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

Post 174 of 178

Cable modems use RF (radio frequency) signals to transport data over hybrid-fiber coax (HFC) networks according to the DOCSIS® specification.  This blog will discuss the finer points extracted from the DOCSIS specification related to how cable modems communicate with the headend Cable Modem Termination System (CMTS), allowing two-way transport of Ethernet traffic over a cable TV network.

There are currently three major revisions and one sub-revision of the DOCSIS specification; DOCSIS 1.0, 1.1, 2.0 and 3.0.  With each major revision came significant changes to the cable modem upstream specification because the upstream has typically been the bottleneck in terms of data through-put rates as will be discussed.

The DOCSIS 1.0 and 1.1 specification provided for 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 data communications, which are symbol rate and filter shaping.  Per the DOCSIS 1.0 and 1.1 specification, the allowable symbol rates for upstream data transmission are 160, 320, 640, 1,280 and 2,560 ksym/sec.  Additionally, the specification defines that the cable modem shall use a root raised cosine shaping filter with an alpha=o.25 or 25%.  Great!  So what does that mean to us in terms of data rate?  Let me explain.  First, let's visualize the QPSK and 16-QAM constellations, theoretically:

QPSK and 16-QAM Modulation

Upstream Modulation Constellation Diagram

The above two diagrams show the data symbol mappings for QPSK and 16-QAM modulations.  Notice that a QPSK modulation has four symbols each containing two bits, while 16-QAM has 16 symbols each containing four bits of data.  This implies that 16-QAM transports more data than QPSK, but it may not be immediately apparent that 16-QAM is more susceptible to signal impairment because the symbols are closer together and therefore are more difficult for the receiver to demodulate.  So now that I have you understanding that modulation is made up of symbols containing multiple bits of data, we can talk "data rate", which is very straight forward.

Data rate = #_Bits_per_symbol * Symbol_Rate

Example: For 16-QAM with a symbol rate of 2560 Ksym/sec.

Symbol_Rate = 2560 ksym/sec

#_Bits_per_symbol = 4

Data rate = 4 bits * 2560 ksym/sec = 10.24 Mbits/sec

This data rate, 10.24 Mbits/sec, happens to be the maximum theoretical data rate for DOCSIS 1.1.  The actual usable data rate for subscriber "protocol data units" (PDUs) is roughly 8 Mbits/sec because DOCSIS requires some amount of overhead for DOCSIS protocol messaging, which is communications between the cable modem and CMTS, that will be covered in a later blog.

Now about that root raised cosine shaping filter...  The purpose of that filter is to minimize the RF harmonic energy produced by the cable modem so that it does not produce interference with adjacent devices.  The way it does this is by acting as a low pass filter by removing all of the high frequency components of what would normally be a very "square wave" looking signal.  It is what make DOCSIS upstream signals look like a "haystack" (don't worry, if you haven't seen a DOCSIS haystack yet, I'll show you one in a blog post shortly).  The reason we want to know about the alpha=o.25 factor is because it allows us to calculate the occupied RF bandwidth for any given symbol rate that a cable modem may be using.  This is done by multiplying the symbol rate by (1 + alpha).  So in the case of our example problem above:

Occupied Bandwidth = (1 + alpha) * 2560 ksym/sec

= 1.25 * 2560 ksym/sec = 3.2 MHz

See, its all pretty simple stuff and can now be applied to the rest of the DOCSIS specifications.  So some of the changes that occurred in the newer specs are as follows:

  • DOCSIS 2.0 added higher order modulations including 8-QAM, 32-QAM and 64-QAM.  In addition a higher symbol rate was added at 5120 ksym/sec.  So at 64-QAM and 5120 ksym/sec one can achieve a theoretic limit of 30.72 Mbits/sec in the upstream or ~27 Mbits/sec of user PDU after overhead.  DOCSIS 2.0 also added some other advanced features that I will address in a future blog.
  • DOCSIS 3.0 added a fantastically cool feature called "channel bonding", both in the downstream and in the upstream.  Channel bonding essentially allows one to take up to four upstream DOCSIS channels and transmit data over them as if they were one single channel.  So for a 64-QAM, 5120 ksym/sec channel as in the DOCSIS 2.0 bullet above, you can bond four together and get a theoretical maximum data rate of 122.88 Mbit/sec.  I will soon be devoting a whole separate blog to DOCSIS 3.0, so stay tuned.

A big incentive for these changes hass been to increase the upstream bandwidth.  This was successful with each revision.  D2.o increased three times from D1.1 (10.24 Mbit/sec to 30.72 Mbit/sec).  D3.0 increased four times from D2.0 (30.72 Mbit/sec to 122.88 Mbit/sec).  What was the driver?  Both competition from telecom operators such as Verizon's FiOS and AT&T's U-Verse as well as subscribers using more applications on the upstream.  When DOCSIS was first launched most subscribers were downloading web pages, but today peer-to-peer file sharing, Voice-over-IP (VoIP), and many emerging applications are enabling symmetrical data utilization rather than the previous model of asymmetrical-dominant downstream traffic models.

I have now laid the foundation of RF fundamentals for my DOCSIS blogging moving forward.  Please feel free to post comments on any questions you have or areas where I should elaborate a little more on as I am next going to dive into the heart of the DOCSIS specification and start breaking down cable modem registration and how the DOCSIS specification enables data communication while managing hundreds of cable modems in the rather harsh environment of a two-way, outdoor cable TV plant.

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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