University of Wisconsin Stout Learning Technologies Bryan Walhovd

Goals:  The Goal of this Module is to give you a basic understanding of a cable modem and how it operates.  This high level overview will help you understand the following:  What a cable modem is.  The types of interfaces that exist on the modem.  Devices associated with a cable provider network.  How a modem comes online.  Glance into the OSI model and DOCSIS.  Future modules will provide a detailed view of each step a modem takes to come online as well as factors that limit connectivity and speed.

What is a Cable modem?  A cable modem is a broadband device used to connect to the internet over a cable companies Hybrid Fiber Coaxial Network (HFC). Image courtesy of:

Cable modem interface  A cable modem has two main interfaces that allow operation with the cable plant and customer equipment.  A coaxial cable port used to hook up to the cable plant  A RJ45 port which is used to connect to a computer or router via an RJ45 Ethernet cable. Image courtesy of: clipart/rj45-detail Image courtesy of: clipart/coaxial-cable-detail

In your home  Typically a modem is hooked to a router in a home or a business which allows access too the internet for multiple devices including PC’s, Phones, Printers and Gaming consoles. Image courtesy of: bmisitgs.wikispaces.com bmisitgs.wikispaces.com

Connection  Once the cable modem is connected to the HFC plant via the Coaxial connector and your PC, your all ready to go right?  Not exactly! The modem itself has to be set up (provisioned) on the cable providers network in order for it to access the internet.  So what next?

The Cable Modem Termination System  Once hooked up to the coaxial plant, the modem connects back to the cable providers CMTS, located typically in a HeadEnd. Image courtesy of: By Vivien Guéant [CC BY-SA 3.0 ( via Wikimedia Commons

What does all this mean???  A Modulator Demodulator (modem) communicates with the cable provider utilizing Radio Frequency.  This communication is bi-directional as we send and receive data from our personal devices. Images courtesy of: Image courtesy of: By Vivien Guéant [CC BY-SA 3.0 ( es/by-sa/3.0)], via Wikimedia Commons Image courtesy of:

How it works!

Downstream Lock  The modem first locks onto the Downstream frequency(s). The normal frequency range delivered from the cable provider is 54Mhz to 1000 Mhz. Scan for Downstream Channel Power Modem ON Downstream Sync Established

Upstream Lock  The CMTS then sends a UCD (Upstream Channel Descriptor) which tells the modem the Return frequency to talk back at which is typically 5Mhz to 42Mhz. Obtain the upstream parameters UCD acquired

Ranging and IP  Once the modem has successfully locked on downstream and upstream, it completes ranging, IP connectivity, and security. Start Ranging Complete Ranging and Power adjustments IP Complete Establish IP Connectivity Establish TOD/TOD Established Establish Security/ Security Established

Configuration, Registration and Baseline Privacy.  Almost there!! Transfer operational parameters and configuration file. Register with CMTS Initiate and establish Baseline Privacy Registration complete Modem is operational!

I’m ONLINE!!  Your device can now communicate to the internet!!! Image courtesy of: my_standardu_docsis.jpg

Do all cable modems function the same?  The OSI model was defined to standardize the functions of a communication system.  A cable modem, as with most communication devices, uses the Open Systems Interconnection Model.  The OSI model breaks a communication system into different layers with the objective of interoperability between devices without requiring logic changes of hardware and software.

Open System Interconnection? Detail: Image By Dino.korah [GFDL ( or CC-BY-SA-3.0 ( sa/3.0/)], via Wikimedia Commons Each Layer is briefly described in the included diagram. Where do you think DOCSIS modems operate?

OSI and DOCSIS  Data Over Cable Service Interface Specification (DOCSIS) was designed to standardize high bandwidth data transfer between cable providers internationally.  DOCSIS standard changes as the consumer demand for bandwidth increases.  DOCSIS modems operate primarily at the Physical, Data Link and Network layers of the OSI model.

Quiz Click Here to start the Quiz What do you know about DOCSIS cable modems?? Hint: Pick the best answer!

A cable modem is a broadband device used to connect to the internet over which type of Network? HFC Network Fiber NetworkCoaxial Cable Network Wireless Network

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OSI is short for? Open Something Interesting Open System Interconnection Open System Interface Open System Internet Try This one!

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CMTS stands for? Cable Modem Transitional System Cable Modem Television system Cable Modem Telecommunications System Cable Modem Termination System

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The first thing a modem does when hooked to the HFC network once powered on is? Looks for its Downstream Looks for it’s upstream parameters Registers with the CMTS Accesses the internet

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Thank You for Completing DOCSIS Cable Modem Basics Part 1. If you have additional questions or feedback, please click here.click here

Cable Modems are interesting… Try Again