Just more on Headends. The author works for SFA..............
cedmagazine.com
The transition to digital
The transition to digital is a revolution in the capacity and capabilities of headends. Digital broadcast services mirror the analog model, using digital satellite receivers and digital cable modulators. However, compression technologies multiply the carriage capacity of the system by factors of up to 12 in the number of programs carried per channel. The larger changes, however, are in the transition to interactive services.
Wide area data networking capabilities using Internet Protocol (IP) must be installed in headends and hubs, along with appropriate IP routing and switching equipment. These involve new skills in data networking for headend technicians where new data services will be linked to video services. These new services still require other new technologies such as e-mail servers and data gateways.
Among other benefits, digital technology at the headend will be an enabler for revenue-generating video-on-demand. Movie subscribers will be able to issue VCR-type commands from their homes to the headend or hub. Depending on the network architecture, video servers along the way will be able to process these orders and provide an at-home alternative to video store rentals.
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Cable operators are forging ahead
with expanded bandwidth, reverse plant
and the transition to digital technology.
The headend will be at the core
of these advances.
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However, even with the greatly expanded channel capacity of digital transmission, there is still a concern regarding the bandwidth needed to carry hundreds of movie choices to the home. One possible solution is to provide a dedicated fiber DWDM technology to each primary or secondary hub. DWDM will make it possible to deliver literally hundreds of 256 QAM signals to the hub, each containing up to 10 streams of video/audio programming.
Frequency-agile translators (RF processors) would be used to tune the appropriate 256 QAM signal from the DWDM feed and place it in a dedicated "frequency slot" of the main RF feed for each node, although QAM signals may also be carried "on-channel" at times. Frequencies will be allocated to areas where the demand is highest, so no particular slot is overloaded.
Still, other challenges remain. For example, although there is a strong push in the industry and in the government to adopt digital technology, analog business continues to thrive for cable operators. Because of the massive installed base and lower cost of analog TVs, analog transmission will probably coexist with digital in cable networks for a long time.
The provision of digital television will also have a major impact on headend design. To allow subscribers to receive HDTV, conversion devices will be necessary at the headend to receive the off-air VSB signals and convert them to the QAM format. Or, some operators may prefer to receive the HDTV signals off-air and process the native VSB format directly onto the cable, using a UHF frequency plan.
As cable networks are upgraded to deliver telephony, the "overhead" previously associated with telephone company operations, including the ability to meet federal regulations, will be delegated to the headend.
Finally, as new services are activated, the main headend will no longer be a small facility with few personnel, but will likely evolve into a large building that requires highly trained and skilled employees with experience in working with complicated data and telephony networks.
Change is coming
As today's technology continues to roll out, cable operators are forging ahead with expanded bandwidth, reverse plant and the transition to digital technology. The headend will be at the core of these advances. The delivery of expanded channel lineups and additional services will depend on the technological capability of tomorrow's headends and operators' expertise in managing these complex facilities.
Note: Dr. Bill Wall, S-A's technical director for Subscriber Networks, contributed to this article.
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