This part of ISO/IEC 8882 defines an abstract test suite for testing the conformance of the Data Link Layer of an IUT with
respect to ISO/IEC 7776 (1986, 1995) or the ITU-T Recommendation X.25 (1980, 1984, 1988, 1993).
Conformance of a Data Terminal Equipment (DTE) to the above ISO International Standard or ITU-T Recommendations is
tested using a dedicated circuit between the tester and the DTE. It is noted that ITU-T Recommendations X.25 (1980), X.25
(1984), X.25 (1988) and X.25 (1993) are written from the perspective of a DCE and therefore do not always explicitly specify the
DTE's operation. In such cases it is assumed that recommended operation of a DTE is included by implication because of the
need to communicate with X.25 DCE's. This part of ISO/IEC 8882 excludes the testing of the LAP procedures given in the ITU-T
Recommendations.
NOTE Test cases for extended mode operation (Modulo 128), multilink procedure and DTE-DTE operation as per ISO/IEC 7776 are for
further study.
The tests in this part of ISO/IEC 8882 are designed for six possible interworking situations, shown in Figure 1. This part
specifies tests for all six cases shown in Figure 1, but recognizes that not every test may apply to a particular DTE. A test
selection procedure has to be performed to determine the applicability of a test to a particular DTE. Such selection will be based
on the PICS and the PIXIT.
Figure 1 Testing X.25-DTE/DCE and X.25-DTE/DTE Interworking
In the rest of this part of ISO/IEC 8882, the term "X.25 standards" means all four of ITU-T Recommendations X.25 (1980), X.25
(1984), X.25 (1988), X.25 (1993), and International Standards ISO/IEC 7776 (1995) and ISO/IEC 7776 (1986), unless stated
otherwise.