Distribution Network & Head end

Distribution Network
The distribution network is the part of a cable television system that connects the head-end of the system (video and media sources) to the customer’s equipment. Traditionally, the local connection has been composed of a coaxial cable that allows for the one-way transmission of with a maximum of one hundred and twenty 6 MHz analog television signals.

The hybrid fiber coax (HFC) system is an advanced CATV transmission system that uses fiber optic cable for the head-end and feeder distribution system and coax for the customers end connection. HFC are the 2nd generation of CATV systems. They offer high-speed backbone data interconnection lines (the fiber portion) to interconnect end user video and data equipment. Many cable system operators anticipating deregulation and in preparation for competition began to upgrade their systems to Hybrid Fiber Coax (HFC) systems in the early 1990’s. As of 2002, over 35% of the total cable lines in the United States had already been converted to HFC technology.

Figure 1 shows a typical cable distribution system that uses a combination of fiberoptic cable and coaxial cable for the local connection. This diagram shows that the multiple video signals from the head-end of the cable television system is converted into digital form to allow distribution through high-speed fiber cable. The fiber cable is connected in a loop around the cable television service area so that if a break in the cable occurs, the signal will automatically be available from the other part of the loop. The loop is connected (tapped) at regular points by a fiber node. The fiber node converts the fiber signals into RF television signals that are distributed on the local coaxial cable network. The coax network distributes the RF signals to homes in the cable television network.

Figure 15: Hybrid Cable Television Distribution Network

The head-end is the master distribution center of a CATV system where incoming television signals from video sources (e.g., DBS satellites, local studios, video players) are received, amplified, and re-modulated onto TV channels for transmission down the CATV system.

Figure 2 shows a diagram of a simple head-end system. This diagram shows that the head-end allows the selection of multiple video sources. Some of these video sources are scrambled to prevent unauthorized viewing before being sent to the cable distribution system. The video signals are supplied to video modulators the convert the low frequency video signals into their radio frequency television channel. The output of each modulator is combined and connected to the distribution trunk.

Figure 2: Head-end System

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