll look perfectly fine. Take a look at the channel chart, and you'll see that those channels actually run from 90MHz to 120Mhz. Nice try. ;)

3.5 Digital Cable

Do all TVs in the house need to have digital or can some get analog cable?

The "digital" part is actually done by a converter box. In order to recieve digital channels, you have to have this converter.

Any TV without the converter will just receive what they always have (regular analog TV).

Why does my digital cable look fuzzy?

It's probably a bad wire between your cable box and your TV (this includes VCRs, etc).

Your digital channels should look much better than your analog channels. They don't quite look like DVD, but they should be somewhere in between.

Your digital box outputs a very nice picture, and if for some reason it looks bad on your TV, it's usually just a loose connector or bad wire AFTER the cable box.

Those "push-on" wires that come with VCRs are notorious for doing this. I know we all hate those screw on ones, they are a pain in the butt to use, but use them anyway. You'll notice a difference.

Of course, there's always the possibility that your TV is no good or a connector on the TV or digital box may be bad.

ALSO: Just a recommendation. If your TV has an S-Video input or RCA jacks (Yellow, Red, White), use them for your digital cable instead. It looks much nicer.

My digital cable tiles up a lot.

This is usually a low signal issue or there is a lot of noise in your line. Check your connections.

Note by liamsdad23 See Profile: Digital channels can also freeze and tile if the signal is too high!

Please refer to my modem troubleshooting section regarding low/bad signal.

How does a digital cable descrambler/filter work?


There are TONS of ads for these "digital descramblers". They are NOTHING MORE THAN A HIGH PASS FILTER! They do not work!

What these things do is prevent a cable modem or digital cable box from talking back to the cable company, therefore it will not report Pay-Per-View purchases back to the cable company. Here's the catch though: The cable company is constantly polling the digital box for signal levels, purchases, and troubleshooting information. If it doesn't hear back from the box, they will flag your account and roll a service call truck on it. Since there is no way for you to clear the PPV purchases out of the boxes' memory, you're going to end up getting charged someday anyway.

Don't waste your money.

3.9 Member Questions

Why does every TV in my house have a horizontal bar that moves up the screen?

I had this problem in my house and after a lot of experimenting was able to track it down to the cable running into the house was running too close to the electrical. The cable was wire-tied to the main electrical feed for the house on the outside, and running over the top of the electrical box in the basement. After replacing the feed into the house with RG-6U, drilling a new hole into the house, and running the cable a few feet from the electrical, the problem went away. My advice for troubleshooting, try attaching a single TV to the cable feed on the outside of the house first. I did that by simply running some RG-6U out the door on the ground and connected it directly to the feed from the pole and to a TV. If the horizontal bar (hum bar) goes away, the problem is most likely caused by a cable wire somewhere running too close to electricity.
(Thanks for fmook for this one.)

Editor's Note: That horizontal bar is called a "hum bar".

Note by Tech3912 See Profile: Also bad crappy amps will do this as well. We see it alot as techs becuase ppl go to radio shack and get screwed... :)

Do cable modems die slowly or quit like a light bulb?

I think in most cases, anything electronic will die suddenly when a component quits working.

However, say you have a component in the cable modem that only malfunctions when it gets hot....and each time it malfunctions sooner and sooner, it's eventually going to stop working.

I had a cable modem that I had to reset all the time.

Most likely a signal issue. See modem troubleshooting section.

Would a converter purchased on E-Bay or elsewhere work properly with my cable?

No. Comcast, like almost all U.S. cable companies, will not activate a box that is not rented from them. This is due primarily for security concerns.

What is a ground block?

A ground block is a item generally placed on the outside of a residence between the cable drop and the internal wiring of a home. A ground block bonds the sheild of coax to the electrical ground of the residence. A ground block is usually connected by solid copper to either a cold water pipe or to the ground of the electrical service drop.

In the attached image the large arrow points to the ground block itself, while the smaller image points at the ground connection.

Click for full size

4. Splitters

What kind of splitter do I want?

If you can flag down a cable guy and convince him to give you some free splitters, then good for you. If you can't, then be prepared to spend some money.

Those cheap looking splitters they sell at Radio Shack are NO GOOD. Do NOT use combiner/splitters. Combiners trash digital signal and make it almost impossible for cable modems and digital boxes to talk back to your provider.

You want at least a 1GHz splitter. 900MHz usually work as well (unless your cable company actually goes above a gig, which I seriously doubt).
The 1GHz means that the splitter is designed to pass signals all the way up to 1GHz. You can use a splitter that says signals up to 2.5GHz instead of 1ghz however you wont need that expanded bandwidth for cable signals.

You may also have the option of buying power passing or not. You most likely do not need a power passing splitter. These are generally for special purpose applications (such as powering a signal amplifier that's in your attic from the outlet in your bedroom without running a power cord to it).

Also, do not get a bigger splitter than you NEED. The more ports a splitter has, the less signal it passes along to each port. Plus, they are more expensive.

Splitters are NOT made to connect wires together. I've been to many houses where people used an old crappy splitter they had laying around to hook up 2 or more short wires to each other because they didn't want to go out and buy a longer wire. SPLITTERS LOSE SIGNAL.

Good brands that I personally have dealt with are Monster (expensive), Regal, and Cablevision. I'm not incredibly fond of Magnavox splitters (usually found at K-Mart).

What is the best configuration for my splitters for TV service?

Basically, you want to even out the signal distribution as much as possible between all of the TVs.

If you have a loop system, there isn't much you can do, but if you have a junction, spend a few minutes looking at the numbers on the splitter and see what you can come up with.

Remember that cable loses signal over distance.

What's the best configuration for HSD with two other TVs?

You should have a 3 way splitter here. Put the modem on the 3.5db leg of the splitter, and the TVs on the 7db legs.

What's the best configuration for HSD with three to four other TVs?

Usually you'll have a two way splitter at the top, with one 3.5 leg going to your cable modem. The other 3.5 leg will go to either a 3 or 4 way splitter, depending on your needs.

What's the best configuration for HSD with five to six other TVs?

In this case I'd usually use a three way at the top.
The 3.5 leg on the first three way will go to a four way. The first 7 leg would go to the cable modem, and the second 7 leg would go to any TV.

The 3.5 and 7 leg can be swapped if the cable modem doesn't like it, but your TVs might be getting a little fuzzy at this point.

After 6 TVs, an amplifier is usually a good idea.

What's the best configuration for HSD with seven or more TVs?

This one is easy:

A two way at the top, one leg going to the cable modem, and the other going to an 8 way. Cable modems do NOT like 8 way splitters, so try and keep it away.

You will most likely need an amplifier in order to have decent TV service (or you can just have some of your unused outlets turned off).

Why don't you discuss 5, 6, 10, or 16 way splitters?

Mostly because they are not common. Although there are some situations where these rare splitters will fit in great, you'd probably have to order them online or from a special supply store.

You can do a pretty good job by just using 2, 3, 4, and 8 way splitters.

What do the numbers on a splitter mean?

Each splitter is different, but here is what most of them mean:

The numbers on each OUT leg of the splitter show how much signal is lost after passing through that leg.

For example, a 2 way splitter has 3.5 on each leg. That means that 3.5db was lost as the signal passed through that leg.

Note: The signal loss written on splitters is how much is lost at around 50 to 100Mhz. The higher a frequency is, the more is lost over distance. This applies to splitters too. In reality, a cable modem is probably losing more like 4 to 5db when passing through this same splitter.

The following info provided by RadioDoc:
On the back of a splitter, you may see a number like 130db EMI etched into it. That's the ingress/egress suppression (shielding) spec for the splitter, e.g. if a +10 dBmv signal is present inside at the input, the maximum leakage from the splitter itself would be 10-130=-120 dBmv, which is very low. Same the other way...

What's the difference between a Passive and an Active Splitter?

Passive Splitters are the ones you see every day. They have an "In", and 2 or more "Outs". You hook them up, they do their thing, and you never touch them again. They do not require any kind of power to run.

Active Splitters run on power. Most active splitters also boost the signal a tiny bit to compensate for the signal that would usually be lost after passing through a passive splitter.

Do not confuse an Active Splitter with an Amplifier.

Why did the installer put a 2-way split on my CM? 1 is open.

You may have been getting too MUCH signal. We do this sometimes to lower the amount of signal you are getting.

Also, he may have just been trying to be nice by leaving something you can plug your TV into.

Note by twisted2736 See Profile: or he/she (Tech) was out of barrels.

Is it important to use a terminator on a unused splitter output?

This really can depend on your system and the type of splitter that you are using.

Here in Gainesville, we use terminators 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.

Honestly, I doubt it would make much difference to have a terminator on a splitter in the home. If you want to do it anyway, you may be able to pick up a pack or something from Radio Shack.

You cannot use the ones that cable companies use, because they require a special tool to install/remove them (prevent cable theft).

Please see the definition for terminators for pictures of them.

When a signal goes in an output, what happens?

How can I configure 17 seperate outlets?

I'm Going To Be Wiring My Home W/ (Wall Plates) RG6 In 8 Rooms
2 Per Room(In Case We Move TV To The Other Side Of Room), So 16 Splits + A Cable Modem

What's The Best Way To Do This?
Will I Need Terminators On The 8 Unused Splits,
Should I Terminate At The Splitter Or The Jacks?

This is a special situation and I highly recommend you get a 2 port AMP from your cable company.

My suggestion: Input going into 2 way splitter, one which goes to the cable modem, and the other going into the 2 port amp, and each port of the amp going into an 8 way splitter. This is of course if you want to have all 17 outlets active at once.

If you're just going to have 9 active, just use a one port amp going to a single 8 way. In this case you might not even need to use an AMP, depending on how strong of a signal the cable co pushes into your house.

Hope this helps!

How do I connect 2 cable modems to a 3-way splitter.

The options are really limited here, so just plug one into the 3.5 leg, and the other into a 7 leg.

4.1 Common Passive Splitters

Two-Way Splitter

Signal loss per leg is 3.5db.

Three-Way Splitter

Signal loss is 3.5 on one leg, and 7db on the other two.

Note that this is the same as hooking up two two-way splitters to each other.

Four-Way Splitter

Signal loss per leg is 7db.

Eight-Way Splitter

(picture soon)

Typical loss per leg is 12 to 14db. If you have to use an eight-way splitter, you probably need an amplifier too.

4.2 Active Splitters

What's the db loss on Active Splitters?

Here is a table of the average gain/loss for an active splitter:

Type Gain at 400MHz Gain at 1GHz
2 Way +2db gain +4db gain
4 Way 0db (no gain and no loss) 0db (no gain and no loss)
8 Way -5db loss -5db loss

These are average numbers that I found. Check the documentation for your splitters for numbers

Active splitters can be a great help if you are having signal problems and don't want to install an amplifier. Remember, active splitters need power, so you'd have to locate it near an electrical outlet.

Active splitters cost around the same as an amplifer.

4.3 DC Taps (special splitters)

What are DC Taps? Where can I get them? You don't talk about them much.

DC Taps are a special type of splitter that lose a ton of signal on one leg, but very little on another. The model number of the model generally determine how much the bigger number is. For example:

DC-4 Tap: Loses 4db on one leg, around 1db on the other.
DC-6 Tap: Loses 6db on one leg, around 1db on the other.
DC-9 Tap: Loses 9db on one leg, around 1db on the other.
and so on...

These splitters have one leg labeled "tap", and the other "out". The "tap" leg is the one with a large amount of loss.

These are usually used in hotels that are wired in a loop system, to save a few hundred dollars in cable and labor costs. There will be one wire running from the central location, to the first room, which then "out"s to the 2nd room, which then "out"s to the 3rd room, and so on. Each room "tap"s into the line. Make sense??

You can get these from different stores online. Here are a couple:
(Note: I am not endorsing these stores. These are suggestions from site members and I cannot guarantee the customer service provided.)

»www.mcminone.com (Thanks, alphapointe See Profile)
»www.cencom94.com/gpage.html (Thanks, egnlsn See Profile)
»www.channelplus.com/product_deta ··· uctId=18 (Thanks, nizbit See Profile)
»www.mjsales.net/items.asp?Family ··· at2ID=93 (Thanks, droberson See Profile)
»www.accessorywarehouse.com/insta ··· s-p1.htm (Thanks, droberson See Profile)

(Note: I think we have enough suppliers here, and thanks to those who have sent me other stores!)

4.4 Amplifiers

What is an amplifier?

A CATV amplifier is used to boost an existing GOOD signal so that it can travel further.

You can find these at Radio Shack (get a good quality one), and Circuit City has begun selling them too (at a high price).

The best place to put these is at the junction, BEFORE the first splitter. Putting an amplifier behind your TV or before your cable modem will NOT help at all, because at that point the signal is already bad and weak. You will just end up amplifying a BAD SIGNAL.

There are several varieties of Amps, but you'll probably be okay with a one port. These usually have a gain of 12 to 15db. Make sure you get a 900MHz (or better) amp. These will run you from $30 to $120.

Two port amps usually have a gain of 9 to 13 db.
Four port amps usually have a gain of 6 to 10 db.

NOTE: Amplifiers are to boost EXISTING GOOD SIGNAL in order to increase the distance that it will travel. Do NOT use an Amp to fix a noisy or bad signal; You'll just end up amplifying a bad signal.

How can I tell if an amplifier will work with my cable modem?

IMPORTANT: There are a lot of amplifiers that will block your cable modem from talking back to your provider! This obviously defeats the purpose of having one.

Look for this when buying an amplifier:
"Cable modem compatible"
"5-42MHz passive return"

Amplifier / Cable Modem Compatibility List

The following Amps are KNOWN to work with cable modems:

Electroline EDA2100 (Thanks, bouchecl See Profile)
Electroline EDA2400 (Thanks, bmusgrove See Profile)
(I'm assuming that the above are 1 and 4 port models, respectively)
Motorola Signal Booster Drop Amp - Model 484095-001-00 (Thanks, Cory Cooper See Profile)
Scientific Atlanta Surge-Gap(r) Drop (Thanks, Me)
Pico Macom CDA-1A (Thanks, Charter tipster. currently $40 shipped).

The following Amps DO NOT work with cable modems:
The inline amplifier sold at Radio Shack prevents 2-way communication. (Thanks, Chakthi See Profile)

I do not have access to many amps. If you have an amp installed in your home, please provide me with the information for it and whether it works or not!

Will an amplifier make my connection faster?


Simple analogy: Adding more gas to your car does not make it run faster.

Adding a better QUALITY gas to your car may make it run more efficiently. Same with amplifiers.

Thanks for Mike for the question.

Can I use a satellite amplifier for my cable modem?

All that matters is that the amplifier boost from 54MHz to 1GHz up, with a passive return in the 2 to 54MHz range.

What to look for in a cable amplifier.

When you determine that you need to apply a cable TV amplifier within your CPE (Customer Premise Equipment) cable structure for increasing your signal level to travel an extended amount, it is very important to install the correct style of amplifier. With many different manufacturers offering a variety of amplification products, it is often difficult for the end user to know what is right for them. The first step in determining the amplifier type that you are going to use is to look at what broadband services (such as video, data, and telephone) you will be using, or will be using now or in the future. There are four basic house amplifiers, Forward Gain, Return Gain, 2-Way Active, and Bypass.

Forward gain cable amplifiers can by used in a variety of different situations, from compensating for lower signal strength from your cable provider, raising the strength of the signal from your cable provider, or increasing the signal before multiple line are connected. The forward gain amplifier works in providing an increased signal level into your home on the upstream band of 54-1000 MHz, and still allowing the 5-42MHz return band to be passed on back to the cable provider with little or no loss in signal level. Things to look for when purchasing a forward gain amplifier are, Low noise of 3 dB or lower, low distortion and excellent return loss which should be about 20 dB.

Return gain cable amplifiers are used to correct for weak signal strength on the return path with in the 5-42 MHz band being sent back to the service provider from your home for your high speed internet, interactive television, and telephony services. Some cases that would require a return gain amplifier are attenuation caused from passives install in the home and long cable runs from the service providers tap to your home. Things to look for when purchasing a return gain amplifier are a noise figure level of 5.5, SCTE compliant water sealed connectors, RFI Shielding of 100 or greater, and the housing material.

2-Way Active cable amplifiers are used to compensate for weak signal strength in where both the forward path of 54-1000 MHz and the return path of 5-42 MHz would need to be increased for your high speed internet, Interactive television, and telephony services.

Bypass cable amplifiers work in the same way as forward gain amplifiers to increase the signal level on the 54-1000 MHz band for high speed internet, interactive television and telephony services, and still allowing the 5-42MHz return band to be passed on back to the cable provider with little or no loss in signal level. The difference from the forward gain amplifier is that the bypass amplifier has a built in relay switch designed to bypass the amplification circuitry to bypass mode and provide constant uninterrupted telephony service in the event of a power outage.

5. Cable Modems

READ FIRST: Troubleshooting Cable Modems

My first suggestion of all is going to be:


The easiest way to do this is by taking the cable modem out to the ground block (where the cable attaches to your house) and plugging the modem DIRECTLY into the drop (the line coming from the cable company). If you still have problems, you either have a bad drop (or bad ground block) or something is wrong in the plant (the cable company's system).

Yes, you will need to borrow a laptop, or run a long network cable to your computer to test the modem. Do NOT run a long cable wire from the ground block to the modem, unless it's 100% brand new.

Any recommendation on brands of cable modems?

I would have to update this question every week, so I won't provide a list here.

Let's just say that Motorola Surfboard line seems to be the favorite among a lot of people right now.

What kind of signal levels do I want on my cable modem?

Downstream Power:
You generally want between -12db and +12db. Most modems are rated from -15 to +15. Anything less or more than that and you may have quality issues.

I personally prefer to not have less than -7db. If you want to raise your signal level a bit, check my troubleshooting and splitter section.

Downstream SNR:
This number is best over 30, but you may not have any problems with down to 25. Anything less and you will probably have slow transfers, dropped connections, etc.

See my "Downstream SNR" definition for more information on this.

Upstream Power:
The lower this number is, the better. If it is above 55, you may want to see if you can reconfigure your splitters. Anything above 57 is not good and should be fixed ASAP. (This is getting pretty close to not being able to connect.)

Upstream SNR:
Anything above 29 is considered good. The higher this number is, the better. If this number is below 25 and 29, you have a minute amount of noise leaking in somewhere. If it's anything less than 25, you want to get it fixed as you may have a lot of packet loss or slow transfer rates.

See my "Upstream SNR" definition for more info.

What kind of bandwidth will I see with a cable modem?

Cable modems can theoretically run up to 38Mb/sec downstream, and 10Mb/sec upstream.

Most providers cap (limit) you depending on your service plan. A common number is 1.5Mb(down)/128Kb(up).

Ask your provider for their numbers.

What advantage is there in connecting the cable modem via USB vs pci card?

The USB port is controlled mostly by the processor, which means that if there is any activity on the USB port, the CPU has to take time to process it.

A NIC card has it's own processor, so it will do all the processing and just hand the data to the CPU or show it into memory without the CPU needing to do any work. Basically, PCI is more efficient, but in most mid to high end computers, you won't notice much of a difference.

Can I take my cable modem to my friend's house and use it?

That depends on your system. Most likely, yes, assuming that they have not installed a high pass filter on your friend's line or limited your particular modem to a particular node.

Why am I not getting the advertised speeds?

This is a hard question to answer, as it could be MANY different things. However, we'll go with the most common issues:

1.) Computer not setup properly. Make sure your computer is tweaked by going to »/tweaks/.
2.) Usually, the reason why you don't see your speed test hitting the limit is because the site you're trying to run the speed test from really isn't capable of giving you a good speed test, or it's just one of those busy times of the day. If you were to try your speed test at 1AM or so, you'll probably get full speed. This is the nature of the internet. Make sure you do a speed test from several different sites to get accurate results.

3.) At this point, some of the other possible reasons why you won't get full speed is because of a bad signal level or some noise in the line. Let me try to explain it better: Say you have a 1.5MB/s connection, but for some reason you're only getting 1.15MB/s transfter. If there is noise in your line, then your modem is probably having to retransmit some of it's data. Those retransmission use up bandwidth, but it won't show up in a speed test. Remember, you're allowed to use exactly 1.5MB per SECOND. If you have to repeat something due to bad noise level, what you're repeating counts toward the 1.5MB limit.

For those who are having a hard time getting this, let me put it this way: Say you're having a conversation with someone, and you, for some strange reason, are only allow to say 12 words per minute. Here's how it'd go:

You: Hi, how are you? (4 words)
Them: Good, and you?
You: Not too bad, just learning some technical stuff. (8 words)

In the above example, you got full bandwidth, you were able to speak the full 12 words per minute.

Now, let's stick some noise in there:

You: Hi, hou? (4 words)
Them: What?
You: Hi, how are you? (4 words)
Them: Good, and you?
You: Not too bad, just (4 words, oops, we're out of bandwidth!)

See? In the example above, we still "sent" the same amount of "data" (12 words) in the alloted time, but due to a retransmission because of noise, it looks like we didn't get the full bandwidth. Make sense? Good.

Check your signal levels or ask the cable co to check the noise levels on your line. Then, replace some of your lines if necessary.

4.) Now, at this point, if you're still not getting full speed, it just may be the cable company's fault. They could have oversold your node, they may have equipment misconfigured, or in some cases, they may have just flat out lied about the speed service they were going to provide you. Unfortunately, at this point there's probably not much you can do except complain for a year until they get it fixed. However, you need to note that the cable co's policy probably states that those speeds are "NOT" guaranteed.

Good luck.

Moving on...


Will the cable modem interfere with the television signal?

From lackatee: No. Your televison should work better now that the tech is there installing new splitters and changing out old fittings. Also, Cable modems, Digital Phone, and cable tv, and run on different frequency's so they do not interfere w/ each other. You can rest assure your cable modem will not affect your television.

From Raydr: There have been cases of the cable modem interfering with TV signal, but this is almost always a case of very bad wiring or defective equipment somewhere. As stated above, each of the devices run on a different set of frequencies and theoretically should not affect each other.

How to re-register a new MAC address with cable modem?

At times you will need to change your original computer of which the mac address of the nic had been registered by the cable modem during the initial installation. It could be that you bought a new computer or simply want to register a different terminal (computer or router).

One way is to clone the original registered mac. Many broadband routers have this capability. However, you would need to remember or write down the original mac for consecutive switches.

Second is to actually call the cable provider and have them reset the modem and connect your new terminal to register its new mac address with the modem. Some cable companies are willing to do this only for one ore two times. If you switch often, please read on:

In order to register a new mac --should you change your pc or it's nic-- you could turn your modem off for at least 3-4 hours. Over night is even better. This clears the mac cache in the modem. Then first make sure you perform the necessary adjustments on your router --wan ip should be dynamic in my case and make sure you release any existing IPs --and power your router down. Power up your cable modem, wait until all blinking goes away and the cable led is steady. Then turn your router on and wait for configuration. This usually takes about 1minute with my Linksys Broadband router and check the status page. If the dynamic ip, gateway and dns servers are assigned and present, you have registered your new mac address --in this case router's-- with your cable modem. In some rare cases you might want to check the status page and if no ip, gateway or dns is assigned, you can renew your request by clicking on the appropriate buttons on this page.

I cannot SNMP my cable modem using Docsdiag, what can I do?

Many Internet Service Providers (ISP) block SNMP from communicating with your cable modem. When your cable modem connects to the cable provider while getting the internet settings such as upstream/downstream and ip address(es), the cable modem receives commands. ISP's may often do this for security reasons or to allow self-configuration of the modem. Calling the cable provider or ISP may solve your problems when troubles occur, as they would rather have you do that then use docsdiag. Proprietary applications such as from Comcast or Time Warner (Road Runner Medic) may "phone home" by communicating with your modem and giving the company details. Be warned that if you are violating your terms of service by having NAT (network address translation) enabled and are hooking more than one computer or running servers (if the Terms of Service prevent such activity) you might land in hot water. However, most servers on the network may be detected from the ISP head on rather than the modem itself.

5.1 Provisioning (Activation)

How does my provider activate my modem?

Please note that the following information applies mostly to DOCSIS systems only. I do not know how other cable systems work, only the one I work in.

Usually, if the cable modem is being installed by a technician, when he arrives at your residence, he calls his dispatcher and tells him or her the HFC MAC address of your modem. If you look at your cable modem, you will see a string of numbers on it somewhere starting with 00-90, 00-20, 00-00, or something similar. (Those numbers vary with the manufacturer of the modem).

If you are doing a self install, you are usually instructed to call your cable company and read the number off to them. They then bind it to your account and activate it.

Every cable modem manufactured has a unique MAC address (just like every network card or network device). This MAC address is bound to your account, and if your account is active, the cable modem is allowed to talk on the network.

After your modem is provisioned or activated, you then hook it up to your cable and plug it in.

(Continue on to Modem Boot Up...)

Is there any way to get free cable modem service?

This really depends on where you live.
Some systems place High Pass Filters and/or Traps on a customer's line if they are not an active HSD customer. This prevents the cable modem from being able to talk back to the head end and/or from receiving a cable modem signal.

If this is the company's ONLY way of protection, and they forgot to put one on your line, then you may be able to hook up a cable modem and surf away.

This is unlikely though.

Most cable companies now authorize your cable modem by serial number and/or MAC address. Attempting to use a non-authorized modem will just end up in your modem being denied access to the network.

If I copy a Valid MAC address in my Router could I get free service?

No, and here's why:

As far as I know, there are three major ways that cable companies allow cable modems to access their network:

1.) Filters/Traps:
Some cable companies don't care about the MAC address of your cable modem. They prevent your modem from talking back to them by installing a filter and/or trap on your cable line at the tap.

When you get your service activated, they come out and remove whatever they need to.

This is not used often because it's very easy for people to steal cable modem service.

2.) HFC (Cable Modem) MAC Authentication:
In this case, the cable company would authorize your cable modem by it's MAC. Every DOCSIS cable modem has one, and if you look at yours, it should have a sticker with an "HFC MAC". When your modem boots up, it attempts to talk to your cable provider, and the provider will either grant or deny it access based on it's MAC address.

3.) HFC (Cable Modem) AND NIC (Network Card) MAC Authentication:
Some cable companies, for some reason, also authenticate the network card in your computer. First it will authenticate the Cable Modem, and then when your computer attempts to get online, they'll check and make sure that you're still using the same computer/network card/etc that you've always used. If you've changed your network card or installed a router, they will block your access.

I'm not sure exactly why they do this, but I'm guessing that this is partially how they prevent you from sharing your cable modem on a network.

This is also why most routers have a "MAC Address Clone" feature. If you tell your router to clone your network card's MAC Address, it makes your cable provider think that you're still using the same computer/network card to access their services, and therefore they allow your router on the network.

Now, the reason why it wouldn't get you free cable: When you clone an address in your router, you're cloning a NETWORK CARD, not a CABLE MODEM.

The only way you could get free cable modem service this way is by somehow changing the MAC address of the CABLE MODEM to one that has been provisioned (authorized) already. DOCSIS cable modems cannot be modified in this way.

Now that I think about it, if any cable companies out there authenticated by Network Card MAC Address only, and not cable modem, this might work, but I doubt there are any providers out there that dumb.

5.2 Modem Boot Up

What's happening when I plug my modem in for the first time?

The DOCSIS specification allows for your provider to send the cable modem signal at just about any frequency they want.

When you get your new cable modem, it does not know what frequency it's supposed to be running at.

When you plug in your modem, and it just sits there, it is actually going through the following steps:
(If your modem has indicator lights, you can look at the manual and determine which stage it is at by looking at the flashing LEDs.)

1.) Staring at a very low frequency (around 100MHz), the cable modem starts scanning for a cable modem signal. It will keep going higher and higher until it "hears" a cable modem stream. (Check your cable modem's documentation to see what frequencies it supports).

2.) Once it has found a cable modem signal, it listens for a few seconds for a broadcast from the cable company. This broadcast contains the information that the cable modem needs in order to talk back to the head end (a machine at the company which manages the cable modem network). If the cable modem does not hear this information after a while, it will just continue scanning listening for another cable modem signal (go back to 1.).
Part of this information is what frequency to talk back on (between 5 and 42Mhz on DOCSIS systems).

3.) Once the cable modem has learned how to talk back, it will actually start trying. The cable modem will start broadcasting at a very low signal level (usually around 8db). This is where the cable modem is ranging, or determining how hard it has to "yell" through all of your splitters in order for your provider to hear it. It will continue increasing it's broadcast power until it hears back from your provider. Usually this ends up being approximately 25 to 50db. If it turns out to be more than 56db, our system considers that to be too much.

4.) Once the CMTS (head end) has acknowledged the modem, the modem then identifies itself. The head end will then either grant or deny access to the network. If it is denied, the cable modem goes back to step 1.
The cable modem at this point will check and see if it needs to upgrade it's internal software (flash ROM).
4a.) If so, it will begin the software update from your provider (usually you'll have a flashing TEST light or something during this process), install it, and reset itself. Go back to step 1.
4b.) If the modem does not find an update, the headend will tell the modem to download a configuration file via TFTP from a server. This file contains the configuration for the cable modem, such as maximum speed, ip address (if any), and other data the cable modem needs to access the network properly. The modem will then load the configuration, and viola:

5.) You are now connected. (Cable/Link light goes solid.)

(Thanks to NoVA_CoxUser See Profile for a correction)

Whoa, wait up. So can I hack the configuration file the modem loads?

I don't know. That's beyond the scope of this FAQ, and it's probably against your provider's TOS.

If you had access to this file, you could probably remove any bandwidth cap you have and/or do some other not so legal things.

You could probably find more info on this elsewhere on the net (this site does not contain this information, and such questions are usually removed very quickly from the forum).

Note by Mr Xaine See Profile: If you live in the United States, this is extremely illegal. The FCC can fine the provider because of your "tweaking" of the modem. Obviously the provider doesn't like the FCC hugging their tail for your mischef, so their hardware watches for this. Your MAC will be flagged, they'll cut you off, then either ban you from access, or ban and fine, or ban and take you to court. To risky for a quick boost in your connection... If you want to call your provider and see what the fine would be, you simply asking may make them start watching your connection speed closer. But if you never plan on doing this, then go ahead and call to see how the corporation's full wrath will smash you.

5.3 Modem Troubleshooting

It seems like my modem never finds a cable modem signal! (Modem won't sync)

If your cable modem doesn't seem to be able to find a signal, I would check splitters first, and make sure that any splitter along the line is at least 900MHz rated.

If you suspect that there may be a wiring problem, take your cable modem out to the ground block and hook it up there. If it still acts up, it may be time to call your provider for a service call.

It looks like the head end doesn't hear my modem. (Modem won't sync)

Make sure that you have the modem through as little splitters as possible. Remember that EVERY device you put inline with the cable modem is another obstacle it has to overcome. The cable modem will only try up to a certain amount (around 60db), and then give up.

Put it on the lowest db leg of the first splitter in your residence, if possible. If it doesn't work then, take it to your ground block and see if it works then. If not, then call your service provider.

Some providers can tell you the signal level that THEY [the head end] are receiving. If it's below a certain point, the system will refuse to acknowledge you.

Also see: Upstream Power too High (>55)

My modem is being rejected by the head end! (Modem won't sync)

Make sure that the modem has been activated on your account, and that you've given them the correct MAC address.

Some modems will have two MAC addresses written on it, one is HFC (this is the one you give the cable company), and one is LAN.

Please note that you won't be able to tell if your modem is being rejected by the headend unless you've read the manual and it explains what all the flashing lights mean. Not all cable modems will give you this indication.

My connection randomly drops in and out. What do I do?

Well, the obvious action is to make sure that all your wiring looks good. Make sure you have your splitters configured properly for cable modem service, and that everything looks like it's in good shape. Read the other modem sections to get your signal levels and troubleshoot from there.

If you call technical support, they generally keep a log of your cable modem status, such as downstream power (signal it's receiving), upstream power (how hard it's working), and upstream and downstream snr (signal to noise ratio).

They can look at this chart and tell you if your modem has always had bad signal or if it's something recent.

If they tell you that the line has too much noise on it, this usually means that the shielding has been damaged on a cable somewhere, or you have a really nasty connection somewhere.

If your modem drops in and out even though all your signal levels are good, then it's most likely a loose connection somewhere or a plant (outside of your house) problem.

Please note that if your cable light never goes out but you lose connectivity, it's most likely something down at the cable company, but CAN be a bad modem, or something wrong with your computer. This is not an RF (signal) issue.

If the cable light DOES go out, then it's an RF issue and you need to start looking at the other troubleshooting sections.

How can I check the signal levels on my modem?

Some modems provide a diagnostics page which will give you this information. If not, the easiest way to get signal levels is to call tech support and just ask for them.

Modems that provide diag pages:
(Click each modem name to get to it's diag page)
NOTE: You may need to disconnect the cable line from your modem in order to access the modem's diag page! Also, your cable provider can DISABLE this page, so if it doesn't work, try another method.
Ambit Cable Modems
Motorola Surfboard Line

Most other modems
(Thanks to hsdcable for inspiring me to create this list.)

If you end up having to call Tech Support, they should be able to provide you with the following information:

Downstream Power
Downstream SNR
Upstream Power
Upstream SNR

Fortunately, a lot of providers offer a tool for the techs to use and check signal levels while they're at a job.

While I cannot give you a link to this, I can show you what it looks like. Click here to see my modem levels.

There is also a program you can use to get your modem levels. It's not user friendly, so unless you are familiar with working with DOS, it may be confusing to you.

There is a post in another FAQ with some help on using this program at:
http://www.dslreports.com/faq/2815 (Thanks to Bobcat for the link.)

You can find this program here:
»homepage.ntlworld.com/robin.d.h. ··· ocsdiag/
(Thanks to Justin for the link.)

Here is some sample output from running this program on MY cable modem:

DocsDiag v011209 Copyright 2001 Robin Walker rdhw@cam.ac.uk

Toshiba DOCSIS Cable Modem: HW 6.62; SW 1.7.007

Downstream channel ID = 4
Downstream channel frequency = 705000000 Hz
Downstream received signal power = 10.2 dBmV
Upstream channel ID = 2
Upstream channel frequency = 24752000 Hz
QoS max upstream bandwidth = 128000 bps
QoS max downstream bandwidth = 1550000 bps
SigQu: Signal to Noise Ratio = 33.5 dB
Cable modem status = Operational
Upstream transmit signal power = 48.2 dBmV
Date and Time = 2002-02-28,22:16:27.00-04:00
Configuration filename = modemcfg/generic-std.bin

Downstream Power too Low (-12)

Make sure to read my section on splitters and configure your house accordingly.

There are some cases in which a house is just really huge and a ton of signal is lost before it reaches the outlet. Usually, in a case like this, an amplifier or an active splitter will do the trick.

If your cable line is currently RG59, you should REALLY replace it with RG6.

Upstream Power is Too High (>55)

There are two common things that will cause this:
1.) Be absolutely sure that you are not running your cable modem through any extra splitters, and that you are on the lowest db ports possible. (Read my splitter section on configuring your splitters.)

If you have access to DC Taps, use them, they will most likely fix your problem in this case.

2.) If your upstream line is noisy, your cable modem provider will instruct your cable modem to raise it's upstream power in order to compensate for all the noise on the line. In this case, I suggest you review the Common cause for Fuzzy Pictures on Low Channels (2 through 7) section of my FAQ. It may seem unrelated, but Channel 2 and your cable modem return frequency are very close to each other.

If your upstream power is just "barely" too much, but runs fine, I wouldn't worry about it, but if it's so high that your cable company is threatening to take your kids away, then obviously you need to do something.

Maybe the line running to your cable modem is actually bad? Are you sharing it with a TV? Have a new line run.

Installing an AMP in this case will not fix the upstream power, it will just add another device the modem has to work through.

There ARE two way amps for situations like this. I do not know where you could find one, but I do know that they run in the hundreds of dollars range ($250 to $400).

Good luck with this one.

Note by habu187 See Profile: This is also caused by improper return path padding in amps and LEs or the return path at the node could wrong.

Generic Upstream Power Problems

If you're not in a huge house, but are still having upstream power problems, here are a few suggestions from Jer Dog

Ask your service provider to run a separate drop for the cable modem.
Not all cable companies will run more than one drop to your house though. (The company I work for only allows one drop per address, which means that the only reason why there'd be more than one drop to a house is if the house is actually split in two and rented out as separate addresses.)

Run a new RG-11 Drop and/or cable line.
Another excellent suggestion. RG11 is an even thicker cable than RG6, and has much less signal loss. If your drop is over 150 feet, then it's a really good idea to have RG11 running to your house.

Although I personally have never seen any outlets run with RG11 cable, this could be done if you had access to it. If your cable company makes a habit out of using RG11 inside the home, then good for you, but if not, I doubt you could get a technician to do it. RG11 is much harder to work with, weighs a LOT, and is very unflexible.

Please note that to properly crimp an F11 fitting on, you need special strippers and a special crimper, AND a special fitting. I haven't seen any stores which carry any of this stuff.

Use a DC instead of a splitter.
This is a GREAT suggestion. DCs are really a different type of splitter, which when used correctly, can really help in loop systems or in other problematic situations.

I will have be adding a section on DCs shortly.

7. Off Topic

I'm having problems playing pogo.com games since I got my Cable/DSL connection.

If your computer is locking up or rebooting (restarting) when you try to load a game, it is probably due to the networking card in your computer. Linksys, NetGear and some SMC brand Networking cards have compatibility issues with many of pogo's games.

All three brands are very common, and frequently are distributed by many Broadband Internet providers when you sign up for DSL or Cable Modem Internet Access. If you're connected to the Internet via DSL, Cable Modem, or through a private home network, chances are good that one of these cards is installed in your computer.

Our suggestion is to try downloading and installing the Java Virtual Machine from Sun Microsystems (this is different from the one from Microsoft). This has fixed the problem for many of our users who had this problem, and it may work for you. Here's how to get Sun's Java VM:

- Go to »java.sun.com/getjava/download.ht ··· oad.html
- Follow the directions listed on that page for installation
- Restart your computer before trying to play again

Unfortunately, if this doesn't work for you, the only resolution we have left to offer is to replace the Linksys or NetGear Networking card with one from another brand like 3Com or D-Link.

Thanks to Service Tech for this submission.

I changed the MTU for my cable connection. service got worse. why?

MTU ( Maximum Transmission Unit ) is the greatest amount of data that can be transferred in one physical frame on the network.

By raising the MTU, you can achieve faster data transfer (more data is sent at once), HOWEVER, if you raise it too high, you'll end up trying to push more data through the pipe than it can handle, causing dropped packets and lost data (which causes retransmissions).

Think of it this way:

Say you have a water line that can handle 1.5 gallons per second. Now, you start pushing 3 gallons per second. Chances are, the pipe will now start "dropping" or leaking water.

Of course, if you set your MTU too low, this can limit how much data is transferred in a certain amount of time.

MTU isn't the only thing that affects your data speed, however this FAQ does not cover this topic.

8. Out of curiousity...

What does a node look like?

Well, not all of them will look the same, but here in Gainesville, if the node is on the ground, it's hidden inside this:

See the second box in the background? That one contains the battery backup for the node. I didn't have a chance to take pictures of the batteries, but just imagine 4 or 5 car batteries inside that box and that's kind of what it looks like. ;)

This is the inside of the node box:

This is a picture of a node that TACSPEED posted in the forums a few weeks ago:

What is that weird cylinder thing they put on my splitter?

That's a filter.

How is my modem working at -18db if it's supposed to be at least -15?

As far as the -15 to +15 range goes, that's not actually mandatory for the modem to work. It's the ideal range, because in most cases, at -15 the noise is starting to get bad.

Here's an analogy:
Say you're in a filled room having a conversation with someone. The volume they are speaking at will be the signal level. The volume of the people around you is the noise level.

Now, if the person speaking to you is at a low volume, you're going to have a hard time making out what they're saying because of all the noise.

HOWEVER, remove that noise from around you, and you can hear that guy just fine. If you have very little noise, you can operate at lower signal levels reliably.

I've had a modem running at -32db before, because it was running through a 300MHz splitter.

Do you get better connection closer to the node?

I think there is a Node at the pole in my back yard. Do you get better results the closer you are to the node?

Not necessarily. The farther away you are, there is a greater chance of noise getting into the line, but the difference in speed is quite negligible, especially since the LEs (Line Extenders, or amplifiers) boost signal both ways all the way down the line.

Is there a optimal range for the receive window?

Yes, there is an optimal range for every class and type of service, every operating system, etc.

Go here:


Thanks to StillLearn See Profile for pointing out a spelling mistake.

Do I need a surge protector for my cable modem?

You can never have too much protection. Lightning can enter your home through your cable lines just as easily as the phone/electric lines. Of course, the NEC requires that we ground the line to prevent lightning damage, but that does not guarantee that lightning will not enter your home.

I recommend a surge protector for your cable modem if you're in an area with lightning storms (especially in the south). Make sure you get one that is cable modem compatible.

A surge protector shouldn't affect bandwidth. From what I've found, it will either work or it won't. Make sure you use a surge protector that is rated for cable and/or DSL modems.

Sharing bandwidth - how many users per node?

Check out this FAQ for a detailed description

9. Terminology


Tap - Usually the connection point at the utility pole or ped where your cable is hooked up.

Pedestal / Ped

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.



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.

Input Line

Generally, this is the line running from your ground block to your junction or first splitter in a loop system.

Star / Home Run system

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.)

Loop System

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.


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.

Downstream Power

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 See Profile for a correction.)

Downstream SNR / Signal to Noise Ratio

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".

Upstream SNR / Signal to Noise Ratio

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".

Upstream Power

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.


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.


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.


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 See Profile: .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 .


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.)

Insertion Loss

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.


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. ;)


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.

10. Tables/References

What are the frequencies for channels in the US?

There is a complete list of CATV and Air channel frequencies at:

What are the limit sets for a cable modem?

Min Video level 0.0
Max Video Level 15.0
Min Delta V/A (dB) 10.0
Max Delta V/A (db) 17.0
Min Digital Level -10.0
Max Digital Level 10.0
Signal Quality 64 QAM 28.0
Signal Quality 256 QAM 32.0
Max Pre BER 1.0E-7
Max Post BER 1.0E-9
Min CM Headroom (dB) 5.0
Max Ingress -30.0

*These are suggested levels for the downstream plan for a cable modem.