Comments on: DOCSIS and Cable Modems – How it works :: Advanced RF

By: RF cable

RF cable — Tue, 15 May 2012 11:39:32 +0000

Thanks to share this awesome post.

By: DOCSIS and Cable Modems - How it works :: Upstream RF | The Volpe Firm

DOCSIS and Cable Modems - How it works :: Upstream RF | The Volpe Firm — Tue, 01 May 2012 19:43:46 +0000

[...] two upstream modulation profiles; QPSK and 16-QAM. I covered modulation profiles in detail in my Advanced RF blog post and thus will not review it here, but I will cover a couple of new concepts regarding [...]

By: Brady

Brady — Sat, 23 Jul 2011 13:04:08 +0000

Thanks Ash. Appreciate the feedback.

-Brady

By: Ash

Ash — Fri, 22 Jul 2011 22:07:09 +0000

Nice stuff – really helpful for a quick rampup of DOCSIS required for my project needs at work

I also like the way you make it clear for everyone to understand but at the same time do not miss out the technical details

By: Bernard Price

Bernard Price — Thu, 21 Jul 2011 07:52:57 +0000

A simple explanation of a complex subject; congratulations.

By: Brady

Brady — Thu, 07 Apr 2011 00:53:16 +0000

Hi,

QAM in the HFC network can be a little confusing because we take the QAM baseband signal and them RF up-convert to a higher frequency with yet another “carrier wave”. So you are pretty much right in your thinking, there is just a little more to it.

QAM Modulation does involve two carrier waves which are modulated by the data. The carrier waves are sinusoidal and 90 degrees out of phase. The QAM modulation is at what is called a base band frequency, which means centered around 0 Hz. In order to make it compatible with the HFC network we need to get it up into the 88-1000 MHz range. To do this we use a device called a mixer and a carrier wave centered at the desired frequency, say 550 MHz. Input the baseband QAM signal with the 550 MHz carrier wave and now we get the upconverted QAM signal at 550 MHz.

So as you see, a QAM signal in the HFC plants is a combination of the baseband modulated signals that is then upconverted to a higher frequency. Every QAM signal gets upconverted to its respective channel on the HFC channel plan, but they always start at baseband. The CMTS or set top box will then downconvert it back to baseband (0 Hz) before demodulation.

Hope that clears it up.

Regards,
-Brady

By: NOC Tech82

NOC Tech82 — Mon, 28 Feb 2011 03:32:17 +0000

Brady,

While reviewing QAM it seems I have hit a little bit of a brick wall that has me second guessing what I presumed to have already known.

In its most simplistic nature, QAM allows us to modulate the amplitude of a given RF carrier so we can move digtal content through it. You mentioned that QAM modulates the amplitude of “two” carrier waves. Im a little confused because I always perceived the RF portion as the quote carrier wave and digital content was the other signal that was involved or have I been wrong all this time and there is actually two RF waves used to allow digital content to pass over an RF frequency?

I hate to ask such a silly question but I just want to make sure Im on the right page with the rest of the world.

Thanks!

By: Michael

Michael — Mon, 22 Nov 2010 15:42:10 +0000

This is a great blog – really helped me understand RF modulation principles. Thanks !

By: Gary

Gary — Tue, 09 Nov 2010 17:28:53 +0000

Great site Brady!
>
I’m hoping you can answer a couple of questions relating to DOCSIS channel spacings within the RF spectrum?
>
Firstly, I understand that the downstream utlises the 108MHz to 862MHz, using 8MHz channels (EuroDOCSIS) and upstream utilising the 5MHz to 65MHz, using upto 6.4MHz channel bandwidths. I would like to know what degree of separation (i.e. frequency gap) exists between each adjacent channel in botht he upstream and downstream direction as I cannot seem to find this referenced anywhere?
>
Secondly, are you able to confirm what the actual usable bandwidths or frequencies are for DOCSIS/EuroDOCSIS, Analogue TV, and Digital TV in both the upstream and downstream frequency ranges? i.e. upstream spectrum states DOCSIS/EuroDOCSIS (Data) uses the 5MHz to 42 or 65MHz range, but doesn’t have that entire range to play with. What are the actual defined frequency bands open to these services within the entire Cable TV/HFC spectrum plan (5Mhz-862MHz range)?
>
Thanks in advance…
>
Gary

By: Cyrus Winnen

Cyrus Winnen — Tue, 10 Aug 2010 04:22:26 +0000

So glad I stumbled upon this! I am new to all this but I am little bit confuse by the first point you made. Nevertheless, these are good suggestions!