WO2004056096A1

WO2004056096A1 - Method of announcing sessions

Info

Publication number: WO2004056096A1
Application number: PCT/IB2003/005468
Authority: WO
Inventors: Juha-Pekka Luoma; Dominique MÜLLER; Toni Paila
Original assignee: Nokia Corporation
Priority date: 2002-12-18
Filing date: 2003-11-27
Publication date: 2004-07-01
Also published as: TW200419393A; JP2011045093A; CA2510709A1; US9485044B2; TWI268436B; BR0317540A; KR100742244B1; EP1574047A1; JP2006512027A; US20060253544A1; KR20050085702A; JP5542592B2; AU2003280200A1

Abstract

An electronic service guide (ESG) is provided by transmitting announcements describing multimedia sessions, such video streams. Sessions are organised into a session directory (28) which is split into two parts: a full session directory (291) and an updated session directory (292). A first kind of announcement describes all sessions in the full session directory. A second kind of announcement describes sessions in the updated session directory. Once a client has received a description of the full session directory, it need only listen to announcements of the second type so as to learn of any updates to sessions.

Description

Method of announcing sessions

Field of Invention

The present invention relates to a method of announcing sessions particularly, although not exclusively to a method of announcing multimedia service sessions through a multicast network.

Background Art

Audio, video and other types of data may be transmitted through a variety of types of network according to many different protocols. For example, data can be transmitted through a collection of networks usually referred to as the "Internet" using protocols of the Internet protocol suite, such as Internet Protocol (IP) and User Datagram Protocol (UDP).

Data is often transmitted through the Internet addressed to a single user. However, it can be addressed to a group of users. This is known as "multicasting".

One way of multicasting data is to use an IP datacasting network.. Through such an I -based broadcasting network, one or more service providers can supply different types of IP services including on-Une newspapers, radio, television and download of music songs, videos, pictures, games and software. These IP services are organised into sessions, each session comprising one or more media streams in the form of audio, video and/or other types of data.

To determine when and where these sessions occur, users refer to an electronic service guide (ESG). One example used in DNB is an electronic program guide (EPG). The electronic service guide is usually divided up into parts and transmitted to users.

This approach, however, has several drawbacks. On the one hand, if any sessions are updated, then the user usually has to wait until a new version of the service guide has been received before they receive notification of updated sessions. On the other hand, few sessions are usually updated. Therefore, much of the data received by the user is superfluous. This is wasteful both in terms of processing power and electrical power, both of which tend to be in short supply in battery- powered mobile terminals.

The present invention seeks to provide an improved method of announcing sessions transmitted through a network.

Summary of the Invention

According to a first aspect of the present invention there is provided a method of announcing sessions transmitted through a network, the method comprising providing a first set of announcements describing a plurality of sessions and providing a second set of announcements describing at least one updated session.

This has the advantage that it is possible to choose whether to be provided with the first set of announcements describing the plurality of sessions or to be provided with the second set of announcements describing any updated sessions. This allows updated sessions to be announced more quickly and efficientiy.

An updated session may be a new session which is added to the pluraUty of sessions, a one of the pluraUty of sessions in which content is added, changed or deleted or a session which is deleted from the pluraUty of sessions.

Providing the first set of announcements and providing the second set of announcements may comprise providing the first set of announcements through a first channel and providing the second set of announcements through a second, different channel.

Providing the first set of announcements and providing the second set of announcements may comprise providing the first set of announcements through a first address, preferably a destination address, such as a first multicast IP address, and providing the second set of announcements through a second, different address, preferably a destination address, for example a second, different multicast IP address, respectively. Providing the first set of announcements and providing the second set of announcements may comprise providing the first set of announcements through a first port number and providing the second set of announcements through a second, different port number respectively.

Providing the first set of announcements and providing the second set of announcements may comprise providing the first set of announcements through a first logical channel and providing the second set of announcements through a second, different logical channel respectively.

Providing the first set of announcements and providing the second set of announcements may comprise including in each announcement of the first set of announcements data for identifying the announcement as an announcement which describes a one of the pluraUty of sessions and in each announcement of the second set of announcements data for identifying the announcement as an announcement which describes a one of the at least one updated session.

Providing the first set of announcements and providing the second set of announcements may comprise including in each announcement of the first set of announcements respective data for specifying a position of a corresponding session within a first portion of a session directory and including in each announcement of the second set of announcements respective data for specifying a position of a corresponding session within a second portion of the session directory.

Providing the first set of announcements and providing the second set of announcements may comprise providing the first set of announcements through a first physical channel and providing the second set of announcements through a second, different physical channel respectively.

Providing the first set of announcements and providing the second set of announcements may comprise providing the first set of announcements through a first network and providing the second set of announcements through a second, different network respectively.

The method may further comprise providing a third set of announcements describing another pluraUty of sessions including the at least one updated session.

The method may comprise providing the first set of announcements through a first channel, providing the second set of announcements describing at least one updated session through a second, different channel and providing a third set of announcements describing another pluraUty of sessions including the at least one updated session through the first channel.

The method may comprise arranging the providing of said second set of announcements after the providing of said first set of announcements.

The method may comprise arranging the providing of said first set of announcements and the providing of said third set of announcements at substantially during an overlapping or same time periods.

Providing the first set of announcements and providing the second set of announcements may comprise transmitting the first set of announcements through the first channel and transmitting the second set of announcements through the second, different channel.

The method may comprise transmitting the first set of announcements according to a session announcement protocol (SAP), unidirectional hypertext transfer protocol (UHTTP), asynchronous layered coding (ALC) protocol or similar unidirectional protocol based on user datagram protocol (UDP). The method may comprise including a description of a corresponding session in each announcement, for example arranged according to session description protocol (SDP). The method may comprise providing means for determining whether aU of the first set of announcements have been provided, for example by providing the first set of announcements as a series of Unked messages.

The method may comprise providing the first set of announcements in a first set of time slots and providing the second set of announcements in a second set of time slots, each timeslot of the first set of timeslots being provided at a different time from each timeslot of the second set of timeslots. The method may comprise multiplexing the first and second sets of announcements.

The method may further comprise providing a third set of announcements identifying the at least one updated session. Providing the second set of announcements describing the at least one updated session may comprise providing a set of announcements identifying the at least one updated session. Providing the second set of announcements describing the at least one updated session may further comprise including a description of a corresponding session. Providing the seςond set of announcements' describing the at least one updated session may comprise providing a set of notifications pointing to the at least one updated session.

According to another aspect of the present invention there is provided a method of announcing sessions transmitted through a network, the method comprising providing a first set of announcements describing a pluraUty of sessions and providing a second set of announcements identifying at least one updated session.

The method may further comprise providing a third set of announcements describing the at least one updated session. The method may comprise transmitting at least one of the sets of announcements according to asynchronous layered coding (ALC) protocol. The method may comprise transmitting at least one of the sets of announcements according to a protocol based on asynchronous layered coding (ALC) protocol. The method may comprise defining an asynchronous layered coding (ALC) protocol session and defining at least one ALC channel. The method may comprise transmitting a set of metadata for describing the pluraUty of sessions via a first ALC channel. The method may comprise transmitting a set of metadata for describing at least one updated session via a second, different ALC channel. The method may comprise transmitting a set of metadata for identifying the at least one updated session via a third, different ALC channel. The method may comprise transmitting a set of metadata as a transport object. The method may further comprise defining a respective deUvery table relating to the transport object and transmitting the deUvery table.

According to a second aspect of the present invention there is provided a computer program which, when executed by data processing apparatus, causes the data processing apparatus to perform a method of announcing sessions transmitted through a network.

According to a third aspect of the present invention there is provided a method of accessing sessions transmitted through a network, the method comprising selectively receiving a first set of announcements describing a pluraUty of sessions; and selectively receiving a second set of announcements describing at least one updated session.

The method may further comprise determining whether all of said first set of announcements have been received. The method may further comprise selecting not to receive further said first set of announcements and selecting to receive said second set of announcements. The method may further comprise selecting not to receive a third set of announcements describing another pluraUty of sessions including said at least one updated session. The method may further comprise selecting to receive a fourth set of announcements describing at least one further updated session.

The method may comprise using the second set of announcements to identify said at least one updated session. The method may comprise selecting to receive another set of announcements including a description of said at least one updated session. The method may comprise obtaining a description of said at least one updated session. According to another aspect of the present invention there is provided a method of accessing sessions transmitted through a network, the method comprising selectively receiving a first set of announcements describing a pluraUty of sessions and selectively receiving a second set of announcements identifying at least one updated session. The method may further comprise selectively receiving a third set of announcements describing said at least one updated session.

According to a fourth aspect of the present invention there is provided a method of accessing sessions transmitted through a network, the method comprising Ustening to a first set of announcements describing a pluraUty of sessions, determining whether said first set of announcements have been received; if said first set of announcements have been received, then stopping Ustening to said first set of announcements and Ustening to a second set of announcements describing at least one updated session.

The method may further comprise stopping Ustening to a third set of announcements describing a further pluraUty of sessions including said at least one updated session.

According to a fifth aspect of the present invention there is provided apparatus for announcing sessions transmitted through a network, the apparatus comprising means for providing a first set of announcements describing a pluraUty of sessions and means for providing a second set of announcements describing at least one updated session.

According to a sixth aspect of the present invention there is provided apparatus for performing the method.

According to a seventh aspect of the present invention there is provided apparatus for announcing sessions transmitted through a network, the apparatus comprising a first transmitter for providing a first set of announcements describing a pluraUty of sessions and a second transmitter for providing a second set of announcements describing at least one updated session.

The apparatus may comprise means for managing an electronic service guide for announcing sessions to be transmitted through the network, means for managing content of sessions to be transmitted through the network, means for storing and electronic service guide for announcing sessions to be transmitted through the network, means for storing content of sessions to be transmitted through the network, means for determining changes to an electronic service guide, the changes corresponding to updated sessions to be transmitted through the network, a server for providing information relating to changes to an electronic service guide, the changes corresponding to updated sessions to be transmitted through the network, a server for providing content and/or means for transmitting data.

According to an eighth aspect of the present invention there is provided apparatus for accessing sessions transmitted through a network, the apparatus comprising means for selectively receiving a first set of announcements describing a pluraUty of sessions and means for selectively receiving a second set of announcements describing at least one updated session.

The apparatus may comprise means for determining whether said first set of announcements has been received the apparatus being configured such that if the determining means determines that the first set of announcements has been received, then the means for selectively receiving said second set of announcements is configured to receive the second set of announcements.

The apparatus may comprise means for selectively receiving a third set of announcements describing another pluraUty of session including the at least one updated session, the apparatus being configured such that if said determining means determines that the first set of announcements has been received, then the means for selectively receiving the third set of announcements is configured not to receive or not to forward the third set of announcements. The apparatus may comprise means for receiving data, means for filtering an electronic service guide for announcing sessions to be transmitted through the network, means for storing an electronic service guide for announcing sessions to be transmitted through the network, means for browsing an electronic service guide for announcing sessions to be transmitted through the network, means for filtering content, means for storing content and/or means for browsing content.

The apparatus may be a handheld mobile communications device.

According to a ninth aspect of the present invention there is provided a system for presenting program schedule data on a display, said system comprising at least two announcements, the schedule data being organized at least partly from a first set of announcements describing at least partly a pluraUty of sessions and at least partly from a second set of announcements describing at least one at least partly updated session.

According to an tenth aspect of the present invention there is provided a system for presenting program schedule data on a display, said system comprising at least two announcements, the schedule data being organized at least partly from a first set of repeatable announcements describing a pluraUty of sessions, at least partly from a second set of repeatable announcements describing at least one at least partly updated session and at least session descriptions of at least one of the repeatable announcements for defining whether the at least one of the first and second announcements is received or not.

According to a eleventh aspect of the present invention there is provided a system for deUvering program schedule data to end-user terminals, said system comprising two sets of announcements, each set comprising at least one announcement, the schedule data being organized at least partly from a first set of announcements describing at least partly a pluraUty of sessions and at least partly from a second set of announcements describing at least one at least partly updated session. According to a twelfth aspect of the present invention there is provided a system for presenting program schedule data to end-user terminals, said system comprising at least two set of announcements, each set comprising at least one announcement, the schedule data being organized at least partly from a first set of repeatable announcements describing a pluraUty of sessions, at least partly from a second set of repeatable announcements describing at least one at least partly updated session and at least session descriptions of at least one of the repeatable announcements for defining whether the at least one of the first and second announcements is received or not.

The second set of announcements may include a version number of each updated session for aUowing a cUent to detect if they have missed an earUer update. If a cUent detects it has missed an earUer update and is not currently receiving the first set of announcements, the cUent may start receiving the first set of announcements until it has received a full and latest version of the program schedule data. If the cUent detects that it has received a fuU and latest version of the program schedule data, it may stop receiving the first set of announcements and continues receiving only the second set of announcements. If the cUent detects it has missed an earUer update, it may fetch a fuU and latest version of the program schedule data over an interactive network. Each set of repeatable announcements may be divided into segments before transmission and a location of each segment within a whole transfer may be indicated in a framing field of each respective segment; the indicated location may enable cUents to determine whether they have received aU segments that constitute a given set or whether they need to wait for receiving more segments.

The program schedule data may be viewed either directly by a human end-user or automatically used by a software appUcation. The program schedule data may be presented progressively to a human end-user or made progressively available to an automatic software appUcation as the said data is being received. The program schedule data may be viewed by a human end-user via a graphical user interface. The program schedule data may be used by a personal video recorder. Brief Description of the Drawings

Embodiments of the present invention wiU now be described by way of example with reference to the accompanying drawings in which:

Figure 1 is a schematic diagram of a multicasting system 1; Figure 2 shows content stored in a content database;

Figure 3 shows a session directory;

Figure 4 shows electronic service guide data stored in an electronic service guide database;

Figure 5 shows updated content stored in a content database; Figure 6 shows an updated session directory;

Figure 7 shows updated electronic service guide data stored in an in an electronic service guide database; Figure 8 shows a first embodiment of a session directory before an update according to the present invention; Figure 9 shows the session directory shown in Figure 8 after the update in accordance with the present invention;

Figure 10 shows electronic service guide data before an update in accordance with the present invention;

Figure 11 shows electronic service guide data after an update in accordance with the present invention;

Figure 12 shows a session announcement message using SAP and SDP protocols in accordance with the present invention;

Figure 13 illustrates transmission of a description of a session directory using session announcement messages shown in Figure 12 in accordance with the present invention;

Figure 14 is a process flow of a method of operating a datacast service system in accordance with the present invention;

Figure 15 is a process flow of a method of operating a datacast cUent in accordance with the present invention; Figure 16 shows a second embodiment of a session directory after an update in accordance with the present invention;

Figure 17 shows spUtting electronic service guide data into data segments in accordance with the present invention; Figure 18 shows another session announcement message using UDP and UHTTP protocols in accordance with the present invention;

Figure 19 illustrates transmission of a description of a session directory using session announcement messages shown in Figure 18 in accordance with the present invention;

Figure 20 shows notification of update data in accordance with the present invention;

Figure 21 shows another session announcement message using UDP and ALC protocols in accordance with the present invention; Figure 22 iUustrates transmission of electronic service guide data using ALC channels in accordance with the present invention;

Figure 23 shows transmission of a description of a session directory using session announcement messages using time division multiplexing in accordance with the present invention; Figure 24 shows a schematic diagram of a terminal used to receive multicast data in accordance with the present invention and

Figure 25 shows an electronic service guide browser in accordance with the present invention.

Detailed Description of the Invention

Multicasting system 1

Referring to Figure 1, a multicasting system 1 is shown. In this example, the multicasting system 1 is an internet protocol (IP) datacast system. The multicasting system 1 may include a datacast service system 2, a datacaster 3, a datacast network A and a pluraUty of cUents 5. For clarity, only one cUent 5 is shown.

An administrator 6 provides scheduled content, such as audio, video and/ or other types of data, for datacasting to cUents 5 and provides metadata for describing the content. The metadata includes information regarding transmission of content.

The datacast service system 2 generates IP streams carrying content items and related metadata for datacasting to cUents 5. The datacaster 3 receives IP streams from the datacast service system 2, provides Layer 2 encapsulation and modulation and transmits the IP data to cUents 5 over the datacast network 4. The datacast network 4 is a point-to-multipoint network for deUvering IP-based data. TypicaUy, the datacast network 4 supports a pluraUty of simultaneous datacasts to cUents 5. In this example, the datacast network 4 does not provide a return data path from the cUent 5 to the datacaster 3. The datacast network 5 may be for example a Digital Video Broadcasting (DNB) network, a Digital Audio Broadcasting (DAB) network, an Advanced Television Systems Committee (ATSC) network, an Integrated Services Digital Broadcasting (ISDB) network or a Wireless Local Area Network (WLAN). The cUent 5 comprises a terminal for receiving content and content descriptions over the datacast network 4 and presenting them to an end-user 7. The terminal may be fixed, such as a desk-top personal computer or a television set-top box, or portable, for instance a lap-top or notebook, personal computer, personal digital assistant or mobile telephone handset which have receiving means for receiving broadcast transmissions.

The datacast service system 2 includes an electronic service guide (ESG) management module 8, an ESG database 9 for stqring metadata for the electronic service guide, a service discovery server 10, a content management module 11, , a contents database 12 for storing content for datacasting and a content server 13.

Electronic Service Guide (ESG) is a set of metadata describing available content such as e.g. streaming media and downloadable files with indication of their transmission schedules. The fuU or partial metadata of a single ESG is deUvered to receiving cUents in an ESG session that may comprise one or more channels.

The ESG management module 8 aUows the administrator 6 to control metadata for describing datacast content. Content items can be grouped into IP services and IP sessions. Content items can be aUocated (or de-aUocated) time slots for transmission. Thus, the metadata describes the structure of content items as a hierarchy of IP services and IP sessions. The metadata may also include information on the transmission schedule of IP sessions and individual content items within IP sessions. The content management module 11 aUows the administrator 6 to add, replace and delete content items in the content database 12.

The service discovery server 10 generates announcements of IP services and IP sessions based on the metadata found in the ESG database 9. The announcements are sent to the datacaster 3 for transmission over the datacast network 4. The announcements may be transmitted repetitively by repeating them in carousel style or by transmitting them multiple times.

As will be explained in more detail later, two kinds of announcements are generated. A first kind of announcement describes a full IP service directory and a second kind of announcement describes updates to the IP service directory.

In one embodiment of the present invention, the second kind of announcements is used to transmit an updated session directory.

In another embodiment of the present invention, the second kind of announcements comprises identification of those parts of the service directory that have been changed. The second kind of announcements may comprise only such identification. Such second kind of announcements may be regarded as a notification of updates. The second kind of announcement comprising only notification of updates can be sent more frequently than the second kind of announcements comprising updates. The second kind of announcements may comprise both one or more notifications of updates and one or more updates, whereby the updates are selected from the set of updates available at the time of the announcement.

The content server 13 retrieves scheduUng information from the ESG database 9 and, based on the scheduUng information, retrieves content from the content database 12 and sends it to the datacaster 3 for transmission over the datacast network 4. The cUent 5 includes a datacast receiver 14, a service discovery cUent 15, an ESG database 16 for storing metadata for the electronic service guide, an ESG browser 17, a content filtering appUcation 18, a content database 19 and a content browser 20.

The datacast receiver 14 receives data over the datacast network 4 whereupon it demodulates and decapsulates the data. In this case, the datacast receiver 14 forwards the demodulated and decapsulated data to an IP stack (not shown). The demodulated and decapsulated data comprises IP packets carrying content streams or metadata describing content. The IP packets are forwarded from the stack (not shown) to IP -based appUcations 15, 18 running on the cUent 5.

■ The service discovery cUent 15 receives the IP packets on one or more given addresses and one or more given ports for carrying IP service announcements. As will be explained in more detail later, the service discovery cUent 15 can receive announcements of the first type describing the fuU service directory and, either alternatively or additionaUy, announcements of the second type describing updates to service directory. The IP packets carry metadata which can be stored in the ESG database 16 or forwarded directly to the ESG browser 17.

The ESG database 16 has an information structure very similar to the server-side ESG database 9. The ESG database 16 is initially empty, for example when the cUent 5 is first switched on, but fills up and is updated as IP session announcements are received from the datacast service system 2.

The ESG browser 17 allows the end-user 7 to view schedules and descriptions of IP services, sessions and content items available from the datacaster service system 2. The ESG browser 17 can retrieve metadata from the ESG database 16 or receive metadata directly from the service discovery cUent 15.

The content filtering appUcation 18 receives the IP packets on one or more given addresses and one or more given ports configured by the content browser 20 or other appUcations running on the cUent. The IP packets carry content which can be stored in the content database 19 or forwarded directly to the content browser 20.

The content browser 20 is loaded and run when the end-user 7 has selected a particular datacast content item for consumption. The content item can be received in real time or retrieved from the content database 19. The content browser 20 can be for example a Web browser, an MP3 player or a streaming video cUent.

The multicasting system 1 may aUow automatic content uploading by external content providers (not shown) and forwarding of Internet-based content. The datacaster 3 can also deUver content to a pluraUty of datacast networks (not shown), each datacast network comprising one or more transponders.

In an embodiment of the present invention, one or more ESG proxies (not shown) may be provided between the datacaster 3. and the cUent 4. Each ESG proxy is capable of receiving and transmitting ESG metadata or parts of ESG metadata, updates and/ or notifications of updates. Each ESG proxy can filter ESG metadata or parts thereof, including updates and notifications of updates from one or more ESG senders and output the filtered ESG metadata to one or more ESG sessions. LogicaUy, an ESG proxy fits in between ESG senders and receivers. A proxy may also cache ESG metadata or parts thereof including updates and notifications of updates and may provide its own bandwidth control or congestion control schemes on the output.

Sessions

Referring to Figure 2, content 21 is shown which is stored in the content database 12 and which includes first, second, third and fourth sessions 22_1? 22₂, 22₃, 23₄. The first, second and third sessions 22_l5 22₂, 22₃ comprise data relating to soccer. For example, the first session 22, may include text relating a game, the second session 22₂ include video streaming and the third session may include audio streaming 22₃. The fourth session 22₄ comprises data relating to hockey. A session 22_l5 22₂, 22₃, 23₄ may comprise a single IP stream or a pluraUty of IP streams. Session Directory

Referring to Figure 3, a session directory 23 is shown according to which the sessions 22_1; 22₂, 22₃, 22₄ are organised. The session directory 23 includes, at a first level, categories such as sports 24,. Further examples of categories include arts, business, computers, games, news and shopping and other categories which are commonly found on web portal sites. Each category includes, at a second level, sub-categories, such as soccer 25, arid hockey 25₂. Each sub-category may be further sub-divided. For instance, the soccer sub-category 25, can be divided into soccer leagues, each of which may be divided' into league divisions and each of which in turn may be divided into players.

Each category, sub-category or further sub-categor may include one or more sessions. For example, the soccer sub-category 25, includes the first, second and third sessions 22,, 22₂, 22₃, while the hockey sub-category category 25₂ includes the fourth session 22₄.

Referring to Figure 4, ESG data 26 is shown which is stored in the ESG database 9. The electronic service guide data 26 includes first, second, third and fourth sets of metadata 27,, 27₂, 27₃, 27₄ for describing the first, second, third and fourth sessions 22,, 22₂, 22₃, 22₄ respectively. The ESG data 26 reflects the structure of the session directory 22.

The ESG data 26 is transmitted to cUents 5 so. s to provide an ESG for users. However, there is a problem if the ESG data 26 needs to be updated, as will now be explained:

Referring to Figures 1, 2, 3 and 4, initiaUy, ESG data 26 is transmitted from the datacast service system 2 to the cUent 5. The datacast service system 2 sends sets of metadata 27,, 27₂, 27₃, 27₄ to the datacaster 3 to be transmitted to cUents 5. The cUent 5 begins to receive the sets of metadata 27,, 27₂, 27₃, 27₄ and starts to fill the initiaUy empty ESG database 16. Eventually, all the sets of metadata 27,, 27₂, 27₃, 27₄ are received and are stored in the ESG database 16. At this point, the ESG is complete. Referring to Figure 5, the content database 12 is updated and corresponding updated content 21' is shown. The updated content 21' includes an updated session 22,' and a new session 22₅. For example, the first session 22, may be updated by replacing a match preview with a match report. The new session 22₅ may be a text file with a hockey fixture Ust.

Referring to Figure 6, an updated session directory 23' is shown and includes the updated session 22,' and the new session 22_s.

Referring to Figure 7, an updated ESG data 26' is shown including an updated first sets of metadata 27',, and a new set of metadata 27₅.

Referring to Figures 1, 4, 6 and 7, the updated ESG data 26' is transmitted from the datacast service system 2 to the cUent 5. The datacast service system 2 sends the updated ESG data 26' to the datacaster 3 for transmission. The cUent 5 then receives updated sets of metadata 27,', 27₂, 27₃, 27₄, 27₅. However, the cUent 5 does not know whether each set of metadata 27,', 27₂, 27₃, 27₄, 27₅ relates to existing or updated sessions. Thus, each incoming set of metadata 27,', 27₂, 27₃, 27₄, 27₅ is compared with stored sets of metadata 27„ 27₂, 27₃, 27₄ to check whether they relate to an updated data session. Processing metadata in this way is wasteful. Furthermore, there can be delay between the first session 22, being updated and the electronic service guide at the cUent 5 being revised.

Therefore, it is desirable to provide an improved session directory and an improved ESG.

One solution to the problem is to spUt the session directory and divide transmission of the ESG accordingly. Description of the session directory is transmitted by sending two types of session announcements one for describing the fuU session directory and another for describing an updated session directory, as will now be described in more detail: Split Session Directory — First Example

Referring to Figures 8 and 9, a first embodiment of asession directory 28, 28' according to the present invention is shown before and after an update respectively.

The session directory 28, 28' is spUt into two parts at a relatively high level, in this example above the category level, and the two parts are referred to as the fuU session directory 29, and the updated session directory 29₂ respectively. Later, in a second example, a session directory is described which is spUt at a relatively low level.

The fuU session directory 29, includes substantiaUy the same categories described earUer, such as sports 24,. Each category includes sub-categories, such as soccer 25, and hockey 25₂. Similarly, there may be further sub-categories. Each category, sub- category or any further sub-category may include one or more sessions. In this case, the soccer sub-category 25, includes the first, second and third sessions 22,, 22₂, 22₃ and the hockey sub-category category 25₂ includes the fourth sessions 22₄.

The updated session directory 29₂ also includes categories which correspond to the categories in the fuU session directory, such as sports 30,. Similarly, each corresponding category includes corresponding sub-categories, such as soccer 31, and hockey 31₂. Similarly, there may be corresponding further sub-categories. Each corresponding category, corresponding sub-category or any corresponding further sub-category may include, if there has been an update, one or more updated sessions.

Before the update, the updated session directory 29₂ does not Ust any sessions.

After the update, the updated directory 29₂ Usts updated sessions. In this case, the soccer sub-category 31, includes the updated, first session 22,' and the hockey sub- category category 31₂ includes the fifth session 22₅. This configuration is used to send two types of session announcements. One type of announcement is used to describe aU sessions. Another type of announcement is used to describe updated sessions.

Thus, the cUent may Usten initiaUy to announcements of the fist type so as to receive a description of aU the sessions, i.e. the fuU session directory. Once the cUent has received the description of aU sessions, the cUent may Usten only to announcements of the second type so as to learn of any updates to the sessions.

Session Announcements using SAP and SDP

Referring to Figures 10 and 11, a first example of ESG data 32, 32' in accordance with the present invention is shown before and after the update.

The ESG data 32 includes first, second, third and fourth sets of metadata 33,, 33₂, 33₃, 33₄ for describing the first, second, third and fourth sessions 22,, 22₂, 22₃, 22₄ respectively.

The updated ESG 32' includes the updated first, second, third, fourth and fifth sets of metadata 33,', 33₂, 33₃, 33₄, 33₅ for describing the updated first, second, third, fourth and fifth sessions 22,', 22₂, 22₃, 22₄, 22₅ respectively.

A Session Announcement Protocol (SAP) is used to transmit sets of metadata 33,, 33,', 33₂, 33₃, 33₄, 33₅ to cUents 5 and a Session Description Protocol (SDP) is used to describe the sessions 22,, 22,', 22₂, 22₃, 22₄, 22₅. Reference is made to "Session Announcement Protocol" by M. P. Maher, C. Perkins & E. Whelan, RFC 2974, IETF, October 2000 and to "Session Description Protocol" by M. Handley & N. Jacobson, RFC 2327, IETF, April 1998..

The use of the Session Announcement Protocol and the Session Description Protocol advantageously permits information describing the structure of session directories to be transmitted to cUents 5. Reference is made to "Describing session directories in SDP" by R. Finlayson, Internet Draft, IETF, January 2001 and "Towards multicast session directory services" by A. Santos, J. Macedo & N. Freitas. Referring to Figure 12, an embodiment of a session announcement 34 according to the present invention is shown. The session announcement 34 comprises an SAP header 35 and payload in the form of an SDP description 36 of a session. The SDP description 36 includes a set of metadata 33 for describing a session.

Referring to Figure 13, in an embodiment of the invention, a description of the session directory 28 is transmitted by sending two types of session announcements 37,, 37₂ each describing a session directory, in this case the full session directory 29, and the updated session directory 29₂ respectively.

The first type of session announcements 37, is used to send descriptions of aU ' sessions, i.e. the full session directory 29,. During an earUer cycle 38,, the announcements 34,, 34₂, 34₃, 34₄ describe all sessions 22,, 22₂, 22₃, 22₄ before the update and, during a later cycle 38₂, the announcements 34,', 34₂, 34₃, 34₄, 34₅ describe all sessions 22,', 22₂, 22₃, 22₄, 22_s after the update.

The second type of announcements 37₂ is used only to send descriptions of sessions that have been added, removed or changed since the transmission of announcements 34,, 34₂, 34₃, 34₄ during the earUer cycle 38,. In this example, no cycle precedes the earUer cycle 38,. Thus, during the earUer cycle 38,, there are no announcements of the second type 37₂. During the later cycle 38₂, the announcements 34,', 34₅ describe updated sessions 22,', 22₅ (Figure 9).

Usually, there will be more than two cycles 38,, 38₂ of announcements.

Furthermore, more sessions may be updated. Thus, each subsequent cycle (not shown) may or may not include announcements of the second type 37₂. Optionally, announcements of the second type 37₂ may be sent repeatedly during a cycle to protect against irrecoverable transmission errors.

The structure of the session directory 28 (Figure 9) may be described using a hierarchy of multicast IP addresses using SDP and SAP. An embodiment of a process of describing the structure of the session directory 28 according to the present invention includes transmitting a first session announcement on a given multicast address. The first session announcement includes a second multicast address and other details relating to a session directory. The process includes transmitting a second session announcement on the second multicast address. The second session announcement includes a third multicast address and other details relating to a session sub-directory. Because subdirectories in turn can be used to announce a succeeding level of a session directory, the session directory hierarchy can be organized as a tree of any depth. In this example, a root or default session announcement (not shown) is transmitted on a widely known address, which specifies a pair of addresses for receiving announcements of the first and second types 37,, 37₂ respectively.

One or more "category" fields may be included in the session announcements for aUowing cUents 5 to filter and organize session announcements.

As described earUer, announcements of the first type 37, are transmitted on a first IP address, such as 224.2.17.0. . ^{' '}

Referring to Figure 13, the first session announcement 34, may include an SDP description 36 of the first session 22, including, for example:

v=0 o=j smith 2890842807 2890844525 IN IP4 10 .47 .16 . 5 c=IN IP4 224 .2 .17 . 12/127 t=2892054126 2892399688 m=data 9875 TTHTTP UDP a=cat : Pull . Sports . Soccer

If the first session announcement 34, is updated, then the updated first session announcement 34,' may include an SDP description 36 of the updated first session 22,' including, for example: v=0 osjsmith 2890842807 2890844526 IN IP4 10.47.16.5 c=IN IP4 224.2.17.12/127 t=2892054126 2892399726 m=data 9875 UHTTP UDP a=ca : Full . Sports . Soccer

Likewise, the second session announcement 34₂ may include an SDP description 36 of the second session 22₂ including, for example:

v=0 o=j smith 2890842807 2890844526 IN IP4 10.47 .16. 5 C=IN IP4 224 .2 . 17 .13/127 t=2892054126 2892399726 ' m=video 9875 RTP/AVP 31 a= cat : Full . Sports . Soccer

Announcements of the second type 37₂ are transmitted on a second IP address, such as 224.2.17,1.

Referring stiU to Figure 13, the updated first session announcement 34,' may include an SDP description 36 of the updated first session 22,' (Figure 9) including for example:

v=0 o=j smith 2890842807 2890844526 IN IP4 10 .47 .16 .5 c=IN IP4 224 .2 . 17 . 12/127 t=2892054126 2892399726 ma data 9875 UHTTP UDP a=cat : Update . Sports . Soccer

The updated session 22,' (Figure 9) may be identified as an updated session in a number of ways:

Firstly, the first session announcement 34, and the updated first session announcement 34,' specify different version numbers in the "o=" field, namely may include comparing version numbers of the first and updated first session announcements 34„ 34,' and noting different version numbers.

Secondly, the updated first session announcement 34,' is provided through a different channel, in this case a different IP address, which is reserved for announcements relating to updated sessions. Thus, identifying an updated session may include receiving an announcement on a different channel.

Thirdly, the updated first session announcement 34,' includes a category field, which identifies the fact that the session announcement relates to an update. Thus, identifying an updated session may include determining whether an announcement identifies itself as relating to an update and/or determining a position within a session directory.

Method of operating the datacast service system 2

Referring to Figures 1 and 14, an embodiment of a method of operating the datacast service system 2 according to the present invention is shown.

The ESG management module 8 identifies whether sessions have been updated in the content database 12 (step SI). If it identifies any updated sessions, then it updates corresponding sets of metadata in the ESG database (step S2). Updating may include adding or deleting metadata. Metadata is passed to the service discovery server 10, which generates updated session announcements for any updated sets of metadata (step S3). The service discovery server 10 forwards a first set of announcements describing a pluraUty of sessions, in other words fuU session announcements, and a second set of announcements describing at least one updated session, in other words updated session announcements, to the datacaster 3 through different channels, such as different IP addresses (steps S4 & S5). The datacaster 3 receives the announcements and transmits them over the datacast network 4 to each cUents 5. Method of operating client 5

Referring to Figures 1 and 15, an embodiment of a method of operating the cUent 5 according to the present invention is shown.

The cUent 5 checks whether it has received aU the session announcements of the first type 37, (step TI). If not, the cUent 5 Ustens to both types of announcements 37,, 37₂ (step T1& T2). However, if^'the cUent 5 has received all the session announcements of the first type 37,, then it can stop Ustening to announcements of the first type 37, and continue Ustening only to announcements of the second type 37₂. This has the advantage that it saves processing power and electrical power because fewer session announcements are received and/or processed.

, The first and second types of announcements 37,, 37₂ may include multicast addresses of announcements relating to other session directories, which in turn may include multicast addresses of announcements relating to further session directories.

Announcements of the first type 37, may be considered as relating to a session directory including sub-directories to a given depth of directory hierarchy. Announcements of the second type 37₂ may Ukewise be considered as relating to a session directory including sub-directories to a given depth of directory hierarchy. If announcements of either type 37,, 37₂ relate to more than one session directory, then they can be used to announce the details of a different sub-tree of the IP session hierarchy. Thus, if descriptions of multiple subdirectories are transmitted using announcements of the first type 37,, then the cUent 5 may stop receiving announcements relating to a particular subdirectory as soon as it has received aU the different session descriptions of that subdirectory.

Split Session Directory — Second Example

Referring to Figure 16, a second embodiment of asession directory 28" according to the present invention is shown. The session directory 28" is spUt into two parts at a relatively low level, in this example above the session level, and the two parts are referred to as the full session directory 29, „, 29, _b and the updated session directory 29_2a, 29_2b respectively. The ESG data 32 and the updated ESG data 32' are modified to reflect the structure of the second embodiment of a session directory 28" according to the present invention.

Session Announcements using UHTTP

A drawback of using session announcements employing SAP and SDP is that it is difficult for a cUent 5 to estabUsh when it has received enough announcements of the first type 37, to describe the fuU session directory 29,. Announcements 34,', 34₂, 34₃, 34₄, 34_s may be lost or corrupted and these protocols do not allow such events to be detected.

In an alternative embodiment, this problem is solved by Unking together session announcements describing the fuU session directory 29,.

A Unidirectional Hypertext Transfer Protocol (UHTTP) is used to implement a concatenated transfer of multiple session descriptions and references are made to the Society of Motion Picture and Television Engineers standard SMPTE 364M- 2001 "Declarative Data Essence — Unidirectional Hypertext Transport Protocol" and to "Appendix C: The Unidirectional Hypertext Transfer Protocol (UHTTP)" in Enhanced Content Specification, Advanced Television Enhancement Forum.

UHTTP supports MIME multipart/related content-type protocol, so allowing a single UHTTP transfer to comprise multiple independent MIME objects and reference is made to "The MIME Multipart/Related Content-type" by E. Levinson, RFC 2387, IETF (1998).

Referring to Figure 17, the ESG data 32 is considered as a single resource 39 which can be spUt into a pluraUty of data segments 40,, 40₂, 40₃. In this example, there are fewer data segments than there are sets of metadata. However, the number of data segments may be equal or greater than the number of sets of metadata. Redundant error correction segments (not shown) may be calculated and interleaved with the data segments 40,, 40₂, 40₃. The updated electronic service guide data 32' is processed in the same way.

Referring to Figure 18, a user datagram protocol (UDP) packet 41 is shown which includes a UDP header 42 and a UDP payload 43. The UDP payload 43 includes a UHTTP packet 44 which includes a UHTTP header 45 and a data segment 40„ 40₂, 40₃ as payload. UHTTP aUows each data segment 40,, 40₂, 40₃ to be numbered.

Referring to Figure 19, ESG data 32 is transmitted as a Unked transfer and updated ESG data 32' is also transmitted as a Unked transfer. For the ESG data 32, first, second and third UDP packets 41,, 41₂, 41₃ are transmitted. Likewise, for the updated ESG 32', fourth, fifth and sixth UDP packets 41₄, 41₅, 41₆ are transmitted. In each case, if one or more UDP packet 41„ 41₂, 41₃, 41₄, 41₅, 41₆ is unsuccessfuUy transmitted or data segment contained therein is unsuccessfuUy retrieved, then corresponding UDP packets 41„ 41₂, 41₃, 41₄, 41₅, 41_s are re-transmitted.

Descriptions of updated sessions are transmitted in a seventh UDP packet 41₇.

As described earUer, a default session announcement may be used to provide details of the full and updated session directories 29,, 29₂. An example of a default session announcement may include:

v=0 o=dcaster 4289098098 4289099125 IN IP4 130.230.3.2 s=Experimental session directory service i=Full and update session directories delivered via UHTTP u= ttp: //www. datacaster. com e=dcaster@datacaster.com c=IN IP4 224.2.17.12/127 t=2873397496 2873404696 m=data 42451 udp uhttp a=X- sessio -director -full m=data 42452 udp uhttp a=X- session-di ectory-updates In this example, the fuU session announcements and updated session announcements are provided on different port numbers. Also in this example, UHTTP is used fiύl session announcements and updated session announcements. However, UHTTP may be used for fuU session announcements, while SAP and SDP may still be used for updated session announcements.

Numbering of data segments 40,, 40₂, 40₃ aUows the cUent 5 to detect when they have received the ESG data 32. Once this occurs, the cUent 5 Ustens for updates.

The use of UHTTP has another advantage. It supports forward error correction

(FEC) which can be used to increase the probability of successful transmission even if bit and burst errors occur in transmission. If, however, FEC fails to recover any errors at the cUent-end, the cUent 5 waits for periodic UHTTP retransmission. Alternatively, if a return path is provided, then automatic repeat request (ARQ) may be used.

Other protocols which provide reUable deUvery of content may be used.

Asynchronous Layer Coding (ALC), or a protocol based on ALC, provides reUable deUvery of content and can be used to deUver full or partial ESG metadata, updates and notification of updates.

Asynchronous Layer Coding (ALC) is a scalable reUable content deUvery protocol for IP multicasting and reference is made to "Asynchronous Layer Coding protocol instantiation" by M. Luby, J. GemmeU, L. Nicisano, L. Rizzo and J. Crowcroft, RFC 3450, IETF, April 2002 and December 2002.

Reference is also made to "ReUable Multicast Transport Building Blocks for One- to-Many Bulk-Data Transfer" by B. Whetten, L. Nicisano, R. Kermode, M. Handley, S. Floyd and M. Luby, RFC 3048, IETF, January 2001. Reference is also made to "Layered Coding Transport building block" by B. Whetten, L. Nicisano, L. Rizzo, M. Handley, S. Floyd and M. Luby, Internet Draft, IETF, February 2002.

Session announcements, updates and notifications of updates using an AUZ-based protocol

ALC provides a unidirectional transport service for binary objects, such as files. ALC is based on the Layered Coding Transport (LCT) reUable multicast protocol building block and so inherits the LCT concept of sessions comprising one or more layered channels.

An ALC/LCT session comprises of a set of logicaUy grouped channels associated , with a single sender carrying packets with ALC/LCT headers for one or more objects. For the deUvery of a fuU or partial ESG, updates and notifications of the updates, a protocol based on the ALC protocol may be used. Thus, an ESG session can be defined which comprises one or more ESG channels. Each ESG channel corresponds to an ALC session.

As explained earUer, an ALC session comprises one or more ALC channels. Each ALC channel may be thought of as "bit pipe" for forwarding packets according to the ALC protocol. In preparation for an ALC session, a sender selects a number of ALC channels and chooses corresponding bitrates for each of them. Each recipient of the ALC session can control the receiving bitrate by selecting to receive either aU ALC channels or only some of them.

An ALC channel is uniquely definable and recognizable by a pair of variables (S, G). S is an IP unicast address of the sender and G is a multicast IP address for a multicast receiving group. G may also be a unicast IP address, but RFC 3450 does not define the use of unicast.

An ALC session is uniquely definable and recognizable by a pair of variable (S, TSI). S is the unicast IP address of the sender and TSI is the value of the Transport Session Identifier field in the header of each ALC packet 47 (Figure 21). Using ALC or an ALC-based protocol, an ESG session may be defined which comprises at least one ESG channel. Preferably, the ESG session comprises three ESG channels: one channel for deUvering a full or partial ESG, one channel for deUvering updates and one channel to notify of updates.

Each respective ESG channel carries data packets having the same value in the Transport Session Identifier (TSI) field. Data packets in the same channel are sent from the same source port and IP address, and may be addressed to a different destination port and/or IP address.

An ESG session may include a full ESG channel, an ESG update channel and an ESG notify channel.

The full ESG channel repeatedly deUvers an ESG metadata set representing the sender's full or partial ESG metadata set. When only a partial ESG is deUvered, cUents may be provided access to the fuU ESG via a different protocol, such as a point-to-point ESG transport protocol.

The update ESG channel repeatedly deUvers an ESG metadata set comprising parts of the sender's ESG that have changed since the current version of the full ESG was assembled.

The notify ESG channel repeatedly deUvers a metadata set consisting of pointers to parts of the sender's ESG that have changed since the most recent version of the fuU ESG was constructed. The pointers are data fields within the metadata set, which identify parts that have changed.

Each of the ESG channels in turn may comprise one or more ALC channels. All ALC channels which constitute an ESG channel are sent on consecutive IP addresses. Only a base IP address used for each ESG channel needs to be signalled to the receivers. This is because a "Next flag" in a Congestion Control Indication field enables receivers to discover the foUowing IP addresses that may have been used for the current ESG channel.

ESG receivers with interactive network connection are able to join and leave transport channels, depending on the type of ESG metadata they need to receive and on the congestion status of the network. ESG receivers that only have unidirectional network connectivity are more restricted, but still have the option of filtering out unnecessary transport channels. Furthermore, a network element such as the ESG proxy (not shown) can reduce the number of transport channels forwarded to a unidirectional Unk, for example when congestion is detected at the feed of such a Unk.

' Referring to Figure 20, when sets of metadata 33,', 33₂, 33₃, 33₄, 33₅ (Figure 11) for an updated ESG 32' is prepared, a set of metadata 45 for notification of updates is also prepared. The metadata set 45 comprises pointers to any updated sessions 22,', 22_s (Figure 9). OptionaUy, metadata set 45 may be divided into a pluraUty of data segments 46,, 46₂. ;

Referring to Figure 21, a packet 47 for deUvering metadata sets or data segments is shown. Preferably, the packet 47 is substantially similar to a UDP or ALC packet and may comprise one or more headers, one or more payload data fields and other data fields. A standard header format, such as a UDP header, may be used.

In this example, an ALC packet 47 is described which comprises a UDP header 48, an LCT header 49, an FEC payload ID field 50 and payload 51 which includes at least metadata sets 33, 45 or data segments 46,, 46₂.

Between them, the headers, preferably the LCT header 49, may comprise a number of fields (not shown) including a version number field and a number of flags (not shown) including a congestion control flag, a transport session identifier flag, a transport object identifier flag, a half-word flag, a sender current time present flag, an expected residual time present flag, a close session flag and a close object flag. Further data fields may be included which are reserved for future use. The transport session identifier flag identifies the field format used for the transport session identifier. The transport object identifier flag indicates the field format used for transport object identifier. The sender current time present flag indicates the presence or absence of a sender current time field. The expected residual time present flag indicates the presence or absence of an expected residual time field. The close session flag indicates the ending of the session and the close object flag indicates the ending of the transmission of the object.

Between them, the headers, preferably the LCT header 49, comprise a number of fields (not shown) indicating the length of one or more of the headers and/ or of the packet, a number of fields (not shown) with information relating to congestion control and one or more fields (not shown) including one or more identifiers for identifying the transport session and the transport object.

Further data fields (not shown) may carry information, for example relating to ALC encoding symbols and to possible header extensions. The further data fields (not shown) may include information on a Forward Error Correction (FEC) scheme used. FEC data is redundant information generated from, and interleaved with, payload data. The use of FEC aUows receivers to reconstruct payload data segments lost or damaged due to transmission errors.

Between them, the headers, preferably the FEC payload ID field 50, includes a source block number (not shown) and an encoding symbol ID (not shown).

The source block number indicates from which source block of the object the encoding symbol(s) in the payload 51 is (are) generated. The encoding symbol ID identifies which specific encoding symbol(s) generated from the source block is (are) are carried in the payload 51.

When a protocol based on the ALC is used, an ALC Protocol Instantiation specific header extension (not shown) is included at least once in the deUvery of each transport object. An FEC Object Transmission Information in the header extension enables receivers to discover, in-band, the FEC parameters used to deUver the associated transport object.

A header extension (not shown) comprises one or more fields such as a type of the header extension, a length of the header extension, an identification for the FEC encoder being used, a transfer length of the object, a source block length of every source block of the current transport object carried in the packet payloads, a length of every encoding symbol of the current transport object carried in packet payloads. In addition the header extension may comprise one or more fields reserved for future use.

The information in the congestion control field may, comprise an indication flag, a ■ sequence number the value of which is increased by one for each packet sent, wherein it can be used by receivers to detect packet loss and a part reserved for future use.

When the indication flag is set to 'V, it indicates that the current ALC session consists of two or more ALC channels including the current IP address and the next consecutive IP address. A value of '0' in this field indicates that the current IP address is the highest IP address in the current ALC session. Receivers may monitor this field to detect dynamic addition or deletion of ALC channels by ESG senders.

Further details relating to ALC packet format can be found in "Asynchronous Layer Coding protocol instantiation", RFC 3450, ibid.

In an embodiment of the present invention, the announcements can be regarded as binary objects and thus be called transport objects. Each transport object is identified by the value of a transport object identifier field (not shown), which is unique within the scope of one transport session. Each ESG metadata set is preferably sent as a separate transport object. For each transport object, additional information may be defined in the form of an ESG deUvery table (not shown). On the sender side, ESG deUvery table can be inserted in every transport session. On the receiver-side, ESG deUvery table information parsing can be provided.

For each type of transport channel in a transport session, a different deUvery table can be transmitted.

The ESG deUvery table (not shown) may be defined as a set of mappings, each comprising a transport object identifier value and the properties of the transport object. The ESG deUvery table may comprise two parts: an ESG header and an ESG payload.

The ESG deUvery table header comprises fields for header extension type, header extension length, ESG deUvery table version and ESG deUvery table expiry.

The ESG header extension is a variable length protocol instantiation specific header extension and it is included in aU packets carrying ESG deUvery table and it is identical for aU packets carrying the same version of the ESG deUvery table. The ESG deUvery table version is the number of the currently transmitted ESG deUvery table. This field has the value of '0' for the ESG deUvery table of a new ALC transport session, and is increased by one whenever an updated ESG deUvery table is constructed for the same ALC transport session. After reaching its predefined maximum value, the version number wraps back to '0'. The ESG deUvery table expiry is a time value, indicating the time after which the ESG deUvery table is not expected to be vaUd. A new version of the ESG deUvery table is preferably sent before the expiry time of the current version. However, receivers should continue using the current version of the ESG deUvery table even after its expiry time if they have not received- a newer' version.

The ESG payload contains the actual mappings between transport object identifiers and the attributes associated with the transport object identified by each transport object identifier. The ESG payload format may be an XML structure represented as ASCII text, comprising one or more fields such as e.g. a unique identifier for the transport object within the current ALC transport session, a URL for uniquely identifying the current transport object, the length in bytes of the transport object, the MIME type of the transport object, an identifier for the encoding used for the transport object, such as ZLIB compression and an MD5 checksum for the transport object. The ESG payload fields may use the syntax and semantics of the corresponding fields defined in the HTTP 1.1 specification.

Referring to Figure 23, deUvery of a full ESG as a first set of announcements 37,, deUvery of updated ESG as a second set of announcements 37₂ and deUvery of notifications of the updates as a third set of announcements 37₃ are shown.

• As explained earUer, announcements comprising notification of updates can be sent more frequently than announcements comprising updates.

The datacast cUent 5 can choose to Usten to announcements comprising notifications of updates in preference to announcements comprising updates. If they receive a notification of an update to a session in which the user may be interested, then the datacast cUent 5 can Usten to announcements comprising updates and/or obtain a description of the session in another way, such by unicast.

Time Division Multiplexing

In the embodiments described earUer, IP packets comprising portions of ESG data 32, 32' can be transmitted by the datacaster 3 to the cUent 5 as-and-when transmission slots become available. However, to ensure that the cUent 5 receives the IP packets, it is preferable that the cUent 5 be configured to receive data at any time. This has the drawback of unnecessarily using processing and electrical power.

A solution to this problem is to employ time division multiplexing (TDM).

Referring to Figure 23, an alternative manner of transmitting a description of the session directory 28 in accordance with the present invention is shown. In this example, only a later cycle 38₂' including the two types of session announcements 37„ 37₂ is shown.

Announcements of the first type 37, for describing ESG data and announcements of the second type 37₂ for describing updates to ESG data are transmitted in different time slots 52,, 52₂. For example, the announcements of the first and second types 37,, 37₂ are transmitted in alternate time slots. However, the time slots 52,, 52₂ need not be adjacent. The time slots may be of variable or fixed length.

Thus, if the cUent 5 wishes to Usten for updates to ESG data, then they do not need to Usten to time slots 52, during which announcements of the first type 37, are transmitted, but may Usten to time slots 52₂ during which only announcements' of the second type 37₂ are sent. This allows the client 5 to switch off its receiver 14 (Figure 1) during time slots 52,. The ESG data contains information when the receiver of the cUent needs to be turned on or off and/ or when the content is on the air within service area defined by datacast operator

Datacast client 5 Referring to Figure 24, an embodiment of a datacast cUent 5 according to the present invention comprises a processor 53, input/output interface 54, memory 55, a receiver 56 and a transceiver 57 which are connected via a bus 58. The input/ output interface 54 is connected to a user interface 59, a display 60, storage 61 and speaker 62.

The datacast cUent 5 may be a handheld mobile communication device for use with first and second wireless communication networks in accordance with the present invention. For example, the first wireless communication network may be a DNB-T or DAB network, or any modification of these or similar networks, and the receiver 56 may be configured to receive and demodulate signals from such a network. The second wireless communication network may be a UMTS network or other 3G, 2.5G or 2G telecommunications network and the transceiver 57 may be configured to receive/transmit and demodulate/modulate signals via a UMTS or sirnilar network.

The datacast cUent 5 may be a set-top box connected to a television set for use with first and second wired and/or wireless communication networks. For example, the first communication network may be a DNB-T network and the receiver 56 may be configured to receive and demodulate signals from a DNB-T network. The second communications network may the Internet and the transceiver 57 may include a modem (not shown) for connection via a pubUc switched telephone network to an Internet Service Provider

Using two networks, sessions and session announcements may be transmitted over ^• different networks. Alternatively, the first and second types of session announcements may be transmitted over different networks.

When two networks are available, the user of the cUent device may control the deUvery of fuU or partial ESG metadata, updates thereof and notifications of updates by using a request-response model, wherein the requests are transmitted to the datacast service system through the second communications network. In the communication between the cUent and the datacast service acknowledgements may be used if required. The cUent may make a request for notifications using the second communications network, wherein selected notifications are transmitted to the cUent when such notifications are created.

Computer programs (not shown) when loaded into memory 55 and run by the processor 53 cause the processor 53, in conjunction with other elements of the device, to provide the service discovery cUent 15, the ESG browser 17, the content filtering appUcation 18 and the content browser 20 respectively. Storage 61 is used to hold the ESG and content databases 16, 19. The user interface 59 allows the user to provide instructions to the ESG browser 17 and the content browser 20, such as instruction to select a session. The display 60 allows the user to view session descriptions and session content. The speaker 62 aUows the user to hear session content. ESG browser

Referring to Figure 25, an example of an ESG browser window 63 is shown. The window 63 includes a first section 64 for receiving instructions for filtering sessions, for example on the basis of date of transmission, whether a session is current being transmitted or search terms. The window 63 includes a second section 65 for displaying a Ust of filtered sessions and receiving instructions to select a session. The window 63 includes a third section 66 for displaying a description of a selected session and receiving instructions to access the session.

It will be appreciated that many modifications may be made to the embodiment hereinbefore described.

Session announcements may be unicast, rather than multicast, to a cUent.

Sessions and session announcements may be transmitted over different networks. For example, sessions may be transmitted over a DNB network and session announcements may be sent via a DAB network.

The first and second types of session announcements may be transmitted over different networks. For instance, announcements of the first type may be transmitted through a DNB-T network, while announcements of the second type may be sent though a 3G network. The first and second types of session announcements may be transmitted through the same network, but over one or more different physical channels, for example at different carrier frequencies. The first and second types of session announcements may be transmitted through the same network and over the same physical channels, but over one or more different logical channels.

From reading the present disclosure, other variations and modifications will be apparent to persons skilled in the art. Such variations and modifications may involve equivalent and other features which are already made in design, manufacture and use of systems for announcing sessions and component parts thereof and which may be used instead of or in addition to features already described herein.

Although claims have been formulated in this appUcation to particular combinations of features, it should be understood that the scope of disclosure of the present invention also includes any number of features or any other combinations of features disclosed herein either impUcitly or expUcitly or any generaUsation thereof, whether or not it relates to the same invention as presently claimed in any claim or whether or not it mitigates any or aU of the scientific problems as does the present invention. The appUcants hereby give notice that new claims may be formulated to such features and/or combination of features during the prosecution of the present appUcation or any further appUcation derived therefrom.

Claims

1. A method of announcing sessions transmitted through a network, the method comprising: providing a first set of announcements describing a pluraUty of sessions; and providing a second set of announcements describing at least one updated session.

2. A method according to claim 1 , comprising providing said first set of announcements through a first channel and providing said second set of announcements through a second, different channel.

3. A method according to claim 1 or 2, wherein providing said first set of " announcements and providing said second set of announcements comprises providing said first set of announcements through a first address and providing said second set of announcements through a second, different address respectively.

4. A method according to any preceding claim, wherein providing said first set of announcements and providing said second set of announcements comprises providing said first set of announcements through a first destination address and providing said second set of announcements through a second, different destination address respectively.

-5. A method according to any preceding claim, wherein providing said first set of announcements and providing said second set of announcements comprises providing said first set of announcements through a first IP address and providing said second set of announcements through a second, different IP address respectively.

6. A method according to any claim, wherein providing said first set of announcements and providing said second set of announcements comprises providing said first set of announcements through a first IP multicast address and providing said second set of announcements through a second, different IP multicast address respectively.

7. A method according to any preceding claim, wherein providing said first set of announcements and providing said second set of announcements comprises providing said first set of announcements through a first port number and providing said second set of announcements through a second, different port number respectively.

8. A method according to any claim, wherein providing said first set of announcements and providing said second set of announcements comprises providing said first set of announcements through a, first logical channel and providing said second set of announcements through a second, different logical channel respectively.

9. A method according to any preceding claim, wherein providing said first set of announcements and providing said second set of announcements comprises including in each announcement of said first set of announcements data for identifying said announcement as an announcement which describes a one of said pluraUty of sessions and in each announcement of said second set of announcements data for identifying said announcement as an announcement which describes a one of said at least one updated session.

10. A method according to any preceding claim, wherein providing said first set of announcements and providing said second set of announcements comprises including in each announcement of said first set of announcements respective data for specifying a position of a corresponding session within a first portion of a session directory and including in each announcement of said second set of announcements respective data for specifying a position of a corresponding session within a second portion of the session directory.

11. A method according to any preceding claim, wherein providing said first set of announcements and providing said second set of announcements comprises providing said first set of announcements through a first physical channel and providing said second set of announcements through a second, different physical channel respectively.

12. A method according to any preceding claim, wherein providing said first set of announcements and providing said second set of announcements comprises providing said first set of announcements through a first network and providing said second set of announcements through a second, different network respectively.

13. A method according to any preceding claim, further comprising providing a third set of announcements describing another pluraUty of sessions including said at least one updated session.

_*

14. A method according to any preceding claim, comprising: providing said first set of announcements through a first channel; providing said second set of announcements describing at least one updated session through a second, different channel; and providing a third set of announcements describing another pluraUty of sessions including said at least one updated session through said first channel.

15. A method according to any preceding claim, comprising arranging the providing of said second set of announcements after the providing of said first set of announcements.

16. A method according to any preceding claim, wherein providing said first set of announcements and providing said second set of announcements comprises transmitting said first set of announcements through a first channel and transmitting said second set of announcements through a second, different channel.

17. A method according to any preceding claim, comprising transmitting said first set of announcements according to a session announcement protocol (SAP).

18. A method according to any preceding claim, comprising transmitting said first set of announcements according to a unidirectional transport protocol.

19. A method according to any preceding claim, comprising transmitting said first set of announcements according to unidirectional hypertext transfer protocol (UHTTP).

20. A method according to any preceding claim, comprising transmitting said first set of announcements according to asynchronous layered coding (ALC) protocol.

21. A method according to any preceding claim, comprising transmitting said first set of announcements according to user datagram protocol (UDP).

22. A method according to any preceding claim, comprising including a description of a corresponding session in each announcement.

23., A method according to any preceding claim, comprising including a description of a corresponding session arranged according to session description protocol (SDP) in each announcement.

24. A method according to any preceding claim, comprising providing means for determining whether aU of said first set of announcements have been provided.

25. A method according to any preceding claim, comprising providing said first set of announcements as a series of Unked messages.

26. A method according to any preceding claim, comprising providing said first set of announcements in a first set of time slots and providing said second set of announcements in a second set of time slots, each timeslot of said first set of timeslots being provided at a different time from each timeslot of said second set of timeslots.

27. A method according to any preceding claim, comprising multiplexing said first and second sets of announcements.

28. A method according to any preceding claim, further comprising providing a third set of announcements identifying said at least one updated session.

29. A method according to any preceding claim, wherein providing the second set of announcements describing the at least one updated session comprises providing a set of announcements identifying the at least one updated session.

30. A method according to any preceding claim, wherein providing the second set of announcements describing the at least one updated session further comprises including a description of a corresponding session. ^»

31. A method according to any preceding claim, wherein providing the second set of announcements describing the at least one updated session comprises providing a set of notifications pointing to the at least one updated session.

32. A method of announcing sessions transmitted through a network, the method comprising: providing a first set of announcements describing a pluraUty of sessions; and providing a second set of announcements identifying at least one updated session.

33. A method according to claim 32, further providing a third set of announcements describing said at least one updated session.

34. A method according to any preceding claim, comprising transmitting at least one of said sets of announcements according to asynchronous layered coding (ALC) protocol.

35. A method according to any preceding claim, comprising transmitting at least one of said sets of announcements according to a protocol based on asynchronous layered coding (ALC) protocol.

36. A method according to any preceding claim, comprising defining an asynchronous layered coding (ALC) protocol session and defining at least one ALC channel.

37. A method according to claim 36, comprising transmitting a set of metadata for describing the pluraUty of sessions via a first ALC channel.

38. A method according to claim 36 or 37, comprising transmitting a set of

^■ metadata for describing at least one updated session via a second, different ALC channel.

39. A method according to claim 36, 37 or 38, comprising transmitting a set of metadata for identifying said at least one updated session via a third, different ALC channel.

40. A method according to any one of claim 33 to 39, comprising transmitting a one set of metadata as a transport object.

41. A method according to claim 40, further comprising, defining a respective deUvery table relating to the transport object and transmitting said deUvery table.

42. A computer program which, when executed by data processing apparatus, causes the data processing apparatus to perform a method of announcing sessions transmitted through a network according to any preceding claim.

43. A method of accessing sessions transmitted through a network, the method comprising: selectively receiving a first set of announcements describing a pluraUty of sessions; and selectively receiving a second set of announcements describing at least one updated session.

44. A method according to claim 43, further comprising determining whether aU of said first set of announcements have been received.

45. A method according to claim 44, further comprising selecting not to receive further said first set of announcements and selecting to receive said second set of announcements .

46. A method according to claim 43 or 44, further comprising selecting not to receive a third set of announcements describing another pluraUty of sessions including said at least one updated session. "

Al. A method according to any one of claims 43 to 46, further comprising selecting to receive a fourth set of announcements describing at least one further updated session.

48. A method according to any one of claims 43 to 47, comprising using said second set of announcements to identify said at least one updated session.

49. A method according to claim 48, comprising selecting to receive another set of announcements including a description of said at least one updated session.

50. A method according to claim 49, comprising obtaining a description of said at least one updated session.

51. A method of accessing sessions transmitted through a network, the method comprising: selectively receiving a first set of announcements describing a pluraUty of sessions; and selectively receiving a second set of announcements identifying at least one updated session.

52. A method according to claim 51 further comprising selectively receiving a third set of announcements describing said at least one updated session.

53. A method of accessing sessions transmitted through a network, the method comprising:

Ustening to a first set of announcements describing a pluraUty of sessions; determining whether said first set of announcements have been received; if said first set of announcements have been received, then stopping Ustening to said first set of announcements and Ustening to a second set of announcements describing at least one updated ^■ session.

54. A method according to claim 53, further comprising: stopping Ustening to a third set of announcements describing a further pluraUty of sessions including said at least one updated session.

55. Apparatus for announcing sessions transmitted through a network, the apparatus comprising: means for providing a first set of announcements describing a pluraUty of sessions; and means for providing a second set of announcements describing at least one updated session.

56. Apparatus for performing the method according to any one of claims 1 to 41.

57. Apparatus for announcing sessions transmitted through a network, the apparatus comprising: a first transmitter for providing a first set of announcements describing a pluraUty of sessions; and a second transmitter for providing a second set of announcements describing at least one updated session.

58. Apparatus for accessing sessions transmitted through a network, the apparatus comprising: means for selectively receiving a first set of announcements describing a pluraUty of sessions; and means for selectively receiving a second set of announcements describing at least one updated session.

59. Apparatus according to claim 58, comprising: means for determining whether said first set of announcements has been received; said apparatus being configured such that if said determining means determines that said first set of announcements has been received, then the trieans for selectively receiving said second set of announcements is configured to receive said second set of announcements.

60. Apparatus according to claim 59, comprising: means for selectively receiving a third set of announcements describing another pluraUty of session including said at least one updated session; said apparatus being configured such that if said determining means determines that said first set of announcements has been received, then the means for selectively receiving said third set of announcements is configured not to receive or not to forward said third set of announcements.

61. Apparatus according to any one of claims 58 to 60 which is a mobile communications device.

62. A system for presenting program schedule data on a display, said system comprising at least two announcements, the schedule data being organized at least partly from a first set of announcements describing at least partly a pluraUty of sessions and at least partly from a second set of announcements describing at least one at least partly updated session.

63. A system for presenting program schedule data on a display, said system comprising at least two announcements, the schedule data being organized at least partly from a first set of repeatable announcements describing a pluraUty of sessions, at least partly from a second set of repeatable announcements describing at least one at least partly updated session and at least session descriptions of at least one of the repeatable announcements for defining whether the at least one of the first and second announcements is received or not.

64. A system for deUvering program schedule data to end-user terminals, said system comprising two sets of announcements, each set comprising at least one announcement, the schedule data being organized at least partly from a first set of announcements describing at least partly a pluraUty of sessions and at least partly . from a second set of announcements describing at least one at least partly updated session.

65. A system for presenting program schedule data to end-user terminals, said

' system comprising at least two set of announcements, each set comprising at least one announcement, the schedule data being organized at least partly from a first set of repeatable announcements describing a pluraUty of sessions, at least partly from a second set of repeatable announcements describing at least one at least partly updated session and at least session descriptions of at least one of the repeatable announcements for defining whether the at least one of the first and second announcements is received or not.

66. A system according to any one of claims 63 to 65 claim, wherein the second set of announcements include a version number of each updated session for aUowing a cUent to detect if they have missed an earUer update.

67. A system according to claim 66, wherein if a cUent detects it has missed an earUer update and is not currently receiving the first set of announcements, the cUent starts receiving the first set of announcements until it has received a fuU and latest version of the program schedule data.

68. A system according to claim 67, wherein if the cUent detects that it has received a fuU and latest version of the program schedule data, it stops receiving the first set of announcements and continues receiving only the second set of announcements .

69. A system according to any one of claims 66 to 68, wherein if the cUent detects it has missed an earUer update, it fetches a full and latest version of the program schedule data over an interactive network.

70. A system according to any one of claims 66 to 69, where each set of repeatable announcements is divided into segments before transmission and a location of each segment within a whole transfer is indicated in a framing field of each respective , segment; the indicated location enables cUents to determine whether they have received all segments that constitute a given set or whether they need to wait for receiving more segments.

71. A system according to any one of claim 66 to 70, wherein the program schedule data is viewed either directly by a human end-user or automaticaUy used by a software appUcation.

72. A system according to any one of claims 66 to 71, wherein the program schedule data is presented progressively to a human end-user or made progressively available to an automatic software appUcation as the said data is being received.

13. A system according to claims 71 or 72, wherein the program schedule data is viewed by a human end-user via a graphical user interface.

74. A system according to claims 71 or 72, wherein the program schedule data is used by a personal video recorder.