Section: A.3.6 Fields with conditional presence
Content: A field with conditional presence is declared with the keyword OPTIONAL. In addition, a short comment text shall be included at the end of the paragraph including the keyword OPTIONAL. The comment text includes the keyword "Cond", followed by a condition tag associated with the field ("UL" in this example): -- /example/ ASN1START LogicalChannelConfig ::= SEQUENCE { ul-SpecificParameters SEQUENCE { priority INTEGER (0), ... } OPTIONAL -- Cond UL } -- ASN1STOP When conditionally present fields are included in an ASN.1 clause, the field description table after the ASN.1 clause shall be followed by a conditional presence table. The conditional presence table specifies the conditions for including the fields with conditional presence in the particular ASN.1 clause. The conditional presence table has two columns. The first column (heading: "Conditional presence") contains the condition tag (in italic font style), which links the fields with a condition tag in the ASN.1 clause to an entry in the table. The second column (heading: "Explanation") contains a text specification of the conditions and requirements for the presence of the field. The second column may also include semantics, in case of an optional presence of the field, under certain conditions i.e. using the same predefined tags as defined for optional fields in A.3.5. Conditional presence should primarily be used when presence of a field depends on the presence and/or value of other fields within the same message. If the presence of a field depends on whether another feature/function has been configured, while this function can be configured independently e.g. by another message and/or at another point in time, the relation is best reflected by means of a statement in the field description table. If the ASN.1 clause does not include any fields with conditional presence, the conditional presence table shall not be included. Whenever a field is only applicable in specific cases e.g. TDD, use of conditional presence should be considered.