For various uses of Trilithic 860 functions please see the following Articles for a quick overview of BER and Spectrum.

http://cabletechs.org/troubleshooting/tracking-noise/

http://cabletechs.org/troubleshooting/troubleshooting-noise-part-two/

http://cabletechs.org/troubleshooting/troubleshooting-ber/

For more Meter training documents or answers please visit the forums Meter Training Section.

http://forums.cabletechs.org/meter-training/

This is less of a “How to get my modem upstream lower” article as we all know the less splits from the tap to the modem the lower the  modem upstream. Pretty basic stuff. What we are going to get into this time around is your upstream at the tap. What you expect and what you get are not always the same thing. There are a few reasons why this is and its important we all understand them.

First off, here is some quick math to approximate what your upstream should be at the tap. Tap Value + 18dbmv = Upstream.  Some areas use 20dbmv, Ive always used 18 as it has always proved the most accurate. ( however I always add 20 to the tap value in my head then subtract two because until you get used to doing it, adding 18 to anything is hard head math.. for dumb people like me..) This number might vary from system to system, lets assume youve asked already or split the difference to 19 as a rough guess. Lets get right to it!

For example:

• 10v Tap + 18= 28
• 10v tap should be at 28dbmv
• 14v Tap, + 18= 32.
• 14v tap should be at 32dbmv
• 23v Tap+18= 41
• 23v tap should be at 41dbmv

So why isnt this always the case? Why when I climb up to a 7v or 10v tap would I NOT have 25 or 28dbmv? If I have 41dbmv isnt that a referral to the line department to balance the run?

“Heck, Ill leave that modem at 54dbmv and wait for the line guys to knock the upstream down from 41 to 28. The modem will be sitting right at 41dbmv when they are done!!”

There are NUMEROUS reasons why what you expect might not be there. That is why it is always best to balance your signal allocation to what is present at the tap as long as it is in spec, forward and return. Never assume and leave a customer out of spec.

Now that we have the math down, and under normal circumstances we know what to expect at any value tap, lets get a little deeper in Plant Design.

Below is a design for a random MDU in a highly concentrated area. Think of it as a home with many rooms and various needs for connections. Your going to have to split something at some point. Most MDU areas are fed with a splitter or DC from the main line. This is why, if you have ever noticed, there is a 14v or lower value tap in the lockbox.

Below you can see the key provided as to what is what. DCs are simply DCs. Just like you would use in the home. Minimal loss on the through leg, higher loss on the “down” leg. This goes for forward and return signal. The splitter functions the same as well, equal loss through both legs, forward and return. Looks busy huh?
Lets break it down…

Lets get rid of the green Pole Line behind the string of 6 unit MDUs. Let get rid of all of the plant except for the very first hop off of the amp. The DC12.

The “down” leg of the DC12 has 12db of loss on the forward and return. So do some math with this one and tell me what that 10v tap should have as an upstream.

10+18=28.  But add in the 12db from the DC12 and your upstream at that 10v tap is 40dbmv. That was quick? I thought I would have 28 no matter what? Hmmm.. I guess that is why I have a 10v tap coming fight off of an amp than.

The through leg continue on as we have MANY MORE 10v taps to hit. Lets see how our math works out down this run?

Ok, this looks a little more reasonable, a DC7!! Now Ill get some headroom for my upstream. Quick math time-

10+18= 28 Now add in only 7db for my DC7. 35dbmv. Higher then you expected but plenty to work with.

But wait.. what is that little red Circle with the yellow legs? Its a 2-way splitter according to the legend. Crap. Add in another 4db of return and forward loss. Now Im at 39dbmv. Luckily, further upstream, Im going through the through leg of that DC12. I might as well add in 1db for insertion loss. Back at 40db.

Well, thats about three 10v taps in a row that all got synch at 40dbmv? Lets look at some more of this run.

No we continue on and hit a 20v tap which hits another DC9.

The Down leg of that DC9 feeds a 10v tap directly, the through leg feeds another 2way. You can do the math from here on in. The upstream at these two taps are a little lower but still higher then your expected upstream. Reference the first print and see if you can figure out the rest of the upstreams!

This is a pretty good example of why, given adequate signal at the tap, forward and return, we allocate with what we have. Upstream is very important these days with the bandwidth intensive products we are rolling out and the sensitivity of Docsis 3.0 and whatever is to follow. And now that you know a little bit about upstream and how much thought goes into system design it might be clear why your local maintenance guy cant just “Swap that tap out with a lower value?!!” or “Change that DC12 to a DC 7!!” There is generally a delicate balance to each and every plant run. Knowing this- can anyone tell me what is wrong with the Amp and Tap set up in the very first Field picture of this article?

If you have any comments, questions or corrections please drop them in the Article Comment section or better yet, start or join a discussion in the Tech forum?!

Recently there was a job referred to a local maintenance dept. The technician arrived on site for Digital Tiling. He performed all of his tests, Home Cert, Spectrum Check, various QAMs and of course spending time on the channels he was having issues with. He found the affected channels MER to be at 29-30 and BER to hover around 5.00-E-06. This would definitely cause tiling. The tech also noted that it was only on a handful of PODs, or Channels. Everything around these channels were in spec, no issues.

I will skip the details of the troubleshooting that brought the line tech to the actual issue as it is repeating another article you can find by clicking this text.  I will say this however, the issue was caused by a Direct TV customer who had fed their Direct TV feed into the homes cabling and also back out to the cable plant. What this did was inject a pretty serious carrier back into the cable plant that caused all kinds of impairments on the QAM channels that were occupying the same spectrum.

In simpler terms, We have a digital carrier on say 549MHz. This carrier holds compressed and encrypted channels like HBO, TLC and we will say ESPN. If another carrier is injected into the plant on this same frequency, or through this frequency it will severely “screw up” the channel we are putting out. This is keeping our plant clean. This is ensuring that cable stays a “closed loop system”. Nothing gets in to interrupt our signal carriage and nothing gets out to affect other services.

The screenshot is the actual carrier. This is a rather large carrier and it occupied the space of about 10 cable PODs which all in all wiped out 50 or so channels. Once the drop was disconnected from the Direct TV sub, the MER popped up to 38, BER was solid at 1.00-E-09 and services were restored. I will try to get a screenshot of our same spectrum and our carriers for the purpose of this example and append them to this article.

This article is less of a “troubleshooting” article and more of a “look out for this” article. Taking care of our spectrum and keeping it clean is every techs jobs. Make sure we are shooting our drops and homes for ingress at every job. Doing our best to keep the plant clean and our customers homes as well.

Keep a look out for another article on how to set up your 860DSPi for accurate viewing of the return and forward spectrum!

Troubleshooting Noise comes up often. Now more then ever noise mitigation is playing an important role in your everyday job. Noise plant wise is a cumulative impairment. A little bit of noise here, a little bit of noise there and the “power” of the noise grows and grows. Its the epitome of the snowball effect. Not only that, it affects the service of single homes as well in a few different ways.

Lets dig in. 

Noise has many causes. Things to look for when tracking noise are things you already look for. Loose connectors, damaged cable, chew on the drop, poorly prepped connectors, you name it.

Tracking noise is actually quite simple, at least from the tap to the source in the home. Service techs generally check the Forward and Return for noise in most systems. You arrive on job, climb up to the tap, belt off and start checking for issues. There are two ways to do this. Its best to start whatever habit you want to cultivate now. Im a big fan of having a core routine to troubleshoot by. If Im being thorough Im disconnecting the cable from the groundblock as I enter the yard and screwing on a barrel. This terminates your drop. Now when you climb the pole and check for ingress you will know if you have an issue on your drop or not since it is all you are checking. If you did find noise without doing this you still wouldnt know if it was the drop or coming from the home. You would have to climb down, disconnect the output of the groundblock and check the houses cabling for ingress. And you might see some, the noise you saw could have been from the house and the drop. You wouldnt know.

So lets say you see noise on the drop. Change it. Do not spend too much time trying to find the source. Trying to patch up the drop, just get off of your but and run a new one. As with anything, the more you do this, the faster you will get. See CableTechs “Tips” Article on Running A Drop for tips and tricks. If you do not see noise than perform whatever other tests you are running and head to the groundblock. While you are unscrewing the barrel from the drop run the Spectrum test at the Groundblock going into the house. BAMMO! Ingress! Great- Lets get to troubleshooting shall we?

In this scenario- a real one- we have a customer who called in for a VOD issue. They are having trouble ordering VOD and this is their 3rd trouble call. You open the housebox and see a brand new splitter and connections, the past techs did their job. Or did they? You run your Spectrum test on the 4 way splitter and you see noise. In the graph to the left, the actual screen shot from this trouble call from over 3 years ago, you see the noise. Most MSOs have a threshold of -30 for noise. You can see the “Peak” is at +1dB around 90MHz. In the screen shot above the marker is at 40MHz to isolate where our return path is. You can see the high noise all the way to the left.. that just so happens to be the same frequency range our cable boxes talk back at. The same range our VOD requests shoot out at.  There are also noise spikes further to the right of the spectrum which is also in our Digital Channel range and can cause tiling. ( this is why we do not just check the return on service calls) This is most likely our issue. So how do we track and fix?

You start by removing the outlets off of the splitter one at a time until the noise disappears. When you unscrew the outlet that causes that noise to drop, that is the one you follow. In this case it lead to another splitter. A two way. We follow the same procedure. Put our meter on the input and disco each outlet until the noise disappears. Follow that outlet. We are lead to a second story bedroom with a crawlspace. In the crawlspace we find a “Radioshack” gold splitter with frayed connectors, loose crimps and quarter turned connectors. Only three of the four ports are being used, the last unterminated. The outlets in this case were also all antennae line used for Cable by the sub. At this point you provide the option- Replace your DIY job for whatever the cost is by your MSO or remove the splitter, barrel up the lines and let them know the noise is gone but if it returns they will need these lines replaced. The image to the right is the actual splitter and connections that caused the noise in the preceeding image. This Trouble call was written up in detail in THIS ARTICLE.

Now that is basically what you will come across all day. Noise sources however can be intermittent. These are the worst. Its at this point you employ a few different techniques. Have the customer turn on all TVs, maybe ask what they do before they watch TV, make coffee, do the laundry etc. It is not uncommon for a loose connector or damaged piece of cable to show no issues until something it is close by injects noise into it. A blender, Refrigerator, TV, Surge Protectors, lights on timers. Another call had me at a customers house at 7pm when they stated it tiled like clockwork. I stood in their living room at 7pm and sure enough it started tiling. What else happened at 7pm that I was able to see out of the corner of my eye? Their porch light turned on automatically. I go outside and see a stapled line running along the eaves above the porch light. Bingo.

Another method is the return ingress test. Ill have to save that for another post however.

If you would like to comment on this article feel free to do so below. If you would like to discuss it further, ask questions or offer insights please join the current discussions in the forum!

Troubleshooting BER- The basics.

BER Troubleshooting? This comes up A LOT. Troubleshooting BER has a bit of mystic magic about it. Its a lot of numbers in an odd scientific notation that baffles a lot of techs. Especially the new guys. First off lets start with breaking down the measurement a BER test spits out. 1.00-E-09

What does that mean? Ask any tech and his answer will be the same.

“If its below 1.00-E09 its bad and I have to find whatever is causing it to be bad…”

“What do the numbers mean though?”

“Dunno…”

Well, here they are-

X amount or errors out of X amount of data sent . That is all.

When you are on a service call for tiling or any issue really and the QAM channel you are testing shows a BER value of 5.00E-05 it means simply this- There are 5 errors for every one hundred thousand bits or packets sent. That doesnt sound like much at all. But if you take into account how many packets are sent per second youd see this is a HUGE amount of errors.

The key to running a good solid BER test is patience and immobility. Trilithic, the makers of the 860DSPi that most techs have these days, suggest to keep your meter completely still when running a BER test. Also, since you are measuring over time its suggested to run the test or “sit on the channel” with your meter for at least a minute. You will not get an accurate test or measurement if you just hit your QAM button and channel up as soon as the carrier locks on. Remember, BER is a measurement of errors over time. So give it some.

Repairing BER however is much easier then extrapolating the scientific notation posted above. Its still cable right? So what causes poor BER? A bunch of stuff.

There are more involved issues that can cause poor BER. One possibility is a poorly balanced run. It has been witnessed that a run just off by 2dB or so can throw errors into your plant. More towards the end of the cascade as well. If your a service tech and have BER issues at the tap then you should be referring it to maintenance. Most MSOs agree that a BER tolerance of 1.00E-08 is acceptable. The differnence however might be if we are reading Pre or Post errors. ( more on that in a second.)

Going back up to the top of the article, when taking a measurement take care for accuracy. It isnt entirely uncommon for a tech to not find an issue at the home, BER or otherwise, while the customer swears it tiles all the time. The tech heads to the tap, climbs up the pole

Minimal Errors. Not the issue.

and starts a BER test. As he tests he bangs on the tap a few times assuming there is something loose or damaged that he can coax out of hiding and low and behold! BER errors!! ( of course there are you rookie!) Please refrain from doing this. Any disturbance can cause a few packets to be thrown asunder. This is normal. A little pop of a correctables or uncorrectables generally is not going to be your issue.  About the most I can advocate at the tap would be reading your QAM and maybe swaying the feeder a bit. Not shaking or rattling, but sway it a bit as it would sway in the wind. This will turn up radial cracks and loose connectors and will show as drops of MER or loss of Low End signal. That can be indicative of an intermittent issue on the plant. For the love of god do not shake or bang on the taps themselves.

In the end tracking BER issues is the same as tracking any issue. Find where its bad, track back to where it is good and troubleshoot in-between. High levels, Low levels, noise, loose connectors. You name it, they cause it. But at least now hopefully you understand what BER is, how to read it and how it is measured.

BER is a measurement of errors over time. So you need a little time to build up some packets so your meter can take an accurate sample. You shouldnt need more then a minute of testing to get an accurate QAM reading. Per spec 33 secs should get you a pretty accurate BER reading.
This takes us into Home Certification. It is not uncommon for a QAM to fail a BER test during a home cert depending on how many channels are set up to be checked for BER. The Cert test does not sit on every channel for 30 secs to 1 min. Therefor its testing threshold is a bit lower. Less bits tested divided by any number of errors will show a BER issue. Then you sit on the channel after the test and see no issue, so you test again. Frustrating I know. This is most noticeable on Docsis channels being tested. The meter is testing for Upstream, Downstream, MER, BER, etc. These are a lot of calculations in a short time. ( although it feels like forever when you are at the tap) Be aware of this when tracking your issue. Do not be thrown down the wrong path.

Do NOT use your Home Cert to troubleshoot. The Home Cert test is simply a verification AFTER your issue is resolved. Running around performing home certs to troubleshoot means we are skipping channels that need to be tested, not spending time studying a channel and possibly not listening to the customer in regards to what their issue really is. How many times have we set up a line call for high upstream when we were there for tiling but found no issue? We hope the line tech will come across something else. And we often STOP troubleshooting when any line issue is identified.

This can take us into many other avenues that Ill try and jot down at a later date. I hope this clears up BER reading and tracking.

If you would like to discuss this further, ask some questions or offer some insights please comment on this article or, even better, join the forum and continue the current discussions regarding BER and troubleshooting. 

Further resources.

JDSUs Definition of BER

Ron Hranac on BER