different channel bandwidths. Television technical standards vary across the world and this affects the way DOCSIS variants develop. International TV standards include the following:

National Television Standards Committee (NTSC) is a North American TV technical standard for analog TV systems.
Phase Alternating Line (PAL) is a color encoding system used in broadcast television systems in most of Europe, Asia, Africa, Australia, Brazil, and Argentina.
Système Electronic Couleur avec Mémoire (SECAM) is an analog color TV system used in France and some Eastern European countries.

European cable channels conform to PAL-based standards and are 7 MHz and 8 MHz wide. North American cable channels conform to the NTSC standard, which specifies 6 MHz-wide cable channels. The wider channels in Euro-DOCSIS architectures allocate more bandwidth to the downstream data path. Note
More information about Euro-DOCSIS is available at http://www.eurocablelabs.com.

Content 2.3 Deploying Cable System Technology 2.3.1 Hybrid Fiber-Coaxial (HFC) Cable Networks Accessing the Internet through a cable network is a popular option that teleworkers can use to access their enterprise network. This lesson describes how modern cable operators deploy HFC networks to enable high-speed transmission of data to cable modems located in a SOHO. The lesson presents the basic steps required to provision a cable modem. A significant drawback of only using coaxial cable is the signal attenuation that happens when the signal travels from the antenna to the subscriber. Amplifiers placed approximately every 2000 feet, boost signal strength and ensure that RF signals have enough power to receive all channels within the spectrum (50 to 860 MHz) for analog TV, digital TV, and digital data cable modem services. Unfortunately, amplifiers introduce noise and distortion and the failure of a single amplifier disrupts service. Modern cable operators use an HFC network that deploys fiber in the trunks. Fiber has several benefits over regular coaxial cable:

Reduces the number of amplifiers
Thin and lightweight—takes less space
Covers longer distances
Induces less or virtually no noise
Less loss of signal
Immune to external influences, such as thunder or RF interference
Easier to handle

HFC architecture is relatively simple. A web of fiber trunk cables connects the headend (or hub) to the nodes where optical-to-RF signal conversion takes place. The fiber carries the same broadband content for Internet connections, telephone service, and streaming video as the coaxial cable carries. Coaxial feeder cables originate from the node that carries RF signals to the subscribers. The effective range or service area of a distribution network segment (feeder segment) is from 100 to as many as 2000 subscribers. Fiber trunks carry downstream traffic at a signal strength above 50 decibels (dB) and reduce the number of cable amplifiers in trunk lines. Coaxial cable is already in place throughout many neighborhoods, so cable operators can build an HFC network without having to replace existing coaxial cable between nodes and subscribers. By upgrading a cable plant to an HFC architecture, cable operators can deploy a data network over an HFC system, offer high-speed Internet services, and serve more subscribers. Cable operators segment their networks into smaller service areas in which fewer amplifiers are cascaded after each optical node, typically five or fewer. The tree-and-branch network architecture for HFC can be a fiber backbone, cable area network, superdistribution, fiber to the feeder, or a ring.

Content 2.3 Deploying Cable System Technology 2.3.2 Sending Data over Cable Delivering services over a cable network requires different RF frequencies: the downstream frequencies are in the 50 to 860-MHz range, and the upstream frequencies are in the 5 to 42 MHz range. Two types of equipment are required to send digital modem signals upstream and downstream on a cable system:

A cable modem (CM) on the subscriber end
A cable modem termination system (CMTS) at the headend of the cable operator

A headend CMTS communicates with CMs that are located in subscriber homes. In addition, a headend incorporates a computer system with databases for providing Internet services to cable subscribers. In a modern HFC network, 500 to 2000 active data subscribers are typically connected to a cable network segment, all sharing the upstream and downstream bandwidth. The actual bandwidth for Internet service over a CATV line can be up to 27 Mbps on the download path to the subscriber and about 2.5 Mbps of bandwidth on the upload path. Based on the cable network architecture, cable operator provisioning practices, and traffic load, an individual subscriber can typically use an access speed of between 256 kbps and 6 Mbps. When high usage causes congestion, the cable operator can add additional bandwidth for data services by allocating an additional TV channel for high-speed data. This addition may effectively double the downstream bandwidth that is available to subscribers. Another option is to reduce the number of subscribers served by each network segment. To reduce the number of subscribers, the cable operator further subdivides the network by laying the fiber-optic connections closer and deeper into the neighborhoods.

Content 2.3 Deploying Cable System Technology 2.3.3 Cable Technology: Putting It All Together Figure shows how these cable technologies work together to deliver video and data. In the downstream path, the local headend (LHE) distributes TV signals to subscribers via the distribution network. TV signals are received through satellite dishes, antennas, analog and digital video servers, local programming, and other headends. A modulator/scrambler appropriate for the specific RF channel assigned on the cable processes these TV signals individually. The CMTS modulates digital data on an RF signal and combines that RF signal with the TV signals. The combined signal is input to a fiber transmitter that converts the signal from RF to light (optical) and transmits to a fiber node further downstream. At the fiber node, the optical signal is converted back to an RF signal and then transmitted over the coaxial network comprised of amplifiers, taps, and drops. At the subscriber end, an RF splitter divides the combined RF signal into video and data portions. The CM receives the data portion of the RF signal. The CM, tuned to the data RF signal channels, demodulates the data RF signal back into digital data and finally passes the data to the computer over an Ethernet connection. In the upstream direction, the CM decodes the digital information from the Ethernet connection, modulates a separate RF signal with this digital information, and transmits this signal at a certain RF power level. At the headend, the CMTS, tuned to the data RF channels, demodulates the data RF signal back to digital data and routes the digital data to the Internet.

Content 2.3 Deploying Cable System Technology 2.3.4 Data Cable Network Technology Issues Because subscribers share a coaxial cable line, some problems may occur:

Subscribers on a segment share the available bandwidth on that segment. The bandwidth that is available to each subscriber varies based on the number of subscribers sharing it. Cable operators resolve this issue by adding RF channels and splitting the service area into multiple smaller areas within the segment.
As with any shared media, there is a risk of privacy loss. Available safeguards are encryption and other privacy features, which are specified in the DOCSIS standard used by most CMs.

Note
A common misconception is that a computer may communicate directly with another computer on the same segment. This is not possible because the CM transmits on a completely separate frequency than the frequency on which