Digital video and transport connectivity options

Thu, 10/31/1996 - 7:00pm
Jay Shuler, Senior Manager of Broadband Applications, Nortel (Northern Telecom), Atlanta

Many cable providers understand the compelling need to migrate to an all-digital network, for reasons including pristine signal quality over any distance, rock-solid reliability (bit error rates of up to 1 error in 1,000,000,000,000,000), compatibility with existing long-haul telecom networks and flexibility to accommodate every conceivable type of service. Simply stated, digital is more efficient than analog — capable of sending a greater number of video signals down the pipe.

What cable providers may not have figured out is how to overcome the higher initial costs of a digital network in order to achieve the long-term revenue potential and operational cost savings.

Cable systems can improve the quality of their service and establish a foundation for new services through the construction of digital video linkages to area programming sources, including local television broadcast stations and satellite downlinks. Expanding this network to include local sports and news venues, production studios and institutional sites — and hubbing these sites to a switching center — can create new business and service opportunities.

Those who have looked into the digital network are beginning to understand that the best short-term fit — and the easiest business case for cable companies to make — for digital technology is above the headend. One such application is known as supertrunking.

Most important in distinguishing a good from a bad VOC network is the software

Another type of digital network, which is fully complementary and compatible with a supertrunk network, is the video operations center, or VOC. A video operations center network is a multi-site, switched digital network configured in a logical star that can interconnect hundreds or even thousands of video source and destination sites throughout a community, state or region. This network can provide on-demand or scheduled connections between headends, TV stations, satellite uplink and downlink sites, microwave relay points, sports venues, cultural centers, hospitals, schools, governmental and business sites for a wide variety of unidirectional and interactive video services.

VOC connections between cable ops and local TV stations improve video quality
Constructing a VOC

Figure 2 illustrates a typical VOC network. At each source or destination site is a digital video codec capable of providing broadcast quality video. These sites are connected in a logical star configuration to a central hub site, where there is a digital video switch. This subnetwork is called a cell. A network may consist of one or several cells. Each cell switch is controlled by a cell controller, and the entire network is controlled by a master control system that allows the switches to function together as a single, seamless video network.

The choice of technology is critical to network performance. The overall network must provide extremely high video quality and network reliability. The video codecs must be mutually compatible, and capable of delivering video at "contribution" quality, so that it is capable of withstanding some post-production in the studio. MPEG-2 digital video codecs are designed to support premium services such as interactive video, digital videotrunking and supertrunking.

Perhaps most important in distinguishing a "good" from a "bad" VOC network is the choice of network software. This software should allow a single network to be shared by many users. It should automate network scheduling and switching to provide 24-by-7 operation, while minimizing the number of network provider personnel required to operate and maintain the network. Users should be able to schedule their own network sessions at any time of day using widely available technology (i.e., IBM-compatible or Macintosh computers), while providing for the security and privacy of each user's sites, schedule and sessions. Users should have the flexibility to grant temporary and explicit access to one or more sites or sessions to one or more other users, to allow sharing of programming. The software (and switch) must allow either bidirectional or unidirectional, point-to-point or multicast video routing. And it should explicitly support multiple switched video applications, such as broadcast trunking, distance learning or video conferencing, in order to gain maximum leverage of the network investment. Figure 3 is a screen shot of one such scheduling system.

Figure 3. VOC remote scheduling. The same network allows telemedicine and distance learning for inmates and correctional staff. Source: Nortel
Async or Sonet?

The transmission system can be either DS-3 asynchronous or Sonet (Synchronous Optical Network) fiber or microwave. Single-mode fiber is the most reliable and highest-capacity choice with the longest unrepeated range, but there may be links in the network that can only be spanned affordably using digital microwave facilities. Broadband digital microwave comes in capacities from T-3 (45 Mbps) to over 600 Mbps, at several different spectral frequencies. Individual links can be redundant, and/or microwave hops can form one or more links in a Sonetring network, for maximum survivability.

The choice of asynchronous or Sonet is one of cost vs. the superior survivability and network management capabilities of the Sonet network. It is not unusual for end sites to gain access to the backbone via unprotected DS-3 or OC-3 "spokes," although Sonet access rings are becoming increasingly popular because of superior reliability in case of cable cuts. The advantages of Sonet become even more pronounced in the interoffice portion of the network, both between switching hubs and between network concentration points and the hub. Depending on the number of sites and the traffic patterns between sites, a broadband digital video network can require a lot of bandwidth in the network backbone — a level of bandwidth that can only be provided by Sonet facilities. Finally, Sonet facilities insure compatibility with adjacent telecom networks for future service interoperability.

Applications and justifications

The first application of a video operations center—the one that provides the greatest and earliest benefits to the cable company and its end users—will depend to some extent on the individual business drivers in each community. Logically, it is easier to start where there is the greatest need, where the users already have an existing budget for the type of service that a VOC provides, and where there is the greatest value in the application. It has been the experience of the author's company that, in most metropolitan areas, the best initial customer for VOC services is the broadcast community, which includes the cable company itself. Once the network is established and generating revenues, interactive video sites can be quickly added to leverage incremental revenues out of the installed base. The network provider should plan from the start for rapid network expansion. Not only does this allow for more end users and potential sources of revenue, but it increases the value of the network to each user by providing more sites with which they can communicate.

Improving existing service

Relying on off-air reception from local TV stations exposes the cable system to many of the same reliability and quality problems that caused their subscribers to come to them in the first place. Severe weather can degrade sign al-to-noise before the signal even enters the cable system. VOC connections between cable providers and local TV stations benefit both parties by dramatically improving video quality and network reliability.

For the broadcaster, improved video quality makes the channel more attractive to viewers. For the cable provider, the ability to achieve 60 dB or better at the headend gives it greater headroom to accommodate losses in the distribution network, allowing larger systems in terms of geographic and subscriber reach.

Serving commercial customers

Ambitious owners also have the opportunity to establish revenue-generating switched digital video networks that can serve a variety of commercial customers, including TV stations, news bureaus, venues and local video production houses.

The microwave spectrum is getting very crowded in many metropolitan areas, and it is often difficult to establish lines of sight between electronic news gathering sites and local TV production studios. Broadcasters typically have elaborate microwave routing schemes, with expensive arrays of microwave receivers and transmitters on top of tall buildings or nearby mountains. These relay points are expensive in terms of equipment, real estate and signal degradation. The same video operations center network that connects the headend to the TV broadcasters can also minimize the need for such facilities, while actually simplifying local sports and news coverage.

Among the first sites to add to the video network are local sports venues. This facilitates live coverage of professional, college, and even high school football, baseball, basketball, hockey and other major sports. Instead of sending an expensive microwave truck to cover the game, the VOC allows the broadcaster to dial into the scheduling system over a standard modem line and set up a connection through a digital video codec.

When the camera crew arrives at the game, it merely plugs into a codec pedestal, and the video is sent directly to the studio. For nationally televised games, this video might go directly to a network point of presence (POP) for transmission to a national network studio. This method is used in many cities today.

Video-linked performance venues such as symphony halls, drama theaters and outdoor amphitheaters enable broadcasters and cable operators to bring local cultural programming to their viewers and subscribers. Museums, aquariums a nd libraries are yet another source of programming, with regular and special events that do not normally get much notice in the community.

The ability to easily air these events provides another tool with which local broadcasters and cable systems can differentiate themselves from direct broadcast satellite and other national-level programming.

Community conferencing services

An intelligently planned video operations center network can even serve local businesses and government by providing the infrastructure for premium video conferencing, distance learning, telemedicine and remote arraignment services.

Local K-12 schools, community colleges, and universities have a need to share facilities, teachers and other educational resources through digital networking. A video operations center network can allow fully-interactive distance learning so that high schools can share specialized math teachers for Calculus, science teachers for physics and biology, language teachers, creative writing teachers, art, etc. Connecting K-12 schools to community colleges and universities allows advanced placement (AP) and vocational education to be provided to high school students, and allows local classrooms to be used after hours to provide adult and continuing education classes to parents and employees who could not otherwise afford the time to take them.

Video links between hospitals, clinics, retirement homes and doctors' offices facilitate remote diagnosis and consultation. Telemedicine can improve patient care and save time and money by keeping patients closer to home in lo wer-intensity care facilities while leveraging highly-paid specialist physicians across a wider geographic area.

Connecting to courtrooms, judges' chambers and correctional facilities allows the provision of remote court arraignment and "virtual courtroom" services, which have been shown to save millions of dollars in transportation and security costs, as well as accelerating the judicial process. The same network also allows telemedicine and distance learning for inmates and correctional staff.

Interworking with long-haul networks

A network that interconnects with all the major media sites in a metropolitan area — and which provides encoded, broadcast-quality video transmitted over standard DS-3 facilities — makes an attractive access network for t he national networks and video network providers. By facilitating easy access and standard network interfaces, the VOC could pre-empt competitive entry by a second provider, and help protect market position.

Interworking with supertrunk networks

If the cable provider has an existing supertrunk network, a VOC network can provide management of program content to the super headend for distribution to the local "mini-headends." Basing both the VOC and supertrunk network o n Sonet technology allows seamless network management of both applications through a single network management system.

It also allows voice, data and private line service to be served off the same network infrastructure, with almost unlimited capacity, reliability and ease of operation, while providing the foundation for the ATM networks of the future.



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