The Design Challenge: The continued development of new broadband services such as video and interactive programming is causing an ever-increasing demand for wider band- width. This quest for bandwidth has been responsible for the telecommunications industryΒs major step from copper based cabling to fiber optics. As fiber optics integrate themselves into most cable networks, they have now taken on a hybrid fiber/coax (HFC) style of architecture. HFC architecture has now become the leading choice of both Cable TV (CATV) companies and telephone service providers. Because of the similar- ity of the HFC architecture to their existing networks, cable companies in particular are embracing HFC as an affordable way to position themselves as telephony vendors in a competitive, multi- service future. CATV systems have a vision of providing a complete networked home package where video on demand (VOD), high speed internet and telephony will all be provided via one system: the HFC network. HFC Architecture: HFC architecture usually consists of a fiber trunk line carrying signals in the form of video or telephony from the headend or central office (CO), to feeder cables serving local neighborhoods. Optical nodes on the trunk line convert the signals from light in the fiber optics to radio frequencies (RF) for the copper cables. The feeder cable, a medium sized coaxial cable, provides the signals from the trunk cable to entire neighborhoods. Individual houses subscribing to the cable services have drop lines connected to the feeder cables via taps and network interface devices (NIDs) attached to the outside of their homes for cable telephony and set top boxes for video. Figure 1 below gives a basic layout of how an HFC network should look. > R1220T705 R1220T505 NIDDrop R1220T505 Node HeadendorCOFeeder Coax CableFiber TrunkCable Tap Figure 1. Hybrid fiber / coax (HFC) architecture Cable Telephony: Unlike cable TV where power to operate the TV is not transmitted along the cables, cable telephony requires applications power to operate the NIDs. In a cable telephony system, the cable transmits the signal information and in many cases the local operating power for the NIDs, in 60V to 90V form. Powering the local NID can be carried out in any of the following ways: >

Power can be transmitted across the center conductor of the drop cable from a power passing tap to the NID.ՕPower can be transmitted on separate twisted pair wires that are bonded to the outside of the drop cable. This drop cable is sometimes called a Siamese cable, and also operates between the power passing tap and the NID.Powering can come from the ac supply in the home. In this case a back-up battery must also be used in order to provide emergency telephone access during power failures.The first option above is the most common form ofpoweringthe NID. Coaxial power passing taps act as gateways...

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