home
***
CD-ROM
|
disk
|
FTP
|
other
***
search
/
Internet Standards
/
CD2.mdf
/
ccitt
/
1992
/
q
/
q773.asc
< prev
next >
Wrap
Text File
|
1991-12-31
|
10KB
|
217 lines
The drawings contained in this Recommendation have been done in
Autocad.
Recommendation Q.773
TRANSACTION CAPABILITIES FORMATS AND ENCODING
1 Introduction
This Recommendation provides the format and encoding of
Transaction Capabilities Application Part (TCAP) messages. Formats
and Encoding for the Intermediate Service Part (ISP) are for further
study. This Recommendation is based on the encoding rules provided in
CCITT Recommendation X.209 and is consistent with that
Recommendation.
2 Description conventions
This Recommendation does not use Recommendation X.209 formal
description language. This Recommendation uses the description method
of other Q.700 series Recommendations. Annex A uses the formal
decription language to supplement this Recommendation.
3 Standard representation
3.1 General message structure
Each information element within TCAP message has the same
structure. An information element consists of three fields, which
always appear in the following order. The Tag distinguishes one type
from another and governs the interpretation of the Contents. The
Length specifies the length of the Contents. The Contents is the
substance of the element, containing the primary information the
element is intended to convey. Figure 1/Q.733 shows an overview of a
TCAP message and an information element.
Fig. 1/Q.773 /T1120490-88 = 4.5 cm
Each field is coded using one or more octets. Octets are
labelled as shown in Figure 2/Q.773. The first octet is
the first transmitted. Bits in an octet are labelled as shown in
Figure 3/Q.773, with bit A the least significant and the first
transmitted.
Fig. 2/Q.773 /T1120500-88 = 4.5 cm
PAGE38 Fascicle VI.9 - Q.733
Fig. 3/Q.773 /T1120510-88 = 4.5 cm
The contents of each eleent is either one value (Primitive) or
one or more information elements (Constructor), as shown in Figure
4/Q.773.
Fig. 4/Q.773 /T1120520-88 = 5 cm
3.2 Tag
An information element is first interpreted according to its
position within the syntax of the message. The Tag distinguishes one
information element from another and governs the interpretation of
the Contents. It is one or more octets in length. The Tag is composed
of "Class", "Form" and "Tag code", as shown in Figure 5/Q.773.
Fig. 5/Q.773 /T1120530-88 = 3 cm
3.2.1 Tag class
All Tags use the two most significant bits (H and G) to
indicate the Tag Class. These bits are coded as shown in Table
1/Q.773.
TABLE 1/Q.773
Coding of tag class
Class Coding (HG)
Universal 00
Application-wide 01
Context-specific 10
Private use 11
a
all CCITT Recommendations, i.e. across CCITT No. 7 ASEs, X.400 MHS,
etc.
The Application-wide class is used for information elements
that are standardized across all applications (ASEs) using CCITT No.
7 TC, i.e. TC-Users.
The Context-specific class is used for information elements
that are specified within the context of the next higher construction
and take into account the sequence of other data elements within the
same construction. This class may be used for tags in a construction,
and the tags may be re-used in any other construction.
The Private Use class is reserved for information elements
specific to a nation, a network or a private user. Such information
elements are beyond the scope of the TC Recommendations.
The Tag codes of the Application-wide class not assigned in
this Recommendation are reserved for future use.
3.2.2 Form of the element
Bit F is used to indicate whether the element is "Primitive"
or "Constructor", as is shown in Table 2/Q.773. A primitive element
is one whose structure is atomic (i.e. one value only). A constructor
element is one whose content is one or more information elements
which may themselves be constructor elements.
Both forms of elements are shown in Figure 4/Q.773.
TABLE 2/Q.773
Coding element form
Element form Coding (F)
Primitive 0
Constructor 1
Fascicle VI.9 - Q.773 PAGE39
3.2.3 Tag code
Bits A to E of the first octet of the Tag plus any extension
octets represent a Tag code that distinguishes one element type from
another of the same class. Tag codes in the range 00000 to 11110 (0
to 30 decimal) are provided in one octet.
The extension mechanism is to code bits A to E of the first
octet as 11111. Bit H of the following octet serves as an extension
indication. If bit H of the extension octet is set to 0, then no
further octets for this tag are used. If bit H is set to 1, the
following octet is also used for extension of the Tag code. The
resultant Tag consists of bits A to G of each extension octet, with
bit G of the first extension octet being most significant and bit A
of the last extension octet being least significant. Tag code 31 is
encoded as 0011111 in bits G to A of a single extension octet. Higher
tag codes continue from this point using the minimum possible number
of extension octets.
Figure 6/Q.773 shows the detailed format of the Tag code.
Figure 6/Q.773 /T1106430-87 = 6.5 cm
3.3 Length of the Contents
The Length of the Contents is coded to indicate the number of
octets in the Contents. The length does not include the Tag nor the
Length of the Contents octets.
The Length of the Contents uses the short, long or indefinite
form. If the length is less than 128 octets, the short form is used.
In the short form, bit H is coded 0, and the length is encoded as a
binary number using bits A to G.
If the Length of the contents is greater than 127 octets, then
the long form of the Length of the Contents is used. The long form
Length is from 2 to 127 octets long. Bit H of the first octet is
coded 1, and bits A to G of the first octet encode a number one less
than the size of the Length in octets as an unsigned binary number
whose MSB and LSB are bits G and A, respectively. The length itself
is encoded as an unsigned binary number whose MSB and LSB are bit H
of the second octet and bit A of the last octet, respectively. This
binary number should be encoded in the fewest possible octets, with
no leading octets having the value 0.
The indefinite form is one octet long and may (but need not)
be used in place of the short or long form, whenever the element is a
constructor. It has the value 10000000. When this form is employed, a
special end-of-contents (EOC) indicator terminates the Contents.
There is no notation for the end-of-contents indicator.
Although considered part of the Contents syntactically, the
end-of-contents indicator has no semantic significance.
The representation for the end-of-contents indicator is an
element whose class is universal, whose form is primitive, whose ID
Code has the value 0, and whose Contents is unused and absent:
EOC Length Contents
00(hex) 00(hex) Absent
PAGE38 Fascicle VI.9 - Q.733