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JUST FOR FUN
DOCSIS Tutorial
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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.
[...] High level information from Brady Volpe. [...]
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[...] For those technically inclined, here is a great tutorial. [...]
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Excellent basics understandable even for a fresher.Thanks and congrats for you.Please keep continuing and educate people like me who always wanted to know more….
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Brady,
Great article and a big thanks to you that you continue to provide valuable insight in your area of expertise as you did at Sunrise Telecom…I will continue to frequent your website for valuable information and it’s nice to know you are doing well these days!
Keep up the good work!
Charles Carson
Senior System Analyst and Product Verification Engineer
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Neat explanation. Thanks!
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Great tutorial. Thanks. Question: my cable ISP has various speed tiers. Where and how is the downstream sending of data being controlled to limit users to their speed tier? Is this a responsibility of the CMTS or is it happening above the CMTS? Is there a particular scheme being followed? Thank you in advance for adding something to your tutorial or pointing me to where to learn how this is being done.
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Kevin,
In most cases, rate limiting or speed tiers, for cable modem subscribers occurs during cable modem registration. Right after the the cable modem performs DHCP and receives its IP address, it will download a configuration file via TFTP (trivial file transfer protocol). Among other things the TFTP config file tells the cable modem what its downstream and upstream “speed” capabilities are. During the final steps of registration, the cable modem lets the CMTS know what its speed capabilities are as well, so the CMTS is now informed.
The CMTS also has built in mechanisms for traffic shaping which may rate limit the maximum amount of data you can download. For example, you be able to download a small file very quickly at up to 20 Mbps (burst), but a longer file will be rate shaped down to 15 Mbps or even 10 Mbps. This rate shaping allows one to get small amounts of data very quickly, but larger files, especially P2P traffic, will be slowed down so that others can share the fat pipe.
Speaking of P2P traffic (i.e. file sharing through things like BitTorrent), cable operators often deploy a third layer of traffic analysis which inspects traffic behavior patterns. If they detect patterns that indicate P2P or similar type traffic which is not considered priority traffic, then they may further limit this traffic data rates.
So thatis a complicated answer to your simple question.
Regards,
-Brady
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Thank, Barry, for the further explanation. I’ve seen the max speed indicated in my cable modems status (196.168.100.1), but didn’t realize that my CM had a role in limiting my speed. So if I understand you right, when I do speed tests and I’m getting withing, for instance, 80 percent or better of my subscriber speed, that it is my CM that is likely limiting me, and otherwise the portion of the HFC is relatively lightly loaded at that time. But when my speed starts dropping, such as at peak times, to a portion of my subscribed bandwidth, that now the CMTS is traffic shaping in response to the overall heavy use of the pipe by me and all my neighbors.
So, a follow-up question might be: when we are at peak times of the day (i.e., slower network performance), ignoring QoS such as VoIP, is the CMTS likely depriving everyone equally, or is it still giving a proportional higher amount of bandwidth to the subscribers paying for the higher-speed tiers?
-Kevin
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Kevin,
Regarding your question “is the CMTS likely depriving everyone equally, or is it still giving a proportional higher amount of bandwidth to the subscribers paying for the higher-speed tiers?”
This is driven by service provider policy and, therefore, device configuration – it’s not inherently related to CMTS behavior. Rate limiting can be done in a way that is sensitive to both subscriber and time context.
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can someone show example of Docsis 3.0 configuration file for Cisco EPC3000 cable modem? Didn’t find on Cisco web site.
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Hello Brady,
Do you have a breakdown of a cable modem config file anywhere on your site. I’d really like to understand the parameters of a Docsis config file more thoroughly. It’s very difficult to find good well explained information on things that have to do with Docsis so I appreciate any advice here.
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JP,
I do have some config files and will try to get this posted in the Downloads section.
-Brady
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Brady, have you any experience in implementing the following behavior under DOCSIS 3? Instead of load-balancing traffic equally across all downstreams in a bonding group, downstreams are utilized only as the capacity of others becomes exhausted.
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Hi Daniel,
I have only statically assigned traffic to downstream channels in terms of load percentages (utilization) as you are indicating. So I have un-equally balanced a system in order to create an intended impairment.
But your question is supported by DOCSIS 3.0 via Autonomous Load Balancing. You configure Load Balancing Groups, must consider issues for non-DOCSIS 3.0 modems and whether you have all Logical channels or note, etc. Of course there are always modems that behave badly, but the balancing occurs via DCC and/or a service flow move. So either the cable modems or the traffic is moved around on the downstreams based upon the traffic loading and what you define as the loading rules. It is not longer static.
-Brady
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cool staff we have just rolled out the network i n Kenya hope to learn more from you guys.
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Hi Brady,
congratulations for site.
I am learning a lot.
Regards
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