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Cable (CATV) Data Networks

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The television cable industry has become a major service provider by allowing data transport over upgraded CATV cable networks. The cable industry has had many plans for delivering unique services to its subscribers. High-resolution digital video has been part of the plan, as well as TV set-top boxes that provide interactive game interfaces, WebTV, and other features. Cable data networks make this possible.

This topic describes how broadband services are delivered over the "cable network." It discusses the architecture of the cable network and several competing specifications that are attempting to define a universal cable network standard.

Cable data networks are one of several residential broadband schemes. Other schemes include DSL (Digital Subscriber Line), satellite systems such as Hughes Network System's DirecPC, and wireless data systems discussed under "Wireless Broadband Access Technologies." In fact, MMDS (Multichannel Multipoint Distribution Services) has been called a "wireless cable data network" solution because of its multipoint characteristics. See "Residential Broadband."

CATV Architecture

The traditional CATV system consists of a shared coaxial cable network that transmits analog television signals to downstream subscribers. It is estimated that over 100 million homes are reachable by CATV cable, of which approximately 75 million of those homes are CATV subscribers.

The cable network mimics the over-the-air radio frequency broadcast signals that you would receive via a TV antennae, but they are carried on cable. Television signals are transmitted in 6-MHz channels.

CATV is a shared cable system that uses a tree-and-branch topology in which multiple households within a neighborhood share the same cable. The topology of the CATV system is pictured in Figure 1.

Figure 1

Note that early CATV systems were all coaxial cable. The figure shows a fiber cable trunk combined with a coaxial cable trunk. This dual system is called a hybrid fiber coax (HFC) system. The fiber-optic cable helps overcome attenuation of signals over long distances and problems related to aging components (aging coax cable is commonly replaced with fiber where appropriate). Fiber also provides more bandwidth for future expansion. In some areas, cable providers have been installing fiber cable all the way to the curb. These efforts are covered under "FTTH (Fiber to the Home)."

Since the CATV network was primarily designed for downstream transmission of television signals, most of the existing network is being refitted to support two-way data transmissions. For example, amplifiers are used at various points along the cable to boost signals, but these amplifiers only work in one direction. Upgrading those components, along with many other components has been a big and expensive task. A temporary solution to provide two-way communications is the dual-path approach, in which subscribers transmit data upstream via a separate telephone connection. This scheme is being phased out as full two-way systems are put into place, as described next.

Cable Data Network Operation

Cable data network subscribers connect to the system via a cable modem. Once connected, subscribers obtain a continuous connection to the Internet via the cable network. The modems are internal devices or connect to PC and home entertainment equipment via USB (Universal Serial Bus) and other interfaces.

The cable modem communicates with the CMTS (Cable Modem Termination System) in the head office. The CMTS provides connections for thousands of cable modems over a network that can stretch to over 100 km with potential data rates up to 50 Mbits/sec. The CMTS also connects to the Internet and other media sources, sending and receiving user packets.

The cable modem performs upstream and downstream conversions. In the downstream process, packet data from the Internet arrives at the cable network provider's head end as shown in Figure 2. A processor module converts IP packets into MPEG packets and then error checks and modulates the packets onto a carrier wave using QAM/FEC (quadrature amplitude modulation/forward error correction). The output is then forwarded downstream to the subscriber. The subscriber's modem converts the radio frequency information back to IP packets and sends them to the end device. The head-end portion of the cable network can typically receive signals from a variety of sources, including terrestrial and satellite wireless transmissions.

Figure 2

The cable network uses FDM (frequency division multiplexing). As mentioned, TV channels are carried in 6 MHz bands, and one or more of these channels is dedicated to carrying data. The upstream channel typically occupies lower parts of the bandwidth not occupied by TV channels. Downstream rates are typically in the range of 30 Mbits/sec or less while upstream rates are in the range of 300 Kibts/sec to 10 Mbits/sec.

The actual downstream data rate available to users will fluctuate because the system is shared and not all users will be downloading information at the same time. There may be brief periods where all the bandwidth is available to just a few users, which would make for near instantaneous downloads for megabit-size files. The cable operator also has the flexibility to allocate more bandwidth by making additional channels available for data.

The upstream channel is a problem. When upstream channels are used on the cable network, they typically occupy lower frequencies that are subject to noise. In addition, the typical system uses TDMA, so users must contend for access to time slots. As more people access the network, performance drops. Some systems are so noisy that providers require users to use dial-up connections for upstream data.

Noise on the upstream connection is caused by electrical interference from home appliances, motors, and so on. The problem is only made worse by poor quality construction, old cable, loose connections, and improperly shielded cable. All combined, data is in for a rough ride on the way back to the head end. In fact, the noise problem puts a severe limit on the upstream bandwidth.

Many of the problems just discussed can be resolved with Terayon's S-CDMA (Synchronous Code Division Multiplexing). S-CDMA is a spread-spectrum modulation technique that uses available frequencies more efficiently and can operate more reliably than TDMA in noisy upstream channels. Because of the way that S-CDMA spreads its signals, it suffers less from external noise. In addition, S-CDMA synchronizes the upstream signals, which reduces mutual interference, thus opening up more bandwidth. See "CDMA (Code Division Multiple Access)" for more information.

At first, one might think that downstream traffic naturally requires more bandwidth than upstream traffic. After all, a single mouse click by a user can unleash an avalanche of data from an Internet server. However, many people are using the Internet for voice calls and to exchange files, digital music, pictures, and video. In addition, many people are setting up Web sites that can potentially overuse the upstream channel. Cable operators have had to deal with this problem by preventing users from setting up Web sites at the far end of the network, where there is the most contention for the upstream channel. These Web sites are moved to data centers that are closer to the core of the Internet.

The shared nature of the cable network introduced security problems. Users armed with network snoopers can watch traffic and attempt to capture valuable information. The cable industry has worked out encryption techniques and other security measures that support privacy. Firewalls are now common in homes.

Cable Network Provider Services

Cable operators can add various types of Internet-related services to enhance their networks. For example, caching ensures that the benefits of high-speed Internet access available to cable network subscribers is not lost when accessing slower links and servers on the Internet. For example, a number of users in the system may frequently access a server that is connected to the Internet via a 56K modem link. The cable operator can cache this information on its local servers to make the information immediately available to subscribers. See "Content Distribution."

IP telephony support allows users to make voice telephone calls over the cable network. This requires a cable modem that provides integrated MTA (multimedia terminal adapter) support, which basically means it has a telephone jack and a computer connector. IP telephony over cable networks supports multiple phone and simultaneous calls, which are set up as virtual circuits. Additional virtual circuits can be created at any time, with available bandwidth and the number of handsets/headsets being the only restriction. Incoming calls are set up as another virtual circuit. At the cable operator end, an IP-to-PSTN (public-switched telephone network) gateway converts and routes IP-based telephone calls into the traditional telephone system. See "Voice/Data Networks."

Cable operators are working to provide a number of services to their customers, including audio and video servers that can serve up music and movies. A big player is @Home, a cable- specific ISP, meaning that it provides content to cable companies throughout the United States. Cable companies such as Cox Communications deploy @Home Network as part of their interactive content for homes and workplaces. Cox is an equity partner in At Home Corporation, along with Comcast Corporation and Tele-Communications.

Corporate users should keep in mind that cable networks are primarily geared toward home users, not companies that want to build high-speed remote office connections, extranets, or other high-usage links. Many cable operators may discourage large organizations from connecting to their cable system. See "NAS (Network Access Server)" for alternatives.

Standards Development

Cable standards are designed to provide interoperability between cable modems and head-end gear. Subscribers should be able to buy off-the-shelf cable modems that are guaranteed to connect over the cable network with equipment installed at the cable operators site. Standards benefit both subscribers and operators by making connection easier and promoting new applications. The most important standards are outlined here:

  • DOCSIS (Data over Cable Interface Specification)    DOCSIS is the result of work done by MCNS (Multimedia Cable Network System Partners Ltd.). This standard has become the most interesting and important, and is covered under the "DOCSIS" heading. Visit

  • DAVIC (Digital Audio Visual Council)    DAVIC was a non-profit group that promoted digital audio-visual applications and services based on specifications which maximized interoperability across countries and applications/services. DAVIC developed a digital video broadcast reference model that is popular in Europe and preferred by the European Cable Communications Association (ECCA), a European cable industry organization. DAVIC is oriented toward delivering digital video to home users, while DOCSIS is better positioned for data delivery. DAVIC completed its work and closed in 2000. See the "DAVIC" topic and visit

  • IEEE 802.14 Working Group    This group is defining the physical layer and a MAC (Medium Access Control) layer protocol for HFC networks. The architecture specifies an HFC cable plant with a radius of 80 kilometers from the head end. The group's goal is to develop a specification for delivering Ethernet traffic over the network. ATM networking was also considered for the delivery of multimedia traffic. There has been some conflict between the work done by this IEEE group and the work done by MCNS, but MCNS is implementing part of the IEEE's physical layer work. Still, a paper about cable standards at the CATV Cyberlab claims that "the IEEE 802.14 effort was a failure." In fact, MCNS began work on DOCSIS because the IEEE was not working fast enough on its specification.

  • IETF IP over Cable Data Network (IPCDN) Working Group    The IPCDN is defining how IP can be delivered over the cable network. Most of its work is centered on DOCSIS and addresses higher levels than the IEEE 802.14 Working Group, which is concentrating on physical and data link layer protocols. IPCDN is defining a specification to map both IPv4 and IPv6 into the HFC access networks. The group is interested in multicast, broadcast, address mapping and resolution (for IPv4), and neighbor discovery (for IPv6). IPCDN is also working on bandwidth management and guarantees using RSVP, security using IPSec, and management using SNMP. More information is at The Web site lists a number of Internet drafts that describe various aspects of delivering IP over cable networks.

Copyright (c) 2001 Tom Sheldon and Big Sur Multimedia.
All rights reserved under Pan American and International copyright conventions.