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

Post 190 of 192

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.

CATV Frequency Allocation

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.


Mr. Volpe has over 25 years of communications industry experience. He is focused on the cable and telecom industry with deep technical and business skills. Mr. Volpe is currently the president and chief technologist of the Volpe Firm and holds an MSEE with honors.

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