Those “T” values

• All ASN.1 types are assigned a default “tag”(for the “T” part of the encoding)

• Sometimes use of the default tag would produce ambiguous encodings, for example:

or

Identification ::= CHOICE {social-sec-no INTEGER, works-no INTEGER}

Research-division ::= CHOICE {product-based NULL, basic NULL}

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If every encoding has a “T” part, what is the numerical code for each type to go into that “T” part?

In ASN.1 these IDs are called tags, and are default values are assigned to every ASN.1 primitive type (and to the construction mechanisms SEQUENCE and SET).

If a field in the encoding is (for example) a single TLV representing a value of:

CHOICE

{first-choice BOOOLEAN,

second-choice INTEGER}

then the default tag for the BOOLEAN and the INTEGER type can be used in the encoding, and we have no problem.

If however, as is illustrated in the slide, the CHOICE involves types which have the same default tag then use of that tag in both cases would produce ambiguous encodings.

The case of INTEGER and NULL is illustrated, but a more common case would be where both alternatives are SEQUENCE types.