Hfc1

Notas del editor

• #13: I - 23 © Copyright 1996 Scientific-Atlanta, Inc. Obviously, there will be no bandwidth to spare in the Return Path for an HFC network that supports all of the services described in the previous pages. RF-IPPV: 2.300 Cable Modem: 6.000 Cable Telephony: 24.000 Status Monitoring: 0.250 Interactive Services: 4.000 Bandwidth Required: 36.550 MHz Bandwidth Available: 35.000 MHz (If system is well maintained) Because the bandwidth is so limited, the Return Path must be carefully planned, managed, and maintained. Module 2 will examine in more detail some of the most significant issues to be dealt with by network technicians in the course of maintaining the HFC Return Path.
• #14: I - 3 © Copyright 1996 Scientific-Atlanta, Inc. The signal path formed by the network elements responsible for the transmission of the Forward Band, is called the Forward Path . ( Note that the Forward Path is also known as the “Downstream Path.”) The Forward Path is responsible for delivering RF signals from the headend, to the subscriber’s homes. The signal path formed by the network elements responsible for the transmission of the Return Band, is called the Return Path . (Note that the Return Path is equally well known by the terms “Reverse Path” and “Upstream”.) For a typical application, the HFC Return path begins at the subscriber terminal. Interactive subscriber terminals will generate RF signals in the Return Band which will travel away from the home on the Drop, through the Tap, where it joins other Return Signals traveling up the Return Path. Return Amplifiers are employed to overcome attenuation, and deliver a strong signal to the Reverse Laser Transmitter. The Reverse Laser Transmitter transmits the Return signals via a dedicated Return Fiber, to the headend where the signal is received by the Reverse Optical Receiver. The output of the Reverse Optical Receiver is connected to a return signal processor that serves as an interface between the HFC Return Path and the Control System, or other application systems.
• #18: I - 8 © Copyright 1996 Scientific-Atlanta, Inc. A common application for the HFC Return Path is RF Impulse-Pay-per-View , or RF-IPPV. RF-IPPV is an analog video service that allows a subscriber to view special “Pay-per-View” events like boxing, wrestling, and popular movies not available through other means. There is a charge for each event which generates revenue to be split between the cable system operator, and the provider of the event. The events are scrambled and then broadcast on the network. A subscriber who wants to watch an event, tunes the settop terminal to the appropriate channel and typically presses a “Buy” button. The settop terminal will then descramble the event and the subscriber watches the movie or sports event. Note that no immediate communication between the settop terminal and the Control System occurred. At some defined interval, 12 hours to about 24 hours, the Control System starts a process called Polling , by injecting control data into the downstream path. The Control System is essentially asking each settop terminal in the network, to respond with information about which events the subscriber purchased during the preceding period. The settop terminal responds on an RF carrier via the Return Path. This technique is known as Store and Forward that is, the settop terminal remembers which events the subscriber purchased, then forwards that information to the Control System when polled.
• #19: I - 17 © Copyright 1996 Scientific-Atlanta, Inc. Status Monitoring Systems are utilized to monitor and control network elements. These systems utilize a software package running on a computer at the Headend, which can communicate with Transponders in each Network Element. Network Elements are amplifiers, laser transmitters and receivers, power supplies, and other active equipment. A Transponder is the optional module, or card, installed in a Network Element that actually performs the measurement of data, and communicates with the computer system at the headend. The communication usually occurs as a “poll and respond” procedure, where the computer system sequentially polls each transponder and waits for a reply. This communication occurs utilizing the Forward and Return paths of the HFC network. When polled, the transponder replies with updated readings of the monitored parameters, and with any alarms that might be in progress. An alarm occurs when one or more of the monitored parameters deviates too far from the pre-set values. Monitoring Systems typically measure AC and DC voltages, temperature, Automatic Level Control status, and other critical operating parameters of the Network Element. A different type of system, called Performance Monitoring, measures the quality of the RF signals on the network.
• #20: I - 11 © Copyright 1996 Scientific-Atlanta, Inc. High-speed Cable Modems are probably the hottest HFC application today. Cable Modems allow very high-speed data communications for Internet access and work-at-home applications. Because of the RF bandwidth available, Cable Modems can provide data transfer at nearly 700 times faster than typical telephone modems. Files that would take about 9 minutes to transfer by telephone modem, take only about 0.8 seconds with a Cable Modem. The data transfer speed provided by Cable Modems, is valuable to Internet users who often spend much of their “surfing” time waiting for graphics and files to download to their system. The speed is of equal value to those who work at home, instead of going to the office everyday. Transferring document and artwork files between the office and home could take hours each day with telephone modems, but with Cable Modems, near instantaneous transfer is available. While Cable Modems are a developing technology, and there are many concepts for managing the data traffic and RF bandwidth, the basic system provides one or more high capacity channels from the headend to a group of subscribers, who share the channels. Each of these “channels” typically have a bandwidth of 6MHz, and are delivered via the Forward Path. Each Cable Modem generates a Return Channel that travels back to the headend via the Return Path. The Return Channel usually does not provide as much capacity as the Forward Channel, but it is still about 50 times faster than a telephone modem.
• #22: I - 14 © Copyright 1996 Scientific-Atlanta, Inc. Cable Telephony is an application that allows a subscriber to use telephone services over the HFC network, instead of over the conventional telephone network. The system would allow an HFC network operator to compete with the local phone company, by integrating telephony service into the same network used to deliver video services. A network interface at the home allows the connection of the subscriber’s standard telephone to the HFC network, and a network interface at the headend allows the connection of the HFC network to the conventional telephone network. The incoming and outgoing voice signals are modulated to RF carriers for transmission over the HFC network.
• #23: I - 20 © Copyright 1996 Scientific-Atlanta, Inc. Interactive Services, like Video-on-Demand, allow subscribes to control service provider hardware, like Digital Video Servers. The game, video, or home shopping application is delivered to the subscriber via the Forward Path, and the subscriber interaction is carried up the Return Path. A highly sophisticated settop terminal and Management system is required. The settop terminal must be capable of simultaneously receiving and reassembling digital video, receiving control signals from the Management System, sending replies to the Management System, and transmitting subscriber interaction to the Digital Video Server. Interactive Services provide the subscriber with instantaneous interactivity with the application. Movies can be controlled with VCR-like functions, such as Pause, Fastforward, and Rewind. Interactive Shopping services allow the subscriber to see options, prices, availability and make purchases in a single integrated activity.