Troubleshooting – Modem Upstream
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!
- 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?!