Building the cornerstone
The impact of convergence and digital technology on the cable industry is leading to major changes in network design and management, and probably nowhere is this more evident than at the headend. The combination of video, voice and data is placing tremendous loads on cable systems that were built just for the delivery of TV programming. When other services, such as telephony and video-on-demand are added, it becomes even more crucial that the networks are structured to handle the huge amount of traffic. Additionally, new types of equipment are showing up in headends. Digital receivers and QAM modulators are now common, and as we evolve in the larger digital world, routers, switches, multiplexers and digital servers will be found in increasing numbers.
The networks must not only be able to carry that traffic; they must also be capable of efficiently managing and routing it. For example, subscriber information for a variety of services-not just video-must be correctly delivered to and from homes. The headend plays a key role in this effort.
Economics are also having a major impact on the headend. As networks have grown in recent years, so have the costs associated with headend operation. Cable operators have developed a keen interest in consolidating their headends to make service and management easier and more cost-effective. But, consolidation also means more traffic is concentrated in the system, and as more channels and services are added, problems resulting from the extra demands on the headends are likely to occur.
The good news is that tomorrow's headends will be able to effectively operate under these heavy traffic conditions, with the help of improved technology and modifications to the networks that will allow the headends to deliver a massive amount of services over even greater distances. The transition to hybrid fiber/coax (HFC) technology began when headends only processed broadcast signals into a coaxial network consisting of long amplifier cascades. Because of the limited distances of this earlier technology, fully equipped headends proliferated in every small town and municipality. The advent of HFC technology allowed cable operators the ability to consolidate these headends by linking them together with 1550 nm or digital fiber technology. In this manner, a single facility could be designated as the "master headend," while other headends would be considered "primary hubs."
Redundant antennas, receivers, processors and sometimes modulators could be eliminated from these hubs, leaving little more than an optical hub converting 1550 nm to multiple 1310 nm forward nodes. This freed up manpower, improved reliability and provided precious space for expansion to accommodate future services.
As many cable operators install additional services such as targeted advertising (zoning), telephony, Internet access and video-on-demand, the functions of the headend in a hybrid fiber/coax (HFC) network must be expanded as these new services are introduced to subscribers.
For example, to enable targeted advertising, modulators are typically used to add advertising content to the existing channels, one modulator, per channel, per targeted zone. Sophisticated RF combining and splitting techniques at the master headend route the targeted programming to the corresponding primary hubs for distribution within that geographic area.
This function is usually performed at the master headend because that is the video/audio origination point for most programming. This allows the operator to consolidate the ad-insertion equipment while maintaining good picture and sound quality. Digital technology, which will also originate at the master headend, will make it possible to target advertising even further-to the degree of focusing on a specific neighborhood or even specific demographic clusters chosen by the operator.
But delivering all these services will require more than new technology at the master headend. It will also call for new technology, such as "fiber deeper," in the system and some redesign of existing network architectures. Both primary and secondary hubs become intermediate points between the master headend and the end-of-line.
(See Figure 1.)
The intermediate positioning of the hub provides an ideal site for services that are more "localized" or work more efficiently when serving smaller quantities of homes passed. Local government and educational channels can be inserted into the downstream path at this point. This is also the intermediate point for reverse (upstream) traffic from the home, and therefore, provides easy access to both downstream and upstream signals in a smaller and more manageable serving area. In addition, this design enhances the reliability of the network, reducing maintenance costs and increasing customer satisfaction.
As demand for interactive VOD, data, and telephony increases, hubs will play a more significant role in keeping down the heavy traffic at the master headend by dividing the traffic into smaller segments at each hub. If subscriber demand dictates, secondary hubs can become primary hubs, driving the fiber deeper until demand and capacity are optimized. Reverse traffic can be either processed locally at the hub or optically transmitted back to the master headend using frequency stacking techniques or the emerging baseband digital reverse technology and dense wave division multiplexing (DWDM) architectures.
As the headend and hub evolves in this scenario, more hubs are installed to bring fiber closer to homes, and therefore, more services to subscribers. Hubs take on additional responsibilities, placing increased demands on them. Because of the large number of hubs and manpower limitations, most of these hubs will typically be unmanned sites. Instead of humans, automated status monitoring and network management equipment will keep the operator informed about how the network is performing and provide alarm and fault recovery if trouble occurs. This not only allows problems to be corrected faster for better customer satisfaction, but also is especially important when telephone service is being provided.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.
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.|