TV Broadcast URI Schemes
11th March 1999
- Warner ten Kate <firstname.lastname@example.org>.
- Gomar Thomas <email@example.com>.
- Craig Finseth <firstname.lastname@example.org>.
- Copyright ) 1999 W3C (MIT, INRIA, Keio), All Rights Reserved.
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This document is an informational document and discusses the requirements
posed to URI schemes for identifying resources in TV Broadcast environments.
The document is the outcome of discussions on this subject by the
W3C TV-Web Interest Group [TVWebIG, TVWebMail].
Typical use cases are summarized where TV Broadcast URIs are involved.
A distinction is made between Global and Local usage.
Also, a hierarchy of resource types is identified.
Requirements related to the Global usage case are listed.
Table of Contents
- TV Broadcast: Definition and scope
- Application Scenarios
- Requirements on Global TV Broadcast URI schemes
- Exceptions in TV Broadcast URIs
TV Broadcast: Definition and scope
Definition of TV Broadcast
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 possibly locate, TV Broadcast content.
In this document "URI" is used to indicate TV Broadcast URI.
Typically, TV Broadcast systems are push systems.
The content streamed along a TV Broadcast transport is scheduled
by the service provider; the user has no influence on that.
In this model the user accesses the stream(s) rather than the server
at the upstream station.
Hierarchy in TV Broadcast content
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 consumes 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 can be grouped in series, e.g., to form a serial.
Events are the typical entities which EPGs list to present program
The term component is used to refer the constituents of
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 these applications are using.
Next to continuous data like audio and video, component also encompasses
discrete data like Web pages and applications describing composition and
The URI identifying an application can constitute the base URI for the further
components referenced by that application.
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.
Due to the push character of TV Broadcast there are two dimensions
of hierarchy, a schedule related and a content related.
The first is the hierarchy of
transport system, transport stream, service, series, event;
The second is the hierarchy of
series, event, component, fragment.
The term resource is to be understood as in RFC 2396, sec.1.1 [RFC2396].
In the context of TV Broadcast a resource refers to the entities
service, event, component, and fragment in particular.
Setting and usage of TV Broadcast URIs
TV Broadcast applications need a mechanism to identify and locate
the components 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 transport
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 transport channel
have been proposed, but most are designed with a particular TV Broadcast
transport environment in mind.
Next to locating components at TV Broadcast transport channels,
another aspect of TV Broadcast URIs concerns referencing the events.
In the first place, events are accessible at the TV Broadcast transport
channel, possibly at several channels and at multiple periods of time.
The above mentioned URI schemes also address this aspect, but all in
their own way.
In the second place, the content may be stored and made available
through another path than the TV Broadcast transport channel.
Most evident are local storage, like VCR-type of devices, and the
Local storage devices can be connected through an in-home network
to the user agent 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 transport protocol but uses the
same physical transmission link.
Application types, further definition and scope
Applications can be distinguished in usage of URIs for Global and
Global refers to URIs contained in documents which can be accessed
anywhere around the world, and which identify content related to
any TV Broadcast system in the world, including storage devices
associated with that TV Broadcast system.
Such a global URI may include identification of the TV Broadcast system
to be used.
Local refers to URIs contained in documents which are accessed
within a certain TV Broadcast system, and which identify content
to be accessed through that TV Broadcast system.
URIs that reference content outside the local TV Broadcast system, are
assumed to be either Global URIs or traditional URIs for locating resources
at the Internet.
This document concentrates on Global URIs, as those have a world-wide
interest for standardization.
It would be nice when Local URIs bear an identical format, but that is
considered not a necessary requirement.
Local URIs can be specified within their respective application domains.
On the other hand, it would be nice when Global URIs can serve as
a base URI for Local URIs, either as direct copy or by some mapping function.
Further, URIs can be distinguished in identifying a service or event,
and in identifying a particular content item (component or fragment) in
such a service or event.
This reflects the two dimensions observed above of schedule related and
content related hierarchy.
The use cases where a content item in a certain service is to be identified
while the context isn't already that service, seem rare.
Consequently, a URI is not required to carry both informations (service
and content item) together.
This distinction suggests that identification of a particular content
item belongs to the Local class of URIs, and that Global URIs typically
identify a service or an event.
However, an exception can be found in the case where the same content item
is referenced in various transport contexts, e.g. in a commercial.
An important class of Global URIs identify their resource in a location
and time independent way, i.e. independent of the particular TV Broadcast
transport system and particular schedule.
For instance, they are also valid after local storage.
As such, they resemble URN behavior, opposed to URL behavior.
As the set of resources and their various locations can scale
to large numbers, it is preferred that the URI scheme imposes a
hierarchical structure, certainly when the URI's purpose is to locate
A hierarchy allows for step-by-step resolution and navigation to the
By that, efficiency and scalability is improved.
Further, implying a hierarchical structure allows to group resources,
and by that to distinguish between, for example, in identifying a serial
and an event in that serial.
Use cases, both Local and Global URI
Below, some representative use cases are listed.
An exhaustive list of application scenarios is provided in [USECASES].
- Basic EPG type of locating:
Reference TV Broadcast services and events from a Web page for
navigating to them.
The references are tolerant to modifications in the actual transmission
schedule, but a coarse indication can be derived.
The broadcast program can be indicated through tuning data or
Next to navigation to the program, the EPG also supports for setting
reminders or recording of programs.
Instead of a single program, the serial of which the program is part,
is referred to, such that setting a reminder or a recording for all
episodes can be accomplished.
It is the year 2002. Fox is broadcasting a World Cup game from
South Korea in both analog and digital formats, with the broadcast
reaching North America, Europe, Africa, Asia, Latin America,
Australia, etc., through a wide variety of local affiliates and
re-broadcast operators. Fox wishes to put a hyperlink to the
broadcast on its web site, so that users of Internet-connected TV
receivers all over the world with the right software (perhaps native,
perhaps downloaded) can click on the hyperlink and have their
receivers tune to the broadcast (or set a reminder for the broadcast,
if the game is not currently on).
- A sports fanatic wants to watch all the above broadcasts by Fox.
Therefore he records all the broadcasts and copies the above Fox World Cup
page to his local disc.
From that page he can access the broadcasts or, when they have been
recorded, view them from his recording device.
At its site Fox also provides the transmitted broadcasts, albeit at
high compression rates.
The page will direct users who haven't recorded the broadcast to
- A Web page is composed for presentation on a TV Broadcast receiver.
The Web page is delivered in association with a TV Broadcast program
(the transmission paths may be physically separated). The Web page
includes an object which refers to the associated audio/video image
of the TV broadcast program.
- In a Web page a TV Broadcast event is referred, but the exact
location is not known at authoring time. The URI is incomplete
in its information. Instead a query is added to retrieve the
missing information. When the available TV Broadcast system
supports the query mechanism, the URI can be resolved and the
identified resource can be retrieved. The query language is
technology-independent, i.e. it is not relying on specific fields,
such as SI data, in the TV Broadcast transmission system.
- A TV Broadcast of a soccer match is data-enhanced; in a data carousel
module or an encapsulated IP datagram a file is contained which gives
up-to-the-second statistics on goals scored, fouls committed, corner
kicks taken, shots at goal, shots on goal, etc. The broadcaster wants
to put a URI on their web site which references this file, allowing
applications on Internet-connected TV receivers all over the world
to get to the file and display it in nifty ways.
- A data file is transmitted along with a TV program, the data file
is containing additional information to that program. It also contains
hyperlinks to the programs and/or data in other data files being
broadcast on the same channel and in other channels, so that
receivers can set reminders for the upcoming game and/or data file.
- A Web page is transmitted with a TV Broadcast commercial.
The commercial is about an upcoming TV Broadcast program.
The viewer can click a hotspot area such as to set a reminder for
The Web page can also be accessed at the broadcaster's Web site.
- A set of three Web page is transmitted with a TV Broadcast commercial.
The viewer can navigate the three pages.
The pages are transmitted frequently along various TV Broadcast systems.
The pages can also be accessed at the advertiser's Web site, where
they are maintained at a particular sub directory.
Therefore, the advertiser uses relative referencing between the pages.
- A live quiz show is enhanced such that the viewer can play along.
The enhancement data are a mixture of Web pages, which compose the
quiz's question and answer environment, element values, which carry
the actual questions and (correct) answers to be inserted in the Web
pages, and procedural cells to control the viewer's score.
The Web pages are provided at a Web site long before the show is aired,
such that viewers can prepare.
The element values are transported along the TV Broadcast transmission
channel during the show.
They are synchronized with the actions in the show such as to complete
and update the application.
There are several levels of play along: some pages provide the viewer
with hints such as to ease answering, and some pages provide less
alternatives in the multiple choice questions.
The viewer can select his level by navigating between these pages.
Upon the actual broadcast an application is broadcast with the program
to initiate the enhancement.
The application references the Web site, such that upon tuning to the
TV Broadcast the Web site's home page gets retrieved.
Triggered by stream events in the TV Broadcast transport stream, the
application also controls the insertion of element values (questions
and answers) and the score management (e.g., no score increment after
- A Web site provides a EPG covering programs transmitted world-wide.
A viewer is visiting this site and browses the EPG.
Upon encountering his favorite movie "Once upon a time in the Cyber"
he clicks the item on the EPG.
Regretfully, the movie isn't scheduled for the 419 TV Broadcast satellites
his receiver is configured to.
Instead of setting a reminder, the receiver informs the user the
movie will not appear on his reception links.
Requirements on Global TV Broadcast URI schemes
Conventions used in this document
In this document three levels of priority are used to indicate
the desirability of a requirement.
- The key word "MUST" is to be understood as an essential
and critical requirement.
- The key word "SHOULD" indicates an important requirement.
- The key word "MAY" indicates a useful feature.
[These key words and their meaning are based upon RFC 2119 [RFC2119].
That RFC specifies similar wording for implementation compliance with
a protocol specification.
In this document the wording reflects specification compliance with
- The URI scheme MUST comply with RFC 2396 [RFC2396].
- Where the URI serves as a name identifier (URN), the corresponding
URN specifications MUST be taken into account, e.g. [RFC2141, RFC2168].
- Where the URI serves as a locator identifier (URL), the definition of
the URI scheme MUST follow the guidelines as set forth in [URLGUIDE].
- The URI MAY support queries to be posed to the TV Broadcast receiver.
The query language MUST be independent to the TV Broadcast system.
- The URI scheme SHOULD support relative referencing such that
a TV-program with all its associated resources can be referenced
against a common base, which is the TV Broadcast URI of that
- Where the URI serves as a locator identifier (URL), the URI scheme
SHOULD include a hierarchical structure either to identify the resoure
as a service or an event from a service, or to identify the resource
as an event, a component from an event, or a fragment from a component.
The structure SHOULD provide optional levels to group events into series
or serials and to group components into composites.
Where the URI serves as a name identifier (URN), the URI scheme MAY
include such hierarchical structure.
- The URI scheme SHOULD support the spectrum of transport protocols
applied and standardized in TV Broadcast systems. This includes
both audio/video and data broadcast protocols.
- A URI MUST be invariant with respect to the normal range of
transport stream transformations along the path from provider
to user, both in referencing the time and the location of the
resource in that transport stream.
- Given a URI, it MUST be possible for a receiver to actually
locate the resource, or conclude that it is not reachable.
- A URI MUST be meaningful when interpreted, independent of the
transmission context in which the URI is called.
Transmission context refers to a coherent set of content streams
as they arrive at the receiver.
An example is a set of TV broadcast services sharing a same physical
connection; another is an Internet connection.
In case the context is the same transmission system as in which
the content is located, the URI MUST be resolvable.
- Where the URI serves as a name identifier (URN), it SHOULD be resolvable
under any of the following network access conditions:
The actual resource's retrieved content data MAY differ in terms
of content encoding, content quality, performance, and edit version.
- TV Broadcast
- In Home/local storage
- Where the URI serves as a name identifier (URN), the scheme MUST support
referencing various instantiations of the same content (encoding,
quality/compression ratio, versions/edits).
- Any actual scheme SHOULD be coordinated with standardisation bodies
such as ATSC, DVB, and DAVIC, and SHOULD be reasonably acceptable
to those bodies.
- The URI scheme MUST interoperate with the Internet access schemes,
such as to enable seamless transition in referencing resources at
TV Broadcast or Internet sites.
Exceptions in TV Broadcast URIs
TV Broadcast differs from the conventional Internet in several ways.
The TV Broadcast URI scheme is affected by that in the following aspects:
- The host is not necessarily a server identifiable through an IP-address.
For instance, the "host" is a transport stream.
- The resource access and retrieval scheme is not necessarily
- The resource's availability implicitly depends on, or at least
relates to, a transmission schedule.
Because TV Broadcast is a resource constrained environment, it is
worthwhile to keep the length of the URI limited.
This document does not pose a requirement on a maximum length of
a TV Broadcast URI.
It is left to the particular application domain to specify such
Key words for use in RFCs to Indicate Requirement Levels,
- RFC 2119, S. Bradner. March 1997.
W3C TVWeb Interest Group,
- Group page of the W3C TV-Web IG. Philipp Hoschka.
TV-Web Mail Archives,
- Threads starting with messages 0040, 0041, and 0046.
Uniform Resource Identifiers (URI): Generic Syntax,
- RFC 2396, T. Berners-Lee, R. Fielding, L. Masinter. Aug. 1998.
- RFC 2141, R. Moats. May 1997.
Resolution of Uniform Resource Identifiers using the Domain Name System,
- RFC 2168, R. Daniel, M. Mealling. June 1997.
Guidelines for new URL Schemes,
- Internet-Draft, L. Masinter, H.T. Alvestrand, D. Zigmond, R. Petke. November 1998.
- Posting to the TV-Web IG, C. Finseth. December 1998.