How Cable Modems Function

For millions, television provides news, entertainment, and educational content directly in their homes. A large number of people receive their TV signal through cable television (CATV), as it offers superior picture quality and a wider selection of channels. (For more information, see How Cable TV Works.)

Many cable TV subscribers now also have access to high-speed Internet via their cable provider. Cable modems are in competition with technologies like asymmetrical digital subscriber lines (ADSL). If you've ever wondered about the differences between DSL and cable modems, or how a computer network can transmit both 100 cable channels and any website over a single coaxial cable, read on. This article will explain how cable modems operate and how they manage to deliver both cable TV and Internet to your home simultaneously.

Additional Space

You may assume that a television channel occupies a significant portion of a cable's "space" or bandwidth. However, in practice, each television signal occupies a 6-megahertz (MHz) channel on the cable. The coaxial cable used for cable television can handle hundreds of megahertz of signals, allowing for more channels than you could ever watch. (For further details, see How Television Works.)

Within a cable TV system, each channel's signal is allocated a 6-MHz portion of the cable's total bandwidth, which is then transmitted down the cable to your home. In certain systems, coaxial cable is solely responsible for distributing these signals. In other cases, fiber-optic cables are used to connect the cable company to various neighborhoods or regions, with the fiber terminating and signals continuing through coaxial cables for delivery to individual homes.

Data Streams

When a cable provider offers Internet service, it can use the same cables as cable TV. The cable modem system places downstream data – information sent from the Internet to an individual computer – into a 6-MHz channel. The data appears on the cable like any other TV channel. Therefore, Internet downstream data occupies the same amount of space on the cable as any single TV channel. Conversely, upstream data – sent from an individual back to the Internet – requires just 2 MHz of bandwidth since most people typically download far more data than they upload.

To facilitate both upstream and downstream data transmission over the cable television system, two pieces of equipment are necessary: a cable modem on the customer side and a cable modem termination system (CMTS) on the cable provider's end. These two systems manage all aspects of computer networking, security, and the overall administration of Internet access via the cable TV infrastructure.

Exploring the Cable Modem

Cable modems can either be integrated within the computer or exist as separate external devices. In some situations, a cable modem is embedded in a set-top box, requiring only a keyboard and mouse for full Internet connectivity. If your cable provider has upgraded to digital cable, the new set-top box they provide will allow you to access the Internet, even if you don't receive Internet service via your CATV connection. Regardless of the physical form, all cable modems contain several essential components:

Inside the Cable Modem: Tuner

The tuner is connected to the cable outlet, often with the help of a splitter, which divides the Internet data channel from the regular CATV programming. Since the Internet data is transmitted via a previously unused cable channel, the tuner simply receives the modulated digital signal and sends it to the demodulator.

In some instances, the tuner includes a diplexer, allowing it to utilize one frequency range (typically between 42 and 850 MHz) for downstream data and another range (between 5 and 42 MHz) for upstream data. In other systems, especially those with fewer channels, the cable modem tuner handles downstream traffic, while a dial-up telephone modem manages upstream data. Regardless of the configuration, the signal received by the tuner is then passed to the demodulator.

Exploring the Cable Modem: Demodulator

Common demodulators typically perform four key functions. A quadrature amplitude modulation (QAM) demodulator takes a radio-frequency signal, which has information encoded by varying both the amplitude and phase of the wave, and converts it into a simple signal for processing by an analog-to-digital (A/D) converter. The A/D converter then transforms the voltage-varying signal into a sequence of digital 1s and 0s. An error correction module checks the received data against a known standard, identifying and correcting transmission errors. Most data is transmitted in MPEG format, so an MPEG synchronizer ensures the data frames stay properly aligned and ordered.

Inside the Cable Modem: Modulator

In cable modems that rely on the cable system for upstream traffic, a modulator is employed to transform digital computer network data into radio-frequency signals for transmission. This component is often referred to as a burst modulator due to the sporadic nature of most user-to-Internet traffic, and it consists of three key components:

Exploring the Cable Modem: MAC

The MAC (Media Access Control) unit is positioned between the upstream and downstream components of the cable modem, serving as the bridge between the hardware and software layers of various network protocols. While all network devices feature MACs, the tasks performed by a cable modem's MAC are more intricate than those of a typical network interface card. As a result, many of the MAC functions are delegated to a central processing unit (CPU)—either the one in the cable modem or the user's system CPU.

Microprocessor

The microprocessor's role varies depending on whether the cable modem is designed as part of a larger computer system or to function independently for Internet access without additional computer support. When an attached computer is involved, the internal microprocessor handles much of the MAC function from the dedicated MAC module. In cases where the cable modem is the only unit required for Internet connectivity, the microprocessor takes on the full responsibility of handling the MAC functions and more. Motorola's PowerPC processor is often a popular choice for system designers in either scenario.

Cable Modem Termination System

At the cable provider's head-end, the CMTS fulfills many of the same roles as the DSLAM in a DSL system. The CMTS receives traffic from multiple customers on a single channel and directs it to an Internet service provider (ISP) for access to the broader Internet. Within the head-end, cable providers either host or lease space for third-party ISPs to maintain servers for accounting, logging, and Dynamic Host Configuration Protocol (DHCP) to assign and manage the IP addresses for all users on the cable system. It also includes control servers for a protocol called CableLabs Certified Cable Modems, previously known as Data Over Cable Service Interface Specifications (DOCSIS), a key standard used by U.S. cable systems to provide Internet access to users.

The downstream data flows to all connected users, similar to how it works in an Ethernet network – each network connection decides whether a particular block of data is intended for it. On the upstream side, information is sent from the user to the CMTS, with other users unaware of this data. The limited upstream bandwidth is divided into time slices, measured in milliseconds, allowing users to send one 'burst' of data at a time to the Internet. This time-based division is ideal for the brief commands, queries, and addresses that make up the bulk of most users' traffic back to the Internet.

A CMTS can support as many as 1,000 users connecting to the Internet via a single 6-MHz channel. Since a single channel can handle 30 to 40 megabits per second (Mbps) of total throughput, users may experience much better performance than with traditional dial-up modems. However, this single channel configuration can also contribute to some of the performance issues that certain users encounter with cable modems.

Advantages and Disadvantages of Cable Modems

If you're among the first to connect to the Internet via a specific cable channel, you might have most of the available bandwidth to yourself. However, as more users, particularly those who use the internet heavily, join the channel, you'll need to share that bandwidth, which could lead to slower speeds for you. During peak times with many users online, performance may be far below its theoretical maximum. Fortunately, this issue can be addressed by the cable provider adding a new channel and redistributing the user load.

Another advantage of using cable modems for Internet access is that, unlike ADSL, their performance is not affected by the distance from the central cable office. A digital CATV system is specifically designed to deliver signals at a consistent quality to customer homes. For the upstream connection, the burst modulator in cable modems is programmed to account for the distance from the head-end, ensuring the proper signal strength for accurate transmission.