DOCSIS and Cable Modems - How it works :: RF Fundamentals

DOCSIS 101

DOCSIS and Cable Modems – How it works :: RF Fundamentals

Most of us are quite comfortable with changing the dial on our FM radios. Don’t like what’s on Soft 95.1 FM, then change the frequency (Kenneth) to 102.5 FM for some classic rock. What you have actually done is changed the RF (radio frequency) tuner in your car stereo from a lower frequency of 95.1 MHz to a higher frequency of 102.5 MHz where there was a different station playing. The fact that two different stations were playing at different frequencies is something call Frequency Division Multiplexing or FDM. FDM is used in cable TV to deliver many television channels to our homes on a single piece of RF coaxial cable. Typically the range of frequencies that are delivered to our homes for television signals is 54 MHz to as high as 1000 MHz (though many current systems only support 750 MHz or 860 MHz).

In DOCSIS, a device at the cable operator’s headend call the CMTS or Cable Modem Termination System, is responsible for managing hundreds or thousands of cable modems residing in subscriber’s homes (aka you and me). The CMTS sends data to the cable modems by transmitting a 6 MHz wide band of information (1′s and 0′s) in an FDM mode, just like all of the other television channels that you receive. Now the 1′s and 0′s are actually converted to Quadrature Amplitude Modulation (QAM) and RF-upconverted, but this will be covered in my next blog post call, “Advanced RF Fundamentals.”

So if the CMTS communicates with the cable modems from 54 MHz to 1000 MHz, how to the cable modems send data back to the CMTSs? We do want to send Internet data in both directions, right?! Well this is a pretty cool, yet seldom known fact about CATV plants; cable plants actually transmit RF signals in two directions. See figure 1, below. The forward (or downstream) path is from the cable operator’s headend to the subscriber and is generally from 54 MHz to as high as 1000 MHz. While the upstream is what is returned from every house back to the cable operator’s headend. This frequency range is typically from 5 MHz to 42 MHz. Now the cable modems can send their data back to the CMTS using FDM in the upstream sending 1′s and 0′s which are also converted to QPSK or QAM and RF-upconverted.

Figure 1: CATV Frequency Allocation

So stop back for next blog where I will digg into the mystery behind all of that QAM mumbo jumbo that you hear about in television advertisements. QAM is actually a very interesting topic which makes the transport of Internet Protocol data over many physical media possible – and its not too difficult to understand.

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About the Author:

Brady Volpe is the author of many industry articles and papers and owner The Volpe Firm, Inc. He has over 20 years of broadband cable and telecommunications industry experience specializing in DOCSIS, EPON, VoIP, video, IPTV, RF, and fiber optic transport.

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17 comments

Scott
February 25, 2009
Just stumbled across your blog. Great stuff thank you!

Reply
Martin
June 3, 2009
I work the data side of the MSO I work for but more and more I am moving into the Docsis and rf side of the buisness and need help understanding the rf side. I am sure your site will help me.

thanks

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JJB
August 15, 2009
Absolutely great job, keep up with the good work. The t3 and t4 Excellent!!

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Paul Kirshner
October 24, 2009
Wow, great blog. I love reading blogs about Voice over IP! It’s such an exciting technology. I have learned a lot in implementing a small VoIP network at home, and am thinking of starting VoIP business in my area. There are a number of small businesses in my region that would benefit from it greatly. Thanks again for this blog – it is really well-done.

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DOCSIS and Cable Modems – How it works :: Advanced RF | bradyvolpe.com
June 30, 2010
[...] well that’s “pi” and is approximated by 3.14 f0 is the frequency of the signal, in my blog on RF Fundamentals I discussed that the DOCSIS downstream would be between 54 – 1000 MHz, so f0 could be 500 MHz as [...]

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Manish
August 28, 2010
Typically the range of frequencies that are delivered to our homes for television signals is 54 MHz to as high as 1000 MHz

then

The CMTS sends data to the cable modems by transmitting a 6 MHz wide band of information

If available frequency is between 54 and 1k, how does CMTS transmits data at 6 MHZ.

Please let me understand.

thanks

Reply

Brady
August 30, 2010
Hi Manish,

In your second paragraph I believe you meant “between 54 and 1 GHz”. But aside from that, a DOCSIS CMTS transmits digital data (1′s and 0′s) by first wrapping it into Ethernet Frames and then into and MPEG-TS Layer 2 188-byte long stream (you can google those terms for details). The MPEG stream is fed into a Quadrature Amplitude Modulator (QAM) device that converts the MPEG stream into either 64- or 256-QAM following the ITU J.86 Annex B standard (Annex A for Europe). This modulation is still considered to be in a digital format, but has a 6 MHz bandwidth centered around 0 MHz (baseband) or 8 MHz bandwidth for Annex A – Europe. But we really need to up-convert it to between 54 MHz to 1GHz as you know (actually, the new DOCSIS 3.0 specification defines the downstream to now be 108 MHz to 1 GHz). The upconversion uses a device called a mixer. The mixer takes a local oscillator (LO) and multiplies the baseband 64- or 256-QAM digital channel up to the desired frequency within the RF spectrum (54 MHz to 1 GHz). It will still be 6 MHz wide. And that is a very basic explanation of how a CMTS transmits digital data. The whole process of QAM and upconversion is actually a little more complex, but takes place all in one box so you never see the gory details. You just tell the CMTS or edge QAM what frequency you want it to transmit on.

I hope that helps.

-Brady

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ROBERTO ANDRADE
September 2, 2010
Perfeito !

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Enola Torok
March 18, 2011
Greetings! This is my 1st comment here so I just wanted to give a quick shout out and tell you I really enjoy reading your blog posts. Can you recommend any other blogs/websites/forums that cover the same topics? Thank you so much!

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Brady
April 6, 2011
Thanks for the feedback. Please take a look at the guys at http://www.docsishelp.com/forums.

Regards,
-Brady

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Phillip
May 23, 2011
Great stuff brad! Your posting about how a cmts was really easy to understand, thank you!!

Hope to hear more stuff!!

Phillip

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Sunoop
July 1, 2011
very informative.Great stuff.Thanks Brady.

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Brady
July 1, 2011
Thanks for the feedback Sunoop.

-Brady

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Dave
July 28, 2011
Dude, this is seriously good stuff.

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Brady
July 28, 2011
Why thank you Dave. Appreciate the compliment.

-Brady

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Raghav
January 16, 2012
Gr8 job brad. I really appreciate it.

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J. Alexander
March 31, 2012
Thanks for the easy to understand and informative story. It helps to have some understanding of DOCSIS. The “radio frequency” tuner analogy was a fine example.

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