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ASN.1 & BER

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ASN.1 Purpose of ASN.1 One of the fundamental problems confronting users communicating with different systems is the efficient transfer of data in such a way that the data received is the same data transmitted. In the OSI model, the representation of data types and structures to facilitate this transfer is a function of the Application Layer; the encoding of the data into a specific sequence of bits for transfer isattributed to the Presentation Layer.

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ASN.1 The fundamental unit of ASN.1 is the module. The sole purpose of a module is to name a collection of type definitions and/or value definitions (assignments) that constitute a data specification.

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ASN.1 A type definition is used to define and name a new type by means of a type assignment and a value definition is used to define and name a specific value, when it is necessary, by means of a value assignment.

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ASN.1 - Type Assignment A type assignment consists of a type reference (the name of the type), the character sequence ::= (‘‘is defined as’’), and the appropriate type.

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InventoryList {1 2 0 0 6 1} DEFINITIONS ::= InventoryList {1 2 0 0 6 1} DEFINITIONS ::= BEGIN { ItemId ::= SEQUENCE { partnumber IA5String, quantity INTEGER, wholesaleprice REAL, saleprice REAL } StoreLocation ::= ENUMERATED { Baltimore (0), Philadelphia (1), Washington (2) } } END

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ASN.1 - Value assignment A value assignment consists of a value reference (the name of the value), the type of the value, ::= (``is assigned the value''), and a valid value notation. A value reference must begin with a lower case letter, but otherwise has the same syntax as a type assignment.

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ASN.1 - Value assignment example gadget ItemId ::= { partnumber "7685B2", quantity 73, wholesaleprice 13.50, saleprice 24.95 }

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ASN.1 – Simple Types

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ASN.1 – Character String Type

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ASN.1 – Structured types

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ASN.1 – Structured types example { "American", "1106", { 320, 107, 213 }, { "BWI", "LAX" }, 10 } 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 ENUMERATED { six (6), eight (8), ten (10) }, cancel BOOLEAN DEFAULT FALSE }

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ASN.1 – Tagged Type TAGGED is used to enable the receiving system to correctly decode values from several datatypes that a protocol determines may be transmitted at any given time. TAGGED has no value notation of its own. Its type notation consists of three elements: a user-defined tag, possibly followed by IMPLICIT or EXPLICIT, followed by the value notation of the type being tagged.

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ASN.1 – Tagged The user-defined tag consists of a class and class number contained in braces. Class is UNIVERSAL, APPLICATION, PRIVATE, CONTEXT-SPECIFIC. The UNIVERSAL class is restricted to the ASN.1 built-in types. It defines an application-independent data type that must be distinguishable from all other data types. The other three classes are user defined. The APPLICATION class distinguishes data types that have a wide, scattered use within a particular presentation context. PRIVATE distinguishes data types within a particular organization or country. CONTEXT-SPECIFIC distinguishes members of a sequence or set, the alternatives of a CHOICE, or universally tagged set members. Only the class number appears in braces for this data type; the term COONTEXT-SPECIFIC does not appear.

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ASN.1 – Tagged

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ASN.1 – Tagged As we indicated in the above discussion of type SET, the representation in a) is invalid in ASN.1 because its instances can be ambiguous. The tagging in representations b), c), and d) overcome the problem and allow instances to be transmitted uniquely. IMPLICIT in c) indicates that an original tag is replaced by any of the three user-defined tags. EXPLICIT tagging would be appropriate when strong-type-checking is more important than compact representation; it can be used when the original tag is accompanied by a user-defined tag. The context-specific tagging in d) is similar to the APPLICATION class tagging in b) except that the class of the tag is not specifically transmitted.