1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 : TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU-!.G13-!#().%G0G0,!.5!%4(%G0G0-%4!G13,!.5!%G0G0 it represents, within asyntax diagram, another syntax diagram by name. Hence it is an abbreviated symbol for a more complex construct(consisting of a set of ter
2、minal and/or non-terminal symbols) used in several places.2.3 Annotation symbols (see 3.7) are used to insert references to descriptive or explanatory notes. For example,they may be used to indicate mutually exclusive paths through a diagram.3 Rules3.1 Every symbol box (terminal or non-terminal) and
3、 consequently each diagram must have one, and one only,entry and one, and one only, exit flowline.3.2 Each diagram must fit on a single page. Thus there is no off-page connector symbol.3.3 Flowlines are always unidirectional. The preferred direction for flowlines which select alternatives is down.Th
4、e preferred direction for flowlines which connect symbols is left-to-right. The preferred direction for flowlineswhich indicate repetitions (loops) is counterclockwise.3.4 An arrowhead is required wherever any two flowlines come together, and wherever a flowline enters a symbolbox. Additional arrowh
5、eads may be inserted wherever it is felt that this will improve the clarity of the diagram.3.5 Terminal symbols are surrounded by boxes with rounded corners. The width of the box is proportional to thenumber of characters contained in the box. For short terminal symbols, the box may become a circle.
6、 Symbolsrepresenting system input are surrounded by a single solid line and those representing system output by a double solidline: for terminal input symbols see Figure 1a)/Z.302 and Figure 1b)/Z.302, for terminal output symbols see Figure 1c)/Z.302 and Figure 1d)/Z.302._1)The syntax diagrams used
7、in MML are based on those used to describe the programming language PASCAL 1.2 Fascicle X.7 - Rec. Z.3023.6 Non-terminal symbols are surrounded by rectangular boxes. The name of the non-terminal symbol must bewritten in lower case characters. Every non-terminal symbol must have an associated syntax
8、diagram except where thesymbol is annotated “Not further expanded in diagram form”. The non-terminal symbol used to name a particularsyntax diagram must appear at the upper left corner of the diagram. Symbols representing system input are surroundedby a single solid line, those representing system o
9、utput by a double solid line and symbols representing a combinationof input and output by an outer solid and an inner dashed line:a) for the non-terminal input symbol see Figure 1e)/Z.302,b) for the non-terminal output symbol see Figure 1f)/Z.302,c) for the non-terminal input/output symbol used in d
10、ialogue procedures see Figure 1g)/Z.302.3.7 An annotation is denoted by the following symbol:where n is a number referring to a descriptive or explanatory note. The text of the note must be located at the foot ofthe diagram.Reference1 JENSEN (K.), WIRTH (N.): PASCAL, User Manual and Report, Springer Verlag, New York, 1975.
