The invoke component

Invoke component             Mandatory indication     Code
Component Type Tag           Mandatory                1 0 1 0 0 0 0 1
Component Length 1)

Invoke ID Tag                Mandatory                0 0 0 0 0 0 1 0
Invoke ID length
Invoke ID

Linked ID Tag a)             Optional                 1 0 0 0 0 0 0 0
Linked ID length
Linked ID

Operation Code Tag           Mandatory          local:0 0 0 0 0 0 1 0
                                               global:0 0 0 0 0 1 1 0
Operation Code length
Operation Code
Parameters  2) 3)            Optional

The component description in ASN.1:

Invoke          ::=     SEQUENCE {invokeID, InvokeID Type,
                                  linkedID [0] IMPLICIT InvokeID Type OPTIONAL,
                                  operationCode OPERATION,
                                  parameter ANY DEFINED BY operationCode OPTIONAL }

                                  -- ANY is filled by the single ASN.1 data
                                  -- type following the key word PARAMETER in
                                  -- the type definition of a particular
                                  -- operation.

The length of a Component ID is 1 octet. An Invoke Component has one or two Component IDs: an Invoke ID and if it is desired to associate the Invoke with a previous Invoke, then the second or Linked ID is provided in addition to the Invoke ID. Return Result and Return Error Components have one Component ID, called an Invoke ID which is the reflection of the Invoke ID of the Invoke Component to which they are responding. The Reject Component uses as its Invoke ID, the Invoke ID in the component being rejected. If this ID is unavailable (e.g. due to mutilation of the message undetected by lower layers), then the Invoke ID Tag is replaced with a universal Null Tag ( lways has length = 0) as shown in Table 12).
Table 12/Q.763 - Coding of Null Tag


H
G
F
E
D
C
B
A
Null Tag
0
0
0
0
0
1
0
1
        If an Invoke containing both Invoke and Linked IDs is being rejected, only the Invoke ID is used in the Reject Component.

5)      Operation Code Tag
        Each operation is assigned a value to identify it. Operations can be classified as local or global operations.
        A local operation code follows an Operation Code Tag and Operation Code Length. The Operation Code Tag is coded as shown in Table 13.
        The Global Operation Code is coded as an Object Identifier, which is described in Recommendation X.209.

6)      Parameter Tag
        The Parameter Tag shall be any valid ASN.1 Tag, depending on the type of the parameter supplied. It can indicate either a primitive or a constructor element and refer to any of the defined tag classes.
        When the parameter element is a collection of several information elements, the associated data types shall be derived from the Sequence, SequenceOf, Set or SetOf types.
        The Sequence and Set Tags are coded as shown in the following:
   Sequence Tag    0 0 1 1 0 0 0 0
   Set Tag         0 0 1 1 0 0 0 1