26th November 1998
This document lists the requirements posed to URI schemes for use in TV Broadcast environments. The document summarizes the outcome of discussions on this subject by the W3C TV-Web Interest Group. Please send comments to the mailing list of this group (email@example.com)
In this document TV Broadcast is used as the generic term to refer to currently existing TV systems, their transport protocols, and their typical operation of content provision and distribution. TV Broadcast concerns both digital and analog systems and includes systems like DVB, ATSC, DSS, NTSC, and PAL. The TV Broadcast 'network layer' is typically non-IP based.
The term TV Broadcast URI refers to URIs which identify and/or locate TV Broadcast content. In this document "URI" is used to indicate TV Broadcast URI.
TV Broadcast applications need a mechanism to identify and locate the content building the application. The URI scheme is a useful tool for that as it opens possibilities for seamless transition in referencing resources at TV Broadcast and Internet sites. URI schemes to locate resources at the Internet are well-known, and are not further observed in this document. URI schemes to locate resources in a TV Broadcast transmission channel have been proposed, but wide spread consensus has not yet been reached. There is no complete scheme set existing which covers all access types at hand within TV Broadcast transmission protocols.
Next to locating resources at TV Broadcast transmission channels, another aspect of TV Broadcast URIs concerns the referencing of the TV Broadcast content itself. In the first place, this content will be available at the TV Broadcast transmission channel, possibly at several channels and at multiple periods of time. The above mentioned URI schemes address this. In the second place, the content may be stored and made available through another path than the TV Broadcast transmission channel. Most evident are local storage, like VCR-type of devices, and the Internet itself. Local storage devices can be connected through an in-home network to the client presenting the application. Local storage in the sense of the client's local file system or in the sense of cache buffering are not observed in this document. TV Broadcast content delivered through a so-called IP-tunnel is considered as content made available through the Internet. An IP-tunnel refers to a forwarding path which is logically separated from the conventional TV Broadcast transmission protocol but uses the same physical link.
TV Broadcast content is modeled in a four-layer hierachy, consisting of service, event, component, and fragment. Service is at the top, fragment is at the bottom of the hierarchy.
The term service is used to refer to a concatenation of programs, all being broadcast by the same service provider. The programs of a service share some tuning characteristics. Service corresponds to the naming "channel" as used in today's analog TV.
The term event is used to refer to a single TV program. An event is a time period within a service and therefore can be characterized with begin and end times. The service provider determines the granularity in which the service is split in events. An event can be a complete program or an episode of a program. Events are the typical entities which EPGs list to present program schedule information.
The term component is used to refer to the constituents of an event. The audio and video of a TV program are obvious examples. In case of multilingual programs there are multiple audio components. In case of interactive programs the components are the application documents and the other data the applications are using.
The term fragment is used to refer to a subpart of a component. For instance, it can be a slice of a video sequence, or a subregion in an image.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as decribed in RFC 2119 [RFC2119].
The actual resource's retrieved content data MAY differ in terms of content
encoding, content quality, performance, and edit version.
[Note the difference with the previous requirement, particularly the use of 'MUST' and 'SHOULD'.]
TV Broadcast differs from the conventional Internet in several ways. The TV Broadcast URI scheme is affected by that in the following aspects: