Video-on-demand
The basic principle of video on demand (VoD) is very simple. Digitised video information stored on magnetic disks is retrieved by a video server and delivered to the home or office, where it is reconstructed using either a PC or a STP and displayed on a television set. One approach to video distribution is to use a number of channels to broadcast the same video piece such as a film with starting times staggered by say 5–10 min. Known as near video on demand (NVOD), this method will allow the viewer to choose the viewing time within specified limits. True VoD allows the viewer full choice of viewing time, together with VCR-type controls such as fast forward, rewind, replay and so on. Modern VoD is based on IP uni- cast stream, which means instead of broadcasting a signal to everyone, each consumer now has a personalised channel.
One of the greatest challenges to VoD is the amount of bandwidth required. Consider a network providing 150 standard definition multi-cast television channels (each requiring 1.5 Mbps) to 3000 consumers. Without VoD, the operator ‘requires 225 Mbps (150 channels X 1.5 Mbps) on the network backbone to deliver television service to all consumers. The num- ber of channels determine the required bandwidth. If every consumer is watching VoD programming, the operator requires 4.5 Gbps (3000 con- sumers X 1.5 Mbps) on the network backbone. Now, the number of con- sumers determines the required bandwidth.
Because of the bandwidth impact of VoD, understanding the expected penetration rate of VoD service is critical when designing the network. Typically the service mix is 90% Broadcast or multi-cast TV and 10% VoD.