Among such premium services are likely to be Digital TV, Video-on-Demand (VoD), near VoD, the World Wide Web (WWW), and virtual channels. Digital TV offers better pictures and with better frequency utilization, a wider programme selection than what over-the-air broadcasters or cable operators can offer over analog networks. In addition, the Internet offers a tremendous variety of information and entertainment for the home. In fact, Digital TV and the Internet, offered separately or in combination, are creating new forms of entertainment and information services to the home that are expected to be among the key market drivers for RBB networking.

The key facilitator in the shift to Digital TV was the development of digital compression techniques. PAL and SECAM would need more than 400 Mbps of bandwidth to deliver pictures with existing pixel density and colour. Now we have High-Definition TV (HDTV) which would require more than a Gbps (109 bps). These bit rates could not fit in a 6 MHz channel without compression.

The solution has been found in Motion Picture Experts Group (MPEG) compression technique. MPEG compression enables HDTV to fit into 19.3 Mbps, a reduction in bit rate of more than 50 to 1, which could fit into a 6 MHz channel.

The general agreement among broadcasters, consumer electronics/computer industry, and the film industry seems to be for video compression using MPEG-2 and audio compression using Dolby AC-3 techniques.

For most content, picture quality improves with digital transmission compared with analog transmission, even with standard-definition digital. This is because fewer problems occur with ghosting, vertical hold, and noise. Broadcasters, both over-the-air and cable, can encode with MPEG-2 at speeds from 3 Mbps to 9 Mbps for Standard Definition Television (SDTV). With this amount of control, broadcasters have a means by which to improve picture quality incrementally as picture and audio content dictate.

Video on Demand (VoD) enables consumers to order movies over a network rather than going to a video rental store. VoD is a convenient and highly customized way to view stored content such as movies, documentaries, and other educational fare. Not only does VoD give the viewer access to larger libraries than are available at a single retailer, but it also is a convenient means of access because it provides search mechanisms to locate specific topics. Furthermore, VoD is highly customized because it enables the viewer to choose what and when to watch instead of having the service provider decide. Storage prices are dropping and access is improving.

VoD service is a "pull mode" service where the subscriber demands and receives data from the provider. The consumer decides what to watch and when to watch it from a range of alternatives and then retrieves the selection, much like pulling information from a database.

VoD includes VCR controls, such as rewind, pause, and fast forward. It also has options for jumping to selected scenes, choosing language and subtitles, and activating captioning.

VoD’s pull mode delivery stands in contrast to "Pay-Per-View" (PPV), which operates in "push mode". In push mode the service provider transmits data to the subscriber on a fixed, predetermined schedule, or in response to some event such as the updating of data in the provider’s database. The consumer simply decides whether or not to partake.

No VCR type controls are available. Given the service characteristics of push mode, it is especially appropriate for live one-time events such as boxing and wrestling matches—the two most popular forms of PPV in the US.

In a pull mode environment, no two subscribers are likely to be watching the same movie. Therefore, separate data flows are established, one for each viewer. Each data flow consumes bandwidth dedicated to a single consumer.

VoD was made feasible by the development of digital compression. Without compression, a single-colour movie would consume perhaps as much as 100 GB of storage and could not be transmitted, even over an E3 link to the home. With MPEG, a movie can be stored in 3 GB (providing broadcast TV quality) and played out at 3 Mbps. These are still large numbers, but achievable by proposed networks.

The financial viability of a VoD offering depends on the "take rate". The take rate is the number of movies rented per month divided by the subscriber base. If a service provider has 1,000 consumers and 2,000 movies were rented in a month, the take rate is 200 percent. Take rates of 200 percent or more are likely to be essential to make VoD financially viable.

An alternative to VoD is "Near Video-on-Demand" (nVoD), also known as advanced or enhanced PPV or "staggercast". With nVoD, the service provider elects to offer a particular movie beginning at certain intervals, say every 15 minutes, on a small number of channels, say four. The interval is called the stagger time, hence the term "staggercast". nVoD provides widespread availability of movies without the need for a dedicated point-to-point connection between the viewer and the video server. This reduces server and network resources when compared with VoD but at a loss of consumer flexibility. Given the experience with PPV, consumers appear to be willing to accept these limitations. With nVoD, the viewer is offered the same content continuously, whereas for PPV, the content is offered once. nVoD is still an experimental service undergoing research.

One example of such services is Karaoke-on-Demand (KoD) which is the networked version of Karaoke. When you pick a song, the music, lyrics, and possibly visuals are streamed to your local Karaoke unit. With KoD, it would be possible to select music from a large centralized library. Moreover, it would be possible to have participants in other locations join in to create a virtual chorus! Lyrics could be available in a number of languages, and, in the case of a single participant, it would be possible to select tempo, reverberation, octave, background music, and background scenes. ISDN-based KoD in Japan is a good example of an actual deployment.

Although not the strongest market driver for RBB as yet, KoD raises some interesting issues. Firstly, it signifies that entertainment is something which people will pay for. In many parts of Asia, Karaoke machines sell well, not only to entertainment establishments but also to high-end residential users. Secondly, KoD represents challenging technical problems, especially with regard to synchronization and conferencing, which also require attention for business conferences and VoD. Most importantly, the virtual chorus represents an application that requires a multipoint-to-multipoint topology. Other such applications are interactive gaming and videoconferencing, both of which have consumer applications. A popular interactive game is Doom, in which players at multiple sites stalk and shoot each other in a virtual combat zone. Videoconferencing also is a way for families to celebrate special occasions together, when family members are geographically distant.

Multipoint-to-multipoint applications raise a particularly difficult Quality of Service (QoS) problem. A single receiver might receive input from multiple sites simultaneously, producing congestion at the receiving site. Techniques are being explored to arbitrate the congestion and yet provide some bandwidth to each sender.

As the new networks are being designed, needs for such applications, which are becoming reality in other parts of the world, need to be addressed. Undoubtedly, these applications are bandwidth hungry. But so will be the information driven society.

By Niraj K. Gupta, Voice and data, April 2000.