Many of us have been there before – one or more cable modems 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, also known
With IPv6 on the way in a number of MSO (Multi-System Operator) networks, I have received numerous questions lately about how home devices such as routers and PCs attached to DOCSIS cable modems will get their IP addresses. Will cable operators suddenly start issuing IPv6 IP addresses to their customers? Will you as a subscriber need to upgrade your equipment to support IPv6? Or does the cable modem act as a Network Address Translation (NAT) device and hand-out IP address to each device attached to it? These are some very good questions and the answers are addressed in the DOCSIS specification as I will outline.
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.
This is the first of a new series of Tutorials focused on the Data Over Cable Service Interface Specifications (DOCSIS) version 3.0. I will make the assumption that you are familiar with the DOCSIS 1.x / 2.0 standards or have already reviewed my DOCSIS Basics Tutorial as I will be using many terms without explanation since they were previously covered.
The DOCSIS 3.0 specification is an extension of the DOCSIS 1.x and 2.0 specification which dramatically increases the data throughput by adding a technology known as channel bonding to the DOCSIS downstream and upstream, adding increased security, adding support for IPv6, and substantially improving the back-office management support (MIBs, SNMP, IPDR, etc.) for DOCSIS. Each of these topics will covered in much greater detail in this DOCSIS 3.0 tutorial in multiple posts yet to come.
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.
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.
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.
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
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
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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