DOCSIS Tutorial

DOCSIS Tutorial Series

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The following links are a series of blogs on my DOCSIS Tutorial.  The tutorial is meant to be read in chronological order to build on one another from an educational standpoint. Enjoy and feel free to post comments on sections that you would like to see further elaboration.

DOCSIS and Cable Modems – How it works :: Introduction

Over the past several years of training people on troubleshooting DOCSIS® networks and explaining how DOCSIS cable modems and CMTSs work, I have been asked numerous times – “Why don’t you write a book on this?” You see, there is no definitive book, guide or manual that explains the Data-over-Cable Service Interface Specification (DOCSIS) other than the several hundred page specification…

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…

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

In high speed data communications, the industry has evolved and implemented complex modulation schemes which vary a signals voltage (amplitude), phase, and frequency in order to transmit more data, faster. The following will cover these methods in order to describe how Quadrature Amplitude Modulation (QAM) works in DOCSIS communications.
We live in an analog world. Sure, [...]

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

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.

DOCSIS and Cable Modems – How it works :: Cable Modem Registration

Through DOCSIS tutorial seminars, I have found the most effective way to bring someone up to speed on DOCSIS communications is by teaching the cable modem registration process. During the registration process, we will cover the RF physical layer, theDOCSIS Media Access Layer (MAC) and the Internet Protocol (IP) layer. Yes, the cable modem exercises the first three layers of the Open Systems Interconnection Reference Model (OSI Reference Model or OSI Model)…

DOCSIS and Cable Modems – How it works :: Anatomy of a Burst

Previously I have discussed that cable modems share the upstream channel by using Time Division Multiple Access (TDMA).  This means that when a cable modem is not transmitting data its RF transmitter is turned off.  In order to transmit data it must transmit a burst of data which contains a REQUEST to the CMTS.  The REQUEST relays the cable modem’s Service IDentifier and the number of bytes of information the modem has to transmit in its buffer.  The CMTS prioritizes all incoming REQUESTS and sends out time slots to each cable modem in MAP messages that contains the SID, when the SID (i.e. cable modem) can transmit, and how many bytes to transmit.  It is not uncommon that the CMTS may force a cable modem to span its transmission over multiple time slots since it is a shared medium.

DOCSIS and Cable Modems – How it works :: UCD

Just as a picture is worth a thousand words, so is a complete Upstream Channel Descriptor (UCD) as seen by a cable modem.  This message, sent every two seconds by the the CMTS, contains an enormous amount of valuable information that every cable modem on a DOCSIS network needs to know in order to communicate.  Often undervalued, the UCD is a virtual Hitchhiker’s Guide to the DOCSIS network!

DOCSIS and Cable Modems – How it works :: Station Maintenance

While the UCD provides the language of the DOCSIS network, the Station Maintenance messaging is the proverbial “heartbeat” of the DOCSIS network.  A station maintenance session consists of a Range Request sent from a cable and a Range Response sent by the CMTS.  The CMTS analyzes the signal quality of the Range Request message and sends back any necessary RF adjustments in the Range Response message.  This “handshake” between every cable modem and the CMTS must occur once every 30 seconds as dictated by the DOCSIS specification.

DOCSIS and Cable Modems – How it works :: Quality of Service

DOCSIS 1.0 enabled data over coax with a “best effort” service using a data request-grant methodology.  DOCSIS 1.1 and subsequent specifications added guaranteed Quality of Service (QoS) by providing Unsolicited Grant Synchronization (UGS) which means that a cable modem does not have to send a data request in order to receive a bandwidth grant from the CMTS.  The new UGS service is an enabling technology which has allowed cable operators to successfully deploy the highly revenue generating Voice-over-IP (VoIP) services. In the following sections I will illustrate the differences between best-effort (request-grant) and QoS (UGS) services

DOCSIS and Cable Modems – How it works :: Wrap-up

If you have followed the “DOCSIS and Cable Modems – How it works” tutorials this far, congratulations!  You now have a basic foundation of how DOCSIS networks operate and the ability to pick up the DOCSIS specification and read and comprehend it – this is hard to do for the novice.  If you are just finding this blog for the first time, then I recommend that you go to the DOCSIS Tutorial Series and start at the beginning before proceeding.