Abstract Syntax Notation

Purpose
Built-in Types
- Simple types
  - INTEGER
  - BIT STRING
  - OCTET STRING
  - NULL
  - OBJECT IDENTIFIER
  - REAL
  - ENUMERATED
  - CHARACTER STRING
- Structured types
  - SEQUENCE (think array)
  - SEQUENCE OF (think record)
  - SET (think set)
  - SET OF (think set, all elements same type)
  - CHOICE
  - SELECTION
  - ANY
Classes and Tags
- UNIVERSAL
- APPLICATION
- PRIVATE
- CONTEXT SPECIFIC
Useful Types
- GeneralizedTime (three forms)
- UTCTime (like GeneralizedTime but only to a minute or second)

Some Examples

Integer

Used as integer is used in any programming language. Optionally, a name may be associated with some particular integer values.
For example

   ColorType ::= INTEGER	
      {
        red    (0)
        white  (1)
        blue   (2)
      }

This defines a type called ColorType. Items of type ColorType can take integer values. The names red, white, and blue are recognized as meaning 0, 1, and 2 respectively.

An ordered sequence of bit values, written as either a binary or hexadecimal string. A name may be associated with the individual bit positions. Think of it as an organized collection of boolean values.
For example:

  StateLocations ::= BITSTRING
      {alaska       (0)
       alabama      (1)
       colorado     (2)
       connecticut  (3)
       delaware     (4)
        ...
       }

Bit string lengths must be divisible by 4 to be written as a string of hexadecimal characters, so this states example would have to be padded with two extra positions. A company with branches in colorado and connecticut only would be described by a variable of type StateLocations having value '3000000000000'H or '001100000......"B

Octet String

An ordered sequence of zero or more octets. Similar to bitstring, but always in blocks of eight bits. Individual bits are not named. Values are represented either as binary or hexadecimal strings.
For example:

  badCharacter ::= OCTETSTRING

Object Identifier

A special type that indicates an information object that is a node on an object identifier tree managed by international standards organizations. The value of an item of type OBJECTIDENTIFIER is a string of numbers describing branches in this naming tree.

Enumerated

Enumerated types allow values that are specific integers. This slight modification of the INTEGER example illustrates:

   ColorType ::= ENUMERATED	
      {
        red    (0)
        white  (1)
        blue   (2)
      }

Unlike in the INTEGER example, variables declared to be of this ColorType can only take the values 0, 1, 2. The values red, white, and blue are equivalent to the numeric values.

Character String

Several different Character String types are available, including

NumericString
PrintableString
IA5String (The 8-bit ASCII code)

Sequence

The SEQUENCE type is an ordered list of zero of more component types. This is the type called "record" in some languages, and called "struct" in c.
For example:

  AirlineFlight ::= SEQUENCE
    {
     airline    IA5String,
     flight     NumericString,
     seats      SEQUENCE
                  {
                   maximum  INTEGER,
                   occupied INTEGER,
	           vacant   INTEGER,
                  },
    airport     SEQUENCE
                  {
                   origin           IA5String,
                   stop1       [0]  IA5String   OPTIONAL,
                   stop2       [1]  IA5String   OPTIONAL,
                   destination      IA5String 
                  },
   crewsize ENUMEATED
              {
                six    (6),
                eight  (8),
                ten    (10)
              },
   cancel   BOOLEAN    DEFAULT  FALSE
   }


 Sequence Of

Set
Set of

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Lillian N. (Boots) Cassel

Department of Computing Sciences

Villanova University

Villanova, PA  19085-1699

+1 610 519 7341

cassel@vill.edu