DOCSIS 101 Archives

5 07, 2012

Subscribers get lower data rates than provisioned?

Mia Colabrese2020-06-09T09:36:58-04:00July 5th, 2012|

Why do my DOCSIS subscribers and their Cable Modems get lower speeds than what my DOCSIS config file says they should get? This is a common question that I am asked and it has more than one answer. All too often the RF plant is the first to blame for low DOCSIS cable modem speeds, but in many cases the root cause of the problem can be traced back to the cable modem configuration file and/or the lack of appropriate speed test applications in the headend. Also, some

21 09, 2011

Cable Modems init(r1) init(r2) Why are they stuck?

Mia Colabrese2021-08-20T09:21:27-04:00September 21st, 2011|

Cable Modems init(r1) init(r2), DOCSIS Cable Modems going offline? DOCSIS Cable Modems init(r1) init(r2) are stuck or offline! Many of us have been there before - one or more DOCSIS cable modems init(r1) init(r2) stuck in one of numerous "init()" conditions - how do we interpret these messages and what do we do? A recent reader wrote in and had just this problem. DOCSIS cable modems going offline and getting stuck in "R1" or "R2" condition, or Cable Modems init(r1) init(r2), because these are the status conditions displayed on the DOCSIS CMTS

24 06, 2011

DOCSIS T1, T2, T3, and T4 Timeout Descriptions

Mia Colabrese2022-06-20T20:55:33-04:00June 24th, 2011|

This is a handy summary of the various timeout errors you will find in a DOCSIS network. T3 and T4 timeouts are described in much more detail elswhere in this blog, however this post provides a high level overview of the various timeouts as a reference.

20 04, 2009

DOCSIS and Cable Modems – How it works :: Tutorial Wrap Up

Mia Colabrese2021-08-17T13:35:17-04:00April 20th, 2009|

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.

9 03, 2009

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

Mia Colabrese2021-08-17T13:22:59-04:00March 9th, 2009|

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.

3 03, 2009

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

Mia Colabrese2021-08-17T15:43:18-04:00March 3rd, 2009|

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.

28 02, 2009

DOCSIS and Cable Modems – How it works :: UCD

Mia Colabrese2021-08-17T10:36:51-04:00February 28th, 2009|

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

22 02, 2009

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

Mia Colabrese2021-08-17T22:35:57-04:00February 22nd, 2009|

DOCSIS Cable Modem Data Burst Defined 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

16 02, 2009

Troubleshooting DOCSIS – VoIP Impairments > Delay & Jitter

Mia Colabrese2021-08-17T15:54:55-04:00February 16th, 2009|

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 most disruptive impairment in VoIP networks. The effects of delay to the caller generally appear as echo and talker overlap. In PSTN communications, echo can arise as acoustic echo between the mouthpiece and

15 02, 2009

Troubleshooting DOCSIS – VoIP Impairments > Packet Loss

Mia Colabrese2021-08-17T15:45:30-04:00February 15th, 2009|

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.