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Reproduction of all or part only with our permission..
This FAQ is edited by: 2kmaro See Profile, Lex Luthor See Profile, big greg See Profile
It was last modified on 2010-12-05 09:33:59

1 General

How does a cable modem work?

Simple. Your cable modem talks to a Cable Modem Termination System (CMTS), the component that exchanges digital signals with cable modems on a cable network. The CMTS is located at the local office of your cable television company.

The downstream information flows to all connected users, just like an Ethernet network. It's up to the individual network connection to decide whether a particular block of data is intended for it or not.

On the upstream side, information is sent from the user to the CMTS. Other users don't see that data at all. The narrower upstream bandwidth is divided into slices of time, measured in milliseconds, in which users can transmit one "burst" at a time to the Internet. The division by time works well for the very short commands, queries and addresses that form the bulk of most traffic from the consumer to the Internet.

A CMTS will enable as many as 1,000 users to connect to the Internet through a single 6 MHz channel. Since a single channel is capable of 30-40 megabits per second of total throughput, this means that users may see far better performance than is available with standard dial-up modems. The single channel aspect, though, can also lead to one of the issues some users experience with cable modems.

If you are one of the first users to connect to the Internet through a particular cable channel, then you may have nearly the entire bandwidth of the channel available for your use. As new users, especially heavy-access users, are connected to the channel, you will have to share that bandwidth, and may see your performance degrade as a result.

It is possible that, in times of heavy usage with many connected users, performance will be far below the theoretical maximums. The good news is that this particular performance issue can be resolved by the cable company adding a new channel, and splitting the base of users.


Where can I find more cable modem information?

CableLabs maintains a long Frequently Asked Questions list on CableModem technology at » ··· em/faqs/

Should I rent or purchase my modem?

The price of modems have come down in price over the past year or so. On one level, it makes sense to purchase the modem as you'll recoup the cost in just over a year. However, if the modem goes bad, you're responsible for it.

Renting has it's advantages as well. If something goes wrong with the modem, all you have to do is get the provider to change it. Even if the modem gets fried by a surge in your home, you'll find that you can change it out very easily.

It comes down to personal preference. Do what you feel is right for you and your situation.

2 Terminology

What is DOCSIS?

DOCSIS is an acronym for "Data Over Cable Service Interface Specification".

DOCSIS provides a set of standards and a certifying authority by which cable companies can achieve cross-platform functionality in Internet delivery. DOCSIS standards are applied to both ends of the connection: the CM (Cable Modem) and the CMTS (Cable Modem Termination System).

The standards are set and verified by CableLabs, a non-profit industry consortium. CableLabs also refers to the DOCSIS specification "CableModem" or "CableModem/DOCSIS".

What is a CMTS?

Cable Modem Termination System

This is the equipment at the cable companies Head-End that interfaces with both the RF plant and the traditional data side of an ISP. This equipment can operate either as a router or as a bridge with your modems. Overall, the Cisco Universal Broadband Router series of CMTS equipment has become the defacto standard in CMTS equipment.

What is SNR?

Signal to Noise Ratio.

The Signal to Noise Ratio is an engineering term for the power ratio between a signal (meaningful information) and the background noise. The higher the SNR, the better.

What is HFC?

HFC stands for Hybrid Fiber Coax.

It is the entire RF link between the CMTS and the CM(cable modem).

What is MSO?

MSO stands for Multiple System Operator. MSO is the industry term for "cable company".

Each cable TV installation serving a community is known as a "cable system", and the operator of the system is called the "system operator". Most system operators run cable systems in more than one community. Therefore, the vast majority of them are Multiple System Operators.

3 Technology

Who sets the DOCSIS standards?

CableLabs is an industry consortium that sets the standards for DOCSIS compliance.

What modulation methods are possible with DOCSIS?

On the downstream side either 64QAM or 256QAM is possible, with 256QAM being the usual case. Both occupy 6MHz of spectrum.

On the upstream either QPSK or 16QAM may be employed with 200Khz, 400KHz, 800KHz, 1.6MHz, or 3.2MHz of spectrum being used.

What are the differences between DOCSIS 1.0 and 1.1?

DOCSIS 1.1 builds upon 1.0, but also includes the following features:

Quality of Service
Dynamic Services
Payload Header Suppression
IP Multicast
CM Authentication
View-based access control and management (VACM)
CM Account Management
Fault Management
Secure Software

Which modems are Certified by CableLabs?

CableLabs maintains the list of modems that are compatible with DOCSIS, CableHome and PacketCable specifications.

The Certified Modem List is a large PDF file.

What are the DOCSIS version specifications?

Currently, there are 3 different version specifications for DOCSIS. DOCSIS 1.0, 1.1 2.0 and 3.0. DOCSIS standards are set by CableLabs »

The most common implementation is DOCSIS 2.0, with 3.0 being adopted quickly.

For details on the different CableModem/DOCSIS specs, please see » and » ··· dex.html

4 Performance

Can I increase my speed?

If you haven't tweaked your computer to its optimum settings, it's possible you may not be getting the best throughput from your connection. You can head over to our »Broadband Tweaks forum for personalized assistance in getting your system optimized for your cable connection.

If you want to try to hack your cable modem to increase your upload or download speed, please see »Cable Users FAQ »Can I uncap my cable modem to get higher speeds?

Which is better to use for a cable modem, USB or Ethernet?

USB 2.0 interfaces can operate at up to 400 Mbps. A fixed amount of USB bandwidth is shared between all devices that are plugged in to your computer. USB is used to connect your computer to a wide variety of devices.

An Ethernet interface, on the other hand, is intended for network traffic only. It connects directly to the computer's bus so that high speed transfers occur with the lowest possible CPU overhead. Ethernet interfaces are included with most modern motherboards. Older machines need a PCI-based Ethernet adaptor. Ethernet interfaces can typically connect at 10 Mbps or 100 Mbps (Fast Ethernet). Newer Ethernet interfaces connect at 10, 100, or 1000 Mbps (Gig Ethernet).

In most cases, you will want to use a wireless or wired router behind your cable modem. In this case, you must use Ethernet for the modem to router connection.

What about speed? Your cable modem will typically operate at 3 megabits per second or less. This speed can easily be handled by the slowest USB or Ethernet connection. Most cable modems can connect with USB 2.0 (up to 400 Mbps) or Fast Ethernet (100 Mbps).

The Ethernet interface is usually preferred, as this frees your USB ports (and bandwidth) for other peripherals.

What's the best RWIN for cable modems?

It depends. Determining your RWIN (TCP Receive Window) is not an exact science, and one size does not fit all.

You can go to our Tweaks Forum for assistance in determining the best RWIN for your computer and connection, or visit the Tweaks FAQ for reference information.

What kind of signal levels do I want on my cable modem?

You can view your signal levels using the cable modem's web interface. You should see readings like this

Receive/Downstream: -15dbmV to +15dbmV
Transmit/Upstream: 30dbmV to 55dbmV
Signal to Noise Ratio (SNR): 30dB or Greater

If your signals are out of spec, you may experience packet loss or retransmissions. Either can lead to slow speeds.

5 Caps

Can I uncap my cable modem to get higher speeds?

If you're on a DOCSIS system, no.

A small configuration file is sent to your modem from your provider. This file is what sets your caps and limits. It can not be changed.

Jan 2009 Update: Reports are that there are kits now available to permit this, however the legitimacy of the use of these kits is probably doubtful and you could potentially be penalized by your provider for uncapping the modem. [2kmaro]

Also see »Cable Users FAQ »What if I uncap?

Why am I capped and other users aren't?

Caps are set by the individual providers. These caps vary from region to region and there's not much you can do about them.

What if I uncap?

If you uncap, you may be kicked off by your ISP. Uncapping is in violation of your ISP's Terms of Service (TOS). If you uncap, you can lose your connection and you may subject to legal action.

Don't do it. It's not worth all of the trouble.