Monthly Archives: February 2009

DOCSIS and Cable Modems – How it works :: UCD

February 28, 2009   Brady   11 Comments
DOCSIS Upstream Channel Descriptor - 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 enourmous 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 Hitchiker’s Guide to the DOCSIS network! So without any further ado, let me show you an example of a full UCD message and then explain its contents further

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

February 22, 2009   Brady   8 Comments
DOCSIS 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

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Troubleshooting DOCSIS – VoIP Impairments > Delay & Jitter

February 16, 2009   Brady   2 Comments

In this blog I will address delay and jitter as they pertain to VoIP in a DOCSIS network.  Delay, jitter and packet loss are the three primary impairment in a VoIP network, but packet loss was addressed in my Troubleshooting DOCSIS – VoIP Impairments > Packet Loss blog. After packet loss, delay is the second

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Troubleshooting DOCSIS – VoIP Impairments > Packet Loss

February 15, 2009   Brady   No comments

In this blog I am going to focus on VoIP packet loss, which is just one of the three (3) primary types of VoIP impairments that are present in a DOCSIS network. I will cover many RF and IP terms in this blog that I have not discussed in my previous tutorials, not to worry! This terminology is all fodder for future blogs. :-)

To review, the three fundamental impairments which impact call quality of VoIP communications are as follows:

* Packet Loss – The complete or partial loss of a packet containing actual voice payload.

* Delay – The time a packet takes to traverse the space between the source and destination of a voice call. The space is comprised of both the physical distance the data must travel in addition to the active network routing and switching elements, which contribute additional delay.

* Jitter – The variance of inter-packet arrival time from one transmitted packet to the next sequential packet.

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Troubleshooting DOCSIS – VoIP Impairments

February 13, 2009   Brady   1 Comment

In this blog I am going to digress for a moment from my standard DOCSIS 101 tutorial and spend a little time on DOCSIS troubleshooting basics, especially with respect to Voice-over-Internet Protocol (VoIP). I am doing this as due to many offline questions specific to this topic. Don’t worry, though I may start to inter-mix some troubleshooting blogs now and again, just check the DOCSIS 101 page to stay on top of the latest DOCSIS tutorial blogs.

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

February 09, 2009   Brady   30 Comments
DOCSIS Network Cable Modem Registration Illustration

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)…

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

February 05, 2009   Brady   23 Comments

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.

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

February 02, 2009   Brady   12 Comments

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, [...]

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