ASN.1 Types.

ASN.1 introduces standard mechanizm of building up complex types. Simple types are used as atoms to buid more complex structured types. And as mentioned before, there are some useful biult-in types.

Simple types

Simple types are intended to be a ultimate basis for any complex type design. They comprise types for representing numerical information (integer, real), finite sets of values (null, boolean, enumerated), transparent data (bit string, octet string), string of characters from various character sets and object indentifiers. Each type has both type and value notation, in which simple type keyword consist of one or two words spelling out the name in English. So here are they:

• INTEGER for any numerical whole-number information
• REAL that funny numbers with mantissa and exponent
• NULL very special type, has only one value which even means (freely) nothing
• BOOLEAN better than null, has two values (wow!) used for logic - true and false
• ENUMERATED much more better than null - give as many values as you want to
• BIT STRING forget all that stupid bytes and words, make your special one
• OCTET STRING but sometimes, bytes are also useful, but I can't remember what is it for
• OBJECT IDENTIFIER for that annoing numbers from ISO

The type notation for each character string type is a predefined type reference as for the useful types. These types simply spell out the name of character set in English:

• NumericString contain just numbers 0-9 and single space
• PrintableString letters a-z, A-Z, numbers 0-9, space and characters '"()+,-./:=? ufff
• TeletexString (or T61String) by CCITT T.61, still haven't got one?
• VideotexString (or ISO646String) by CCITT T.100 and T.101
• VisibleString only visible characters of IA5 (ASCII international version)
• IA5String complete set of IA5
• GraphicString ISO registered graphic characters and unregistered space
• GeneralString registered graphic and control characters plus space and delete

Here you can find some examples about using character string types.

Structured types

Structured types are based on simple types or any subtype derived from them. The term for a single entity of a structure is component. The number of possible structured types is infinite, althoug there is just small finite set of operations to construct them. There are structured types constructed by simple collecting components of the same or different types, these collection can be ordered or not. Then there are structured types constructed by choosing from finite set of components. And, last but not least, special type representing any type. Here are the keywords, as usual spelling out their function in English:

• SET - a collection of distinct type components
• SEQUENCE - ordered collection of distinct type components
• SET OF - like SET, but components are of the same type
• SEQUENCE OF - similarly to SET OF, ordered collection of components of the same type
• CHOICE - a collection of alternatives (no bigamy...)
• ANY - to describe a hole in the specification, any type can be substituted
• TAGGED - how to assign a type an unique identifier

Useful types

Useful types are those which can be normally described by ASN.1 syntax, but they are already defined and available to users because of their general usefulness.

• Generalized time - textual form of date and time: "YYYYMMDDHHMMSS.SSS"
• UTCTime - universal time "YYMMDDHHMM+HHMM" to express time differential
• EXTERNAL - to embed a value from another abstract syntax
• Objectdescriptor - textual object description