Octet-Level Encoding of ASN.1

In ASN.1 all objects actually transmitted are encoded as octets or sequences of octets (see #fn79#1584>. There is a standard encoding schema which allows for extensions, and permits length- delimited and typecast objects to be constructed.

All objects are finally represented as an ordered sequence of OCTETS.
Objects are length delimited - a 'counted' sequence of octets.
New types of object can be defined out of the 'raw' defined objects.

The representation of Octets and Octet-Sequences should always conform to the ISO/CCITT standard to ensure ordering and identification of Octets and Bits within Octets remains correct. This is simply defined as:

#figure1587#
Figure: Bit/Octet Ordering in Concrete Ordering

Octet #tex2html_wrap_inline4222# is to the left of Octet i
Bit #tex2html_wrap_inline4224# is to the left of bit j
;SPM_quot;first;SPM_quot; and ;SPM_quot;last;SPM_quot; refer to leftmost or rightmost depending on whether bits or octets are being referred to.
;SPM_quot;LSB;SPM_quot; and ;SPM_quot;MSB;SPM_quot; refer to least and most significant BIT respectively

The Data Element The basic unit of an ASN.1 specification is the data element or element for short. This is a variable length object but always has three fields (each of variable length): verbatim57

The identifier determines the interpretation of the contents.
The contents may itself be a data element or combination of elements defined by context or specification.
The size and encoding of each of these 'fields' depends on circumstances as laid out below. the Identifier Field
This Field is one or more octets in length, and identifies the 'type' of element. This in turn governs the interpretation of the contents field.
The field consists of a number of sub-fields, bit- sequences which encode various attributes of the identifier and by extension its contents part.
Class (two bits) defining the 'scope' of the identifier within this and other specifications.
Form (one bit) defining the 'nature' of the contents, simple or complex.
Code (upwards of 5 bits) encoding the 'identity' of the identifier, the (possibly context- dependent) mark that allows it's recognition. The context of the Code is determined by the Class.