Cable Modems

January 22, 2012

Getting the internet broadband signal from your local cable TV provider to your computer or home network using a standard Ethernet cable requires what is called a cable modem. In recent years the cable modem has been included with a basic 4 port router and a wireless access point into one single device so that your local cable/internet broadband providers can supply their customers with a simple home network right out of the box.

How Does A Cable Modem Work?

Your cable modem communicates via the neighborhood wiring network to a Cable Modem Termination System (CMTS) located at your local cable company’s office. The network signal used by your modem is electronically sliced into hundreds of 6 MHz channels. Each of these can represent one of your TV channels, but unused channels can also be used to carry your internet broadband data. The CMTS is responsible for connecting a group of customers to an Internet Service Provider (ISP) for connection to the internet.

The downstream information from the CMTS flows to as many as 1000 customers through one 6 MHz channel. Each individual network connection is responsible for capturing the individual blocks of data it needs. The negative of cable broadband is that the signal is shared by all of these users, so actual speed can vary. The good news though is that it is possible for the cable provider to add another channel, splitting the base of users.

Upstream data flows from the modem to the CMTS, but requires only a smaller channel – 2 MHz portion of the bandwidth. The assumption is that people download far more data than they upload.

Different electronic components inside the cable modem are responsible for converting the channel signal into the 1s and 0s required for your computer to understand the signal. This allows you to simply connect a standard Ethernet cable to the RJ45 connection on the back of the device to your computer or home network.

One of the benefits that cable broadband providers have over DSL is that the signal is well suited for traveling over longer distances than DSL. DSL customers can see massive decreases in signal quality the farther away they are from the base station.

Cable Modem Technology Standardization

The cable industry depends on standardization that was brought about in the 1990s. At that time 4 major cable operators formed the Multimedia Cable Network Systems (MCNS) which joined forces with CableLabs®, Rogers Cable and Continental to work at defining interoperable specifications for cable modem systems.

An important step was achieved when CableLabs® was able to reach a harmony agreement between General Instrument and Scientific-Atlanta covering cross licensing of various proprietary technologies including modulation, encryption and forward error correction. This allowed a standards-based approach to digital signal delivery. Combine this with five vendor authors who were enlisted to help create specifications and in March 1997, the group issued the first of what was called the Data over Cable Service Interface Specification, or better known as the DOCSIS® standard.

The standardization of cable modem technologies across different hardware manufactures allowed the cost of the modems to drop considerably. Over a few years modems went from $3,000 to as low as $50. CableLabs® not only certifies hardware manufacturers cable modems but also the cable modem termination system located at the cable companies office.

The DOCSIS® specification itself has gone through different versions 1.0, 1.1, 2.0 and now 3.0. Each version added new technology and capabilities all of which are also backward compatible. DOCSIS® 3.0 is the latest version, bringing about a number of enhancements. The most important is channel bonding, support for IPv6,and support for IPTV.

Channel bonding can be thought of as using the analogies of a road. If the standard 6 MHz channel is represented by a two lane road, what channel bonding provides is a way to combine channels – increasing the two lane road to more lanes which in turn provides capacity for more data to be transmitted.

Bottom line, DOCSIS® 3.0 will allow cable operators to provide data rates in the hundreds of megabits.

Which Cable Modems do Cable Operators Require?

The beauty of the DOCSIS® specification is that it becomes easy for the cable operator to specify which modem standards are required for connection to their broadband. Customers can then shop from different manufactures either online or locally to find the best and most reliable device which meets their needs.

DOCSIS® 3.0 may be the latest specification revision which offers the highest data speeds, but it’s not needed for all customers. It varies based on the download speeds the customer requires.

A good example of how this is done can be seen by using Comcast’s DOCSIS® Device Compatibility and Capability on their DOCSIS® Device Information Center web page. This site makes it easy for customers to determine which are currently approved cable modems. The customer can use this site to ensure the device purchased is compatible with the service level of internet broadband data that are subscribed to.

This site also indicates the level of certification achieved by the modem manufacturer on a scale of 1 to 3 stars, with 3 stars being the highest and most thorough level of testing available. The site gives you the ability to filter devices by End-Of-Life, in case you’re purchasing a used device or via Ebay. The other option is to view only those which are DOCSIS®-3.0 capable.

Reference: CableLabs® DATA 1988-2008