Communications Technology: Archive

How to Boost Your Subscribers' Choices
Video-on-Demand Network Infrastructures
By Yvette M. Gordon

Cable plants have been upgraded to support digital broadcast. Digital set-tops are being installed, and now we look toward layering interactive services. The logical first service, which is coveted as the new revenue generator for the cable industry, is video-on-demand (VOD).

The cable industrys year of awareness and preparation for VOD was 1998. Technological advancements enabled vendors to reduce equipment costs enough to help make the service a reality. Meanwhile, the industry at large became familiar with the technological requirements. Now both sides are looking forward to deploying the killer competitive application.

Generally, VOD is seen as simply installing video servers. This is far from the truth. Although video and audio Moving Pictures Experts Group (MPEG)-encoded files reside on video servers, the control and management of these files, as well as network integration, are critical to the success of VOD. A complete VOD installation will account for total system automation to ensure that the operational costs wont undermine the business model. When analyzing VOD platforms, its critical to ensure all of the pieces are planned for because many solutions require an operator to obtain these components from various vendors. So, what are these components?

First, there is the video server. A video server will store many gigabytes (1 billion or 1,000 megabytes) or terabytes (1,000 gigabytes) of MPEG content and stream or output upon request.

In cable broadband architectures there are two standard output types for video servers: digital video broadcast/asynchronous serial interface (DVB/ASI) and asynchronous transfer mode/optical carrier 3 (ATM/OC3).

The number of video servers depends on the number of titles to be stored and the number of streams to be played simultaneously. The goal is to keep server costs at a minimum and, therefore, multiplex as many streams as possible together into one 6 MHz channel or onto one fiber for hub transport. Some servers perform this multiplexing function on their own, eliminating the cost of an external multiplexer.

Another VOD component is the quadrature amplitude modulation (QAM) modulator. For each multiplexed 27 Mbps or 38 Mbps channel, one 64-QAM or 256-QAM is required for modulating content onto a 6 MHz channel.

The next VOD component is the set-top application, a software program downloaded in-band to a set-top for browsing through the selection of VOD titles. Some VOD set-top applications also are integrated into a program guide application already resident in the set-top. The set-top software allows the customer to browse through and select titles. The content information and graphics are communicated to the set-top either in-band or out-of-band (OOB). Since this information is common to most set-tops, it typically is broadcast to minimize necessary bandwidth.

Movie selections and pause, resume, fast-forward or fast-reverse requests are communicated in real time through the reverse path to another software application at the headend. These typically are called the VOD client and server applications. The selection of the client application is critical to ensuring the look and feel fits what is desired for the area.

Operators may choose to deliver VOD consistent with the program guide or deliver a unique interactive, graphically enhanced experience. Regardless, it is important to keep in mind that the user interface is separate of the video server.

To integrate VOD with the digital network requires interconnect software between the headend application and the digital bandwidth and encryption server.

Most digital networks include one or more headend computers, which manage the digital bandwidth, broadcast schedules/billing and digital encryption control. Since bandwidth control and encryption also are required for VOD, its critical for interactive sessions to employ the same method used for broadcast video. In some cases, the server-side application communicates directly with the encryption server, but for a truly scalable system, this is likely to be a separate software and hardware component.

Some VOD systems also include flexible streaming software as opposed to standard server application programming interfaces (APIs), allowing streams to be controlled by multiple applications in a standard fashion. The streaming software also addresses fault resilience of streams on a software level (re-routing to different QAMs in the event of failure).

Network management is not to be underrated. As technologies advance, having a complete network management system is critical. For VOD, its necessary to account for hardware and software traps, including run-time diagnostics.

With the logical components of a VOD system defined, we now look at content managementthe software component that makes VOD a reality. VOD mandates having video files moving onto and off of servers. If servers reside at hubs, files have to be copied to server complexes all over the city. Performing this manually would introduce extraneous staffing costs and the possibility of frequent errors.

In addition to managing video content, there is the management of the pricing, movie information or metadata (story description, stars, director and so on), and subscriber management. One reason VOD is perceived to be difficult is that until recently no complete and automated content management systems existed.

A good content management system that is tied closely to a server solution can reduce operational cost as well as minimize hardware cost by optimizing storage and more.

In addition to its primary task, a good content management system also will propagate content to servers automatically, making actions such as loading, moving, copying and deleting content files hands-off procedures.

Figure 1 depicts the components described as mandatory for a complete VOD system. (It assumes that a digital network architecture is in place and that VOD will tie into this delivery system.) Depending on which digital set-top box has been selected, the headend architecture can change significantly. Some digital headend equipment includes separate encryption and control servers specific to VOD, whereas others may integrate directly with the same servers that control broadcast bandwidth and encryption.

A generic network architecture for an integrated VOD headend is depicted in Figure 2. Green components represent existing analog broadcast equipment, blue components are existing digital broadcast equipment, and red components are VOD-specific.

Although the concept of every potential VOD subscriber owning his/her own digital channel may seem overwhelming, the amount of bandwidth required for a VOD network is quite reasonable.

Factors for calculating channels required depend on the number of nodes used for combining, size of nodes, encoding rate, VOD penetration rate, simultaneous streams and QAM modulation.

As an example, assume that combining takes place for four 500-home nodes and the VOD penetration rate is 15% of homes passed. Movies are assumed to be encoded at 3 Mbps each. From the many early VOD trials, we have learned that we can support our VOD subscribers by building to 10% simultaneous usage, meaning one stream or bandwidth availability can support 10 VOD subscribers.

So, four 500-home nodes equals 2,000 homes passed. At 15% penetration, we have 300 VOD homes or 30 VOD streams required. Assuming 3 Mbps encoding, this equals 90 Mbps that are required.

If we use 256-QAM (at 38 Mbps per 6 MHz channel), our system would require just over two 6 MHz channels. Since some broadcast bandwidth is required for data and navigation graphics, we can assume three channels are needed in this VOD bandwidth model.

Transport requirements vary according to the network architecture in place and the amount of available space in hubs.

Some servers are physically small and can be installed in hubs but could introduce operational challenges. Other large systems require locating all server capacity in a central headend, but transporting all bandwidth to hubs can be costly. This trade-off must be justified by the mean time between failure (MTBF) and tools available to help manage the servers.

If all video servers reside in the headend, we must transport all required bandwidth to each hub. If a hub has 20,000 homes passed with a 15% VOD penetration and movies are encoded at 3 Mbps, we must have the fiber and transport capacity to send 900 Mbps to each hub.

This typically can be done using one OC-48 connection. It is important to note that scaling via this distribution method also will incur more transport costs.

If all video servers reside in hubs, there is no transport cost involved, although a high-speed local area network (LAN) connection likely will be required to connect the servers for content management. Scaling does not require add-on transport cost and is a more efficient method of installing VOD when hub space is available and the server platform is fault-resilient and efficient enough to run unmanned.

Finally, the key component of a successful VOD network is fault resilience. Fault resilience goes far beyond enabling a server to recover from failure.

To fit into the cable TV model, VOD equipment must operate unmanned. This means that content must be moved around servers automatically as needed and that distribution paths must be automatically redirected in the event of a failure. The most successful VOD platforms available today have fault resilience far beyond the server level, including sophisticated software fault resilience methods. - CT

Bottom Line

Are You Prepared for VOD?

Several video server providers are building various levels of fault-resilient servers, a critical component to video-on-demand (VOD). Some video server vendors also are addressing the many software requirements for managing VOD deployments. Key partnerships for integration with digital network infrastructures are established, and early deployments using standard digital boxes started in late 1998.

The components that are available today allow this technology to finally become the long-awaited new revenue opportunity for cable operators. Are operators prepared for VOD? If we assume that VOD is an add-on service to digital broadcast, then we can directly link readiness to digital deployments: More than 1 million digital set-tops were deployed in 1998, and several operators announced plans to include VOD as a layered service on digital broadband networks. The year of proof that the technology is ready and can be deployed with ease will be 1999.

Yvette Gordon is director of interactive technologies at SeaChange International in Maynard, MA. She can be reached via e-mail at .