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

Head-end
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

Future Enhancements: Packetized Voice, High-Speed Multimedia Services, Fiber Distribution Networks, Soft Switches

The future enhancements to public switched telephone networks include the conversion from circuit switched systems to packet networks, expanded fiber networks, multimedia services, and soft switching systems.

Packetized Voice
Packetized voice is the process of converting audio signals into digital packet format, transferring these packets through a packet network, reassembling these packets into their original data form, and then recreating the audio signals.

By the end of 2001, over 5% of international calls from the United States were over the Internet and more than 9.5% of all inter-exchange telecommunications calls were on managed packet switching networks [7]. Packetized voice transmission allows for key features such as dynamic bandwidth allocation and advanced services. To convert to packetized voice, the EO exchange is either replaced or supplemented by a packet switch.

Various protocols such as resource reservation protocol (RSVP) and real time protocol (RTP) have been developed to ensure the quality of service of voice packets that are transmitted through a packet network.

High-Speed Multimedia Services
High-speed multimedia services is the term used to describe the delivery of different types of information such as voice, data or video. Communication systems may separately or simultaneously transfer multimedia information. High-speed multimedia usually refers to image based media such as pictures, animation, or video clips. High-speed multimedia usually requires peak data transfer rates of 1 Mbps or more.

The providing (provisioning) of multimedia services requires communication lines that can have multiple channels and each of these channels may have different quality of service (QoS) levels. As a result, many emerging multimedia services are likely to use ATM.

Fiber Distribution Networks
Fiber distribution networks use optical fiber to distribute communication channels from the PSTN to end customers. There are three key distribution networks: fiber to the neighborhood (FTTN), fiber to the curb (FTTC), and fiber to the home (FTTH).

Figure 1 shows that public telephone networks have growth options. Initially, they are likely to install (FTTN) and use existing copper lines to reach the home. As demand grows for high-speed data communication services, additional fiber may be installed from the node to the curb (FTTC) to replace copper lines. Eventually, to achieve extremely high data rates to the home or business, FTTH or fiber to the basement (FTTB) may be installed.


Figure 1: Fiber Optic Networks

Soft Switches
Soft switches are interconnection switching systems that can dynamically change its connection data rates and protocols types by software control to provide for voice, data, and video services. Soft switches were developed to replace existing EO switches that have limited interconnection capabilities. Soft switches are packet based and can simulate multiple protocols such as Internet protocol and ATM. This allows for multiple types and simultaneous services to each customer with varying levels of QoS.