Element Structure

From a structural point of view, a TCAP message is a collection of data elements. Each element takes the form of Identifier, Length, and Contents. The TCAP element is the basic building block for constructing a message.

The TCAP element is constructed with a commonly used data encoding scheme, which is often referred to as TLV: Tag, Length, Value format. The identifier specifies the type of element so that the receiving node can interpret its contents correctly. The length is the number of bytes in the element contents, beginning with the first byte past the element length. The contents are the actual data payload being transmitted.

Element Identifier

The Element Identifier is one or more octets comprised of bit fields that creates the class, form, and tag. Tables 10-5 and 10-6 list the values for the class and form. Bit H is the most significant bit.

Table 10-5. Class Values

Class	Bit Value Bits (HG)	Definition
Universal	00	Universal
Application-wide	01	International TCAP
Context-specific	10	Context Specific
Private Use	11	National TCAP/Private TCAP

Table 10-6. Form Bit

Form	Bit F
Primitive	0
Constructor	1

The class defines the identifier's scope or context. The universal class is used for identifiers that are defined in X.690 and do not depend on the application. Application-wide is used for international standardized TCAP. Context-specific identifiers are defined within the application for a limited context, such as a particular ASN.1 sequence. Private Use identifiers can be defined within a private network. These identifiers vary in scope and can represent elements within a national network, such as ANSI, or can be defined within a smaller private network. The tag bits (bits A-E) help to further determine whether the element is national or private. For more information, see "Identifier Tag" in this section.

Constructors and Primitives

The Form bit (Bit F) indicates whether the value is a primitive or constructor, as listed in Table 10-6. A primitive is simply an atomic value.

A constructor can contain one or more elements, thereby creating a nested structure. For example, a Component Tag is a constructor because a component is made up of several elements, such as the Invoke ID and Operation Code.

Identifier Tag

Bits A-E of the element identifier uniquely identify the element within a given class. If all bits are set to 1, this is a special indicator, which specifies that the identifier is octet-extended. In this case, one or more octets follow with additional identifier bits. This format allows the protocol to scale in order to handle a potentially large number of identifiers. If Bit H in the extension octet is set to 1, the identifier is octet-extended to another octet. If it is set to 0, it indicates the identifier's last octet. In the following table, the identifier is extended to three octets using the extension mechanism. As previously noted, the identifier is further discriminated based on the tag bits. When coded as class Private Use, bits A-E are used for national TCAP. If bits A-E are all coded to 1, the G bit in the first extension octet (X13 in the example below) indicates whether it is private or national. The G bit is set to 0 for national or to 1 for private.

Table 10-7. Class Encoding Mechanism

H	G	F	E	D	C	B	A
CLASS		0	1	1	1	1	1	First Octet
1	X13	X12	X11	X10	X9	X8	X7	Second Octet
0	X6	X5	X4	X3	X2	X1	X0	Third Octet

An example illustrates how class, form, and tag are used to create a TCAP element. Figure 10-11 shows an ITU Begin message type in its binary form as it is transmitted on the signaling link. Bit A represents the least significant bit. The ITU Q.773 specification defines the ASN.1 description in the following manner:

Example 10-2. ASN.1 Definition for ITU Begin Message

MessageType ::= Choice {unidirectional [APPLICATION 1] IMPLICIT Unidirectional,

                        Begin          [APPLICATION 2] IMPLICIT Begin,

The message type is defined with a class of Application-wide and a tag of 2. It is a constructor because the message is comprised of multiple elements.

Length Identifier

The length field is also coded using an extension mechanism. If the length is 127 octets or less, Bit H is set to 0 and bits A-G contain the length. If the length is 128 or greater, Bit H is set to 1 and A-G contains the number of octets used to encode the Length field. The additional octets contain the actual length of the element contents. Table 10-8 shows an example using the extension mechanism to represent a length of 131 octets. The H bit is set in the first octet, and the value represented by bits A-G is 1; this means that one additional byte is used to represent the length. The second octet indicates that the element is 131 octets in length using standard binary representation.

Table 10-8. Length Identifier Bits

Length Identifier Bits
H	G	F	E	D	C	B	A
1	0	0	0	0	0	0	1	First Octet
1	0	0	0	0	0	1	1	Second Octet

Message Layout

Now that we have examined in detail how each of the TCAP data elements are constructed, let's take a look at how they are assimilated into a message. There are three distinct sections into which a message is divided: the transaction, dialogue, and component portions. The dialogue portion of the message is optional. Figure 10-12 shows the complete structure of a TCAP message within the context of its supporting SS7 levels.

From the message structure, you can see the TLV format that is repeated throughout, in the form of Identifier, Length, and Content. A single component is shown with a single parameter in its parameter set; however, multiple parameters could exist within the component. If multiple parameters are included, another parameter identifier would immediately follow the Parameter Content field of the previous parameter. The message could also contain multiple components, in which case the next component would follow the last parameter of the previous component. Only the maximum MTP message length limits the TCAP message length.