1、Rec. 807 1RECOMMENDATION 807REFERENCE MODEL FOR DATA BROADCASTING(Question 29/11)(1992)Rec. 807The CCIR,consideringa) that data broadcasting services are growing rapidly in number and diversity of application;b) that the media used to convey data broadcasting services now covers the whole gamut of b
2、roadcast emissionbearers from LF radio through to SHF satellite systems;c) that the reducing cost of computing equipment makes the development of complex receiving terminalseconomically viable;d) that there will be an increasing need to interconnect broadcast transmission and reception equipment toc
3、omputer networks and other data broadcasting equipments;e) that a method needs to be found to ease standardization and facilitate smooth evolution of data broadcastingsystems in the future;f) that it is desirable to encourage the development of compatible interface specifications to rationalize the
4、designof equipment,recommends1. that the layered approach of the ISO Open Systems Interconnection (OSI) basic reference model as describedin ISO 7498 (1984) should be used in the development of any new data broadcasting system;2. that the OSI basic reference model should be interpreted in the broadc
5、asting context, according to Annex 1.ANNEX 1Interpretation of the ISO OSI basic reference modelin the broadcasting context1. IntroductionThe study and development of systems and services of data broadcasting are active in various areas such as thetelevision and sound channels in terrestrial and sate
6、llite broadcasting.In order to ease standardization of data broadcasting systems and to facilitate smooth evolution of databroadcasting services in the future, the development of a common reference model for data broadcasting is necessary.In particular, the use of such a reference model would facili
7、tate the description and introduction of integratedservices digital broadcasting (ISDB) which could include Teletext, still pictures, audio signals, high fidelity audio,facsimile, data and other types of information.2. DefinitionData broadcasting: the broadcasting of coded information intended to be
8、 received by the general public bymeans of appropriate data processing equipment.2 Rec. 8073. A layered model for data broadcastingA hierarchical organization of communication functions for data broadcasting is presented in Table 1 wherethe functional items, listed at each hierarchical level, do not
9、 refer to specific implementation solutions, but to the overalllogical features that are considered sufficient to characterize the service and performance of any typical system.According to this functional model, services may be delivered by arranging the information into logicalgroupings, deliverin
10、g them to lower layers for transmission and, after reception, recovering the information in the properform for use by the recipient.In what follows, the names of the layers are those adopted by the ISO in ISO 7498 (1984) “Basic referencemodel for open systems interconnection”.*TABLE 1Layer structure
11、 of data broadcastingOSI basicreference modelData broadcastingLayer Principal function Classification7 Application Use of information atapplication level6 Presentation Conversion and presentationof informationService informationprotocol5 Session Selection of and accessto information4 Transport Ident
12、ification of groupof data3 Network Identification of logicalchannelData broadcasting system2 Data link Linkage with logicaltransmission unit1 Physical Physical transmissionBroadcasting networks are basically uni-directional. Even where new interactive services are introduced, thereverse path normall
13、y uses a different type of network. This situation is included within the OSI 7-layer model by theconcept of “connectionless” operation. In typical telecommunications, the connectionless class of transmission normallyrefers to a virtual uni-directional protocol, where physical bi-directional data pa
14、ths exist, but are only used in onedirection (an example is the X.25 “datagram”). However, the concept equally covers a physically uni-directionalsituation. In both cases a prior agreement is required as to the purpose and significance of the data transmission. In thedata broadcasting case this agre
15、ement must be set up via other means of communication, although it will frequently beimplicit, such as in the purpose of equipment sold to a user._*The term “network” is used in this Annex in the telecommunications sense of a set of interconnected links sharing the sameprotocol. This is distinguishe
16、d from the use of “network” in the conventional broadcasting sense of set of transmittersbroadcasting the same programme material.Rec. 807 3Layer 1: PhysicalWithin a given broadcast transmission system this layer relates to the electrical transmission of the data signaland includes such items as car
17、rier or sub-carrier frequency modulation method, bit rate and pulse shaping. It may alsospecify the physical details of connectors.Layer 2: Data linkThis layer, which is concerned with an individual link, includes logical functions related to the datatransmission such as digital frame synchronizatio
18、n techniques and associated error control procedures, data formatting,and link-access procedures.Layer 3: NetworkThis layer includes logical functions related to multiplexing, demultiplexing and error control of data packetsbelonging to different communication flows. Examples of such functions are d
19、ata channel addressing and data packetsequencing. It is relevant to the passage of data through a given network. This normally involves data passing throughseveral links of the network.A network in a broadcasting application often consists of a single star. Every link in such a network starts atthe
20、transmitter and ends in a receiver. Thus the process involved when data passes through the network is the same asdata passing over a single link. Where the receiver is a cable-head, the cable operator may choose to distribute data in aform different from that broadcast (such as to extract sub-title
21、data from Teletext and superimpose the sub-titles onto thevision). This means that the tasks performed by Layers 2 and 3 apply to a single transmission path in some cases and inothers to a more complex topology. This does not mean that the two layers cannot be unambiguously distinguished, butit is s
22、ignificant that attempts to apply the OSI model to data broadcasting have sometimes led to difficulties. In practice,the single-star topology of a broadcasting network may make the distinction relatively unimportant.Layer 4: TransportThis layer provides the function of arranging the data in a way su
23、itable for secure transfer from one point toanother, by such means as segmenting data into groups of information (e.g. scrambling where applicable), deliveringthem to the lower layers for transmission to the distant point and there reconstituting the groups of information andarranging them in a prop
24、er sequence. It is the lowest layer which by definition has end-to-end significance, that is, thetransport unit appears intact from entering to leaving the network.Layer 4 may have to take account of some features of the networks it uses. It should not, however, be specificto any particular type of
25、network.In data broadcasting, the network will be uni-directional. Thus the Layer 4 protocol must either operate in theconnectionless mode, or use a different type of network for a “reverse path”. Also, data broadcasting applications areoften implemented by terminals which cover all layers up to 7,
26、the application layer, in one suite of equipment. In thissituation the interfaces to the transport layer can appear unimportant.However, in the definition of data broadcasting protocols the distinction between Layers 3 and 4 is important.Typically, data will be recovered from the broadcasting networ
27、k as the contents of frames or packets. Layer 3 endswhere such framing ceases to have significance. Only the contents of the frames, or packets, are passed to Layer 4.Although this distinction may be theoretical in a complete terminal which implements all layers up toapplication, it becomes vital if
28、 data distributed via a broadcasting network are intended for onward transmission througha different type of network.Layer 5: SessionThis layer includes data handling functions which are intended to assist the user to gain access to services.Examples of such functions are access control and service
29、identification (SI).4 Rec. 807As broadcasting protocols become more complex, multiple services can be carried over the same network, andservices can include additional or optional components. This makes it desirable to provide service identificationinformation within the broadcast and raises issues
30、connected with the OSI 7-layer model which may often be theoretical,but in some situations could be important. For instance, Layers 4 and 5 should not be specific to any particular type ofnetwork, and thus should not in principle perhaps appear in a broadcasting specification. In practice, tradition
31、albroadcasting terminals often implement all layers up to application.Layer 6: PresentationThis layer comprises the functions needed for the presentation of information relevant to each applicationwhich could include text, pictures, sound and other types of processable data.Layer 7: ApplicationThis layer refers to practical use of the potential facilities provided by the lower layers for a given type ofservice. Examples are captioning, telesoftware, cyclic Teletext, stock market data, telemusic, etc._
