How Cable Modems Work

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modem_docsisBy Brian Wilson, CCNA, CCSE, CCAI, MCP, Network+, Security+, JNCIA

So you have a broadband connection at home or work and are wondering how it all works. Well I hope to offer a low-tech level explanation, so you will have a better understanding of how your connection works. Broadband services from cable companies or Multiple Service Operators (MSOs) are normally provided via cable modems and 90% of the cable modems are using DOCSIS (Data over Cable Service Interface Specification).

Basic Explanation of DOCSIS

So let’s now look at what DOCSIS is and what is means. DOCSIS defines interface requirements for cable modems involved in high-speed data distribution over cable television system networks (definition from With DOCSIS comes the different versions (1.0, 1.1, 2.0, & soon 3.0) and the limits for each version. We are not too worried with the versions and limits between the versions as consumers, since we just want speed and security. But the important thing about DOCSIS is that it’s a standard that protects the consumer and vendors by forcing interoperability and keep the cost of equipment down. If the modems were proprietary then the consumers would have to pay a very high price for the modem or would be forced to lease them from the MSOs. Most MSOs will allow you to use just about any cable modem that is DOCSIS certified. Now not all MSOs will do this since they have to support all the modems on the plant with firmware upgrades and other issues with support, but most of your well known modems should be supported.

Now for the CMTS & Node

Cable modems communicate with a device called a cable modem termination system (CMTS). The CMTS is a router that talks to all the cable modems and routes their traffic to the Internet or the MOS’s backbone. There are a few different vendors in the CMTS market, but the most recognized is Cisco Systems. Cable modes are typically grouped into regional nodes. A node is most likely your subdivision but could be a smaller or larger area the MSO uses to support all of your services. The node is a fiber to RF converter that allows the MOS to send services to you area via fiber up to the point of your local service area, and then the services are sent over coax. The size of the node and number of modem customers in that node can make some difference in the speeds of you modem. With cable modems the node is your local access point, and the more users the less bandwidth available for all. Most MSOs over-subscribe nodes but try to make sure there is always 50% available bandwidth at peak hours. What this means is that if you are in a heavily loaded node, and everyone is downloading files, your service can slow some. With how competitive the ISP business is, most MSOs will try to not over-subscribe nodes too much without adding more nodes or splitting then.

Signal Levels and Splitters

Let’s now look at the signal level needed to keep your cable modem online and surfing. Most cable modems have a signal range that they need in order to communicate to the CMTS. The signal levels can differ from vendor to vendor, but as a rule of thumb most modems work well from RX -10dbv to +10dbv and TX 40dbv to 56dbv. A lot of modem vendors provide an informational webpage or diagnostic page you can connect to on the modem to see the messages from the modem and it’s levels. The diagnostic page’s IP is different from modem to modem, but if you lookup your vendors modem specifications, you will be able to see if your modem has this diagnostic page available. Other things to keep in mind is that your cable modem should be on its own coax line from the cable audit box outside or from the main feed you get from the MSO. It’s not a good idea to have your modem on a coax line with a lot of splitters due to each splitter having at least 3.5dbv of loss. And if you have to use a splitter, make sure you read the throughput ratings on the splitter. It should be at least 5-1000 MHz passthough.

Security on the Network

Other things to watch out for is the security of the node. Make sure other subscribers in your node are not able to see your computer and its traffic. This security issue is normally handled by encryption on the modems traffic to the CMTS. Most ISPs use BPI+ encryption to protect your traffic, but you should always use a hardware firewall/router off of any broadband connection. To test the security of the node you can open your windows network places and look to see if any unknown shares are listed. If there is no encryption in the node, and you’re on the same subnet, you might be able to see other
user’s network shares. This is bad if you have pictures or other sensitive data that you do not want others to see. Other things to know are that most MSOs will block ports to your computer network. This is for both their business goals and your protection. MSOs typically block NetBIOS, SMTP, port 80, and a lot of other various ports (business accounts may not have port filters). The port blocking is done mostly to protect the customers from viruses and worms that travel quickly over broadband connections.  

I hope this basic look at cable modems has helped you better understand how it all works, and if you would like to find out more information about DOCSIS, CMTSs, or cable modems try to use Google.

Brian Wilson ( has over 12 years experience in IT starting with a tour in the United States Army. He has worked in and out of the US Government in many different organizations and technical roles including a stint as a Cisco Certified Instructor. Currently he works for one of the largest US broadband providers (ISP) as a Senior Data/Voice Engineer supporting over 3 million High Speed Internet/ VoIP subscribers. He has attained a number of industry credentials covering many aspects of IT including CCNA, CCSE, CCAI, MCP, JNCIA, Network+, Security+, and many DoD Certifications. He also uses his knowledge of IT to benefit a number of charitable organizations. Clearly Brian's knowledge and interests are wide, and his affinity for philanthropy will be the overiding theme of his vast set of articles and videos.

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