Interactive Services in Digital Television Infrastructure

Interactive services in digital television infrastructure

ZORAN BOJKOVIC(1), DRAGORAD MILOVANOVIC(2)

(1)Faculty of Transport and Traffic Engineering, University of Belgrade, Vojvode Stepe 305, 11000 Belgrade (2) Faculty of Electrical Engineering, University of Belgrade, Bulevar Revolucije 73, 11120 Belgrade, Serbia and Montenegro

Abstract: This paper concentrates on interactive services in digital television infrastructure. After an discussion concerning digital television infrastructure for interactive multimedia services, we invoke interactive broadcast data services including data carousel. We then describe digital video broadcast (DVB) with return channel over satellite. The issues for DVB terrestrial return channel system (DVB- RTC) are also analyzed. The network architecture for the analyzed broadcasting networks is presented, too. Key-words: digital television, DVB, return channel, interactive services.

1 Introduction users to the service center. The return channel often Digital television (DTV) technology appears supports the impulse buying of pay-per-view and commercially today in hybrid digital-analog systems, NVOD events. The return channel is usually such as digital satellite and cable systems and it implemented via a narrowband communication link serves as the default delivery mechanism for high- such as a public switched telephone network (PSTN), definition TV (HDTV). All digital sets can display or an integrated service digital network (ISDN). Cable higher resolution of digital format and do not require systems with two-way enabled plants can implement a additional external conversion equipment. The digital return channel over the cable infrastructure. Since both format for TV broadcast transport has several key cable and satellite DTV systems use the same data advantages over analog transport. For service transport mechanism and protocols (MPEG-2 TS) the operators, the key benefit is the high transport physical nature of the underlying distribution network efficiency - digital compression packs five or more is transparent to data services. times as many channels in a given distribution - Data broadband technology enables DTV service network bandwidth. This in turn increases the providers to enhance their customers television operator's revenue potential by delivering more viewing experience by providing a wide range of content and pay-per-view events, including near interactive services as an incremental add-on, to the video-on-demand (NVOD) movies with multiple, DTV broadcast infrastructure. Depending on the closely spaced start times. End-users have a larger underlying digital video broadcast system program selection spaced start times. End-users have infrastructure, the following classes of interactive a larger program selection with CD-quality audio and services are possible: better picture quality potential even when viewed in a  broadcast-only interactive services, hybrid setup with analog TV sets [1].  broadcast with a batch return channel, From the data delivery point of view, the DTV  broadcast with an on line return channel. infrastructure provides a broadband digital Table 1 summarizes types of broadcast-based distribution network, data transport protocols, and services a DTV infrastructure with different return digital terminals (set-top decoders) on the user's channel options can deploy. Note that user-level premises. As such it provides a powerful platform for capability and interactivity is a function of the delivering information and data services that not only network and connectivity infrastructure. An enrich but fundamentally transform the television important challenge facing DTV service designers viewing experience. DTV systems always provide a lies in dividing data services that operate in the most one-way broadcast path for distributing digital video. common, broadcast-only environment and scale up in Optionally a return communication link can be user-level functions with the increased capability of provided to allow the "upstream" flow of data from the underlying DTV infrastructure, when available. 2 Interactive broadcast data services data services (carousel) are multiplexed MPEG-2 Interactive broadcast data services can be broadly transport stream. From the management and categorized as follows [2]: distribution points of view, data service files can be  Interactive data broadcast (IDB) services. Primarily handled in exactly the same manner as any other data-only channels, with optional background audio. DTV stored content (such as NVOD). This lowers When a user selects such a service, a data only screen system acquisition and operation costs for interactive displays. The user may use hot-spot or hyperline-style services because the familiar service center mechanisms. equipment and procedures for TV signal delivery,  Interactive video broadcast (IVB) services. Combine such as the scheduler and NVOD server, also video and data channels to provide an enhanced TV distribute data services. viewing experience. Service delivery and user Directory experience of combined video-data services can be Application Code further categorized in terms of their temporal Repeat for Downloadable Resources carousel relationship. effect Root Object . Unsynchronized video and data. Video and data may be Data Object typically related or unrelated. Common examples include a simple interactive icon overlay, a partial data Data Object overlay such as a ticket display, or a data screen with a Data Object video (broadcast) insert. . Synchronized video and data. In this mode, video and Figure 1. Structural layout of a data carousel. data are both typically related and authorized to be For more detailed information on these topics, the synchronized at playback. references [4, 5] are recommended as research papers Many DTV systems use an intermittent batch return dealing with some theoretical concepts of data broadcast. channel for billing of impulse pay-per-view (IPPV) events. In this approach, the return channel is usually 4 DVB with return channel via satellite implemented using a telephone line with a low- Increased interactivity is a general tendency for speed, dial-up modem. At specified intervals, the telecommunication services today, an aspect which is control billing system polls individual set-top boxes also reflected in the more traditional distribution to retrieve the accumulated information. Optionally services like radio and television. Customers want to the set-top box might dial up the central system when choose, sort, order, store and manipulate what they its local event storage almost fills up or the IPPV receive on their terminal, and ideally also interact viewing credit nearly runs out. from the same terminal. The distribution network becomes an asymmetric interactive network, with a 3 Data carousel possible evolution towards fully symmetric To support low end set top boxes with small amounts communication. This convergence between of application memory and no additional storage communication and broadcasting leads to an capacity, the data streams are broadcast cyclically on evolution from broadcasting to multicasting or point- the DTV network. In effect, the DTV network acts as to-multipoint communication, where the difference a large serial disk for storage. This approach gives lies in the possibility to offer contents/services rise to what is known as the data carousel [3]. The designed for individuals or groups of people with approach allows clients with a local catching restricted access and billing. This evolution will also capability to find the necessary data and code on the have consequences for satellite communications, network at any time, with the worst-case access certainly the most broadcast-oriented medium of all [6]. latency equal to the carousel cycle duration. Fig. 1 shows the basic layout of a data carousel. The There are several ways to design a return channel for carousel data stream consists of an indexing and satellite multicast services, and many believe naming mechanism to locate objects within the data terrestrial return channels to be the most cost carousel, application code to download to the effective and practical. Commonly proposed receiver when the user tunes into the carousel data terrestrial return channels are PSTN, ISDN or GSM. channel, and application data objects that the user However, there is a large worldwide interest for a terminal retrieves at runtime in response of the user's definition of a return channel via satellite, and there interactive requests. are several reasons for that. Firstly, as mentioned A DTV network provides an ideal platform for distributing above, the ”normal” consumer does not want to be data broadcasts in a carousel fashion. The interactive bothered by technical set-ups with interconnections between TV, PC and telephone. A solution where all One of the main challenges for the DVB-RCS group the technical equipment is concentrated within one has been to specify inexpensive terminals. The CR box, and without having to fear blocked telephone indicates ex-factory prices on the order of lines etc. will certainly be appealing for many people. 1000Euros, 3000Euros and 50.000Euros for Another reason to choose satellite services is the Consumer, Prosumer and Corporate terminals increased traffic in the terrestrial networks, which respectively. Cost limitations will also imply EIRP often results in blocking or reduced quality of limitations and possible use of suboptimal service. The instantly available capacity on a satellite components such as nonlinear amplifiers. As soon as link can with efficient resource allocation for interactive services are considered, the delay instance be set to 2Mb/s. A 100Mbyte file will need becomes a matter of concern with implications on about 7 minutes for transfer over satellite whereas the several levels ranging from synchronisation, log-on time required over a 64kb/s terrestrial line will be algorithms to the delay perceived by the user after about 3½ hours. Finally, there is an advantage, both having made a request. This aspect highlights the for the users and the operators, that both channels are need for efficient transport mechanisms, a need to be on the same medium. This enables a better control balanced against the contradicting need for with the QoS and the network management, the flexibility. The NCC is in charge of the network terrestrial infrastructure is often not controlled by the control, which will include several RCSTs, but same operator as for satellite, and this is certainly not maybe also several satellites, feeders, gateways and the case when national boarders are crossed. even several networks. The RCST network to Due to the recognised need for a specification of a manage is a multipoint-to-point structure, far more return channel via satellite, the DVB-TM (Digital complex to administrate than the opposite, the point- Video Broadcasting - Technical Module) created an to-multipoint. Ad-hoc group early 1999, called the DVB-RCS The NCC is thus in charge of the control of every (DVB- Return Channel via Satellite). The DVB RCST in the network as well as the network as a project itself was launched in 1993, under the whole. A terminal will log on after having received auspices of the EBU (European Broadcasting Union) general information by ”listening” to the forward [1]. The motivation was to promote a common, link. The information given there is on the status of standard, European platform for digital TV the network, and most important, the forward link broadcasting, and the idea was supported by all provides the Network Clock Reference (NCR). When players; broadcasters, operators, standardisation the RCST has obtained synchronisation with the bodies, media groups and industry. NCR, it will use one of the designated slots Figure 2 gives the general DVB return channel (indicated in the forward channel) for log-on request reference model. In this model, the interactive in a slotted-aloha manner. If the terminal is network is depicted as independent from the forward successful with this request, the NCC will forward channel. Very often, however, the forward various tables containing general network and interaction channel, or forward signalling channel, is terminal specific information. The specific integrated in the forward transport stream (TS). This information is about necessary frequency, timing and is also the case in the DVB-RCS specification, where power level corrections to be performed by the the forward signalling is part of the DVB-S TS. terminal before transmission start. These tables will Figure 3a shows a simplified diagram of a network also indicate the resources allocated for the terminal, architecture. Actually, the DVB-RCS reference and it is possible to ask for different services or model is far more complex than this, however, the increased capacity during transmission. The NCC has wish to indicate all possible network realisations may the possibility, with certain intervals, to correct the obscure the simplicity of the concept. Usually, transmission parameters of the RCST, and if several RCSTs will be connected to the interactive something goes wrong during transmission, the NCC satellite network, consisting also of the satellite, an shall also have the possibility to force the log-off of earth station and a network control centre (NCC). In the RCST. The continuous signalling from the NCC figure 2, the earth station antenna acts both as a is provided according to MPEG-2 SI. feeder for the forward path and as a gateway for the The DVB-RCS specification is also restricted to the return path. The NCC shall handle the indoor unit, i.e. the signal processing between the synchronisation, give correction messages to the source encoder and the IF conversion. The DVB- terminals and allocate resources. RCS physical layer contains specification of timeslots and frames organised in super-frames. The sequencing is controlled by means of the NCR, the Stations. To allow access by multiple users, the access method is MF-TDMA (Multiple Frequency VHF/UHF radio frequency return channel is Time Division Multiple Access). Otherwise, the partitioned both in the frequency and time domains, specification contains energy dispersion, two types of using frequency division (FD) and time division channel codes (concatenated RS/convolutional (TD). A global synchronization signal, required for coding and Turbo-codes), prefix emplacement, the correct operation of the upstream demodulator at Nyquist filtering and QPSK modulation, most of the Base Station, is transmitted to all users via global which is well known from the DVB-S specification. DVB-T timing signals. Time synchronization signals Many satellite operators have shown their interest in are conveyed to all users through the broadcast the return channel via satellite technology, and channel, either within the MPEG2 Transport Stream concrete plans for operation of such services in the or via global DVB-T timing signals. More precisely, near future exist. The prosumer market has been the DVB-RCT frequency synchronization is derived evaluated to have a potential market of some millions from the broadcast DVB-T signal whilst the time only in Europe, and as soon as higher volumes of synchronization results from the use of MAC terminals are produced, reasonable prices for the management packets conveyed through the broadcast consumer market will be reached. channel. The DVB-RCT system follows the following rules: 5 DVB-T with return channel system  each authorized RCTT transmits one or several low bit The DVB-RCT system is able to provide interactive rate modulated carriers towards the Base Station (INA), service for Terrestrial Digital TV, using the existing  the carriers are frequency-locked and power ranged and infrastructure already used to broadcast DVB-T the timing of the modulation is synchronized by the Base services. The Terrestrial Return Channel system Station (INA), (DVB-RCT) is based on In-Band (IB) downstream  on the INA side, the Upstream signal is demodulated, signalling. Accordingly, the Forward Information using a FFT process, just like the one performed in a path data are embedded into the MPEG-2 TS packet, DVB-T receiver. themselves carried in the DVB-T broadcast channel. For correct operation of the demodulator at the base The Forward Information path is made up of MPEG- station, the carriers modulated by each RCTT shall 2 TS packets having a specific PID and carrying the be synchronized both in the frequency and time Medium Access Control management data. The domains. The frequency tolerance for any carrier Return Interaction path is mainly made up of ATM produced by a RCTT, in regard to its nominal value, cells mapped onto physical bursts. ATM cells include depends on the transmission mode used (i.e. the Application data messages and Medium Access inter-carrier spacing). Control management data. The MAC messages control the access of the RCTTs to the shared 6 Concluding remarks medium [7]. From the data delivery point of view, the digital TV infrastructure provides a broadband digital The interactive system consists of a forward distribution network, data transport protocols and interaction channel (downstream) which is based digital terminals (set-top decoders) on the user upon an MPEG-2 Transport Stream conveyed to the premises. As such, it provides a powerful platform user via a DVB-T compliant terrestrial broadcast for delivery of information and data services that can network, and a return interaction channel based on a not only enrich but fundamentally transform the TV VHF/UHF transmission (upstream). A typical DVB- viewing experience. The data broadcast addition fir RCT system is illustrated in figure 3b. interactive service delivery provides the necessary The downstream transmission from the Base Station movement to elevate the digital video broadcast (INA) to the RCTTs (NIUs) provides network infrastructure into a large-scale, high-speed synchronization and information to all RCTTs. That broadcast-based network computing platform via allows RCTTs to synchronously access the network cost-effective manner. The real proven and a truly and then to transmit upstream synchronized new dimension lies in the combined or converged information to the Base Station. RCTTs can use the medium, when the interactive services are combined same antenna used for reception of the broadcast with and related to the television programming. channel signal. The return channel signal may either Practical deployment of data broadcast have be transmitted directly to a Base Station co-located demonstrated that a surprisingly rich variety of with the broadcast transmitter site, or to a Base information services can be effectively delivered. Station included in a cellular network of Base Many satellite operators have shown their interest in [3] A.Acharya, M.Franklin and S.Zdomi, Prefetching the return channel via satellite technology, and from a broadcast disk, Proc. Int. Conf. Data concrete plans for generation of such services in the Engineering, IEEE CS Press, pp.276-285, 1996. near future exist. It has been evaluated that a [4] T.E.Browen et al., The data cycle architecture, potential market of some millions only in Europe Commun. ACM, vol.30, pp.71-81, Dec. 1992. exists, too. As soon as higher volumes of terminals [5] T.Imielinski, S.Viswanathan and B.R.Badrinath, are produced, reasonable prices for the consumer Data on air: organization and access, IEEE Trans. market will be reached. Knowledge Data Eng., vol.9, pp.353-372, 1997. References: [6] V.Paxal, DVB with return channel via satellite, [1] K.R.Rao, Z.Bojkovic and D.Milovanovic, Telenor, AS 05.04.2000. Multimedia communication systems: Techniques, [7] ETSI 301 958, DVB-RCT interaction channel for standards and networks (Prentice-Hall, 2002.) terrestrial networks, 2003. [2] M.Milenkovic, Delivering interactive services via a digital TV infrastructure, IEEE Multimedia, pp.34-43, Oct.-Dec. 1998.

Table 1. Interactive data services as a function of underlying infrastructure capability [2]. Digital broadcast (one to many) downstream Point-to-point One-way plant (satellite or cable) Two-way plant (cable only) Polled return Rela-time return No return Cable return (real-time) (PSTN) (PSTN) Broadcast Switched Broadcast Broadcast Broadcast Network Phone return, Two-way FTTC One-way Polled return Two-way HFC dial-up ATM One-way (user Interactivity Local Two-way Two-way Two-way response) Browse Browse + Browse + User level Browse + (view Real-time Real-time Full service function Batch transaction interactively) transaction transactions HFC Hybrid Fiber-Coax FTTC: Fiber To The Curb ATM: Asynchronous transfer mode

Figure 2. General reference model of DVB for interactive networks [6].

a) b) Figure 3. Simplified diagram for network architecture for DVB-RCS [6] and DVB-RCT systems [7].