PQM Consultants - Interoperability Testing

It is often assumed that an item of communications equipment that has passed the conformance tests for a particular standard will interoperate with any other similarly tested item of equipment. This is not necessarily so. Successful operation when connected to a protocol tester does not guarantee similar success when connected in a real-life configuration. It is, therefore, important to consider the benefits of interoperability testing.

The purpose of Interoperability Testing is to establish that two or more communicating systems (usually supplied by different manufacturers) can operate effectively when connected together. In this way, customer confidence in the product's ability to exist in a multi-vendor environment is raised. While there are conformance test specifications for many published protocol standards, there are few publicly available interoperability test suites.

Interoperability tests are almost always carried out at the user level without the need for complex test equipment. So, it is generally better to specify the tests as plain English instructions rather than in a machine-readable language such as TTCN. However, PQM Consultants have found the tabular structure of a TTCN test case to be a very effective way of constructing interoperability tests. The following example of a simple QSIG basic service test illustrates this approach. The table clearly identifies the purpose of the test, the fact that it is a mandatory test, what conditions must be established before the test can take place and the test steps and measurements that comprise the test itself.

Test:	BS- 1	Selection Criteria:	Mandatory	Selected:		Yes No
Title:		User A calls User B and User B is busy
Test Purpose		To verify that calling user in an outgoing call receives a busy indication if the called user is busy and that the B-channel reserved for the call is made available for other calls while the busy indication is being received.
Pre-test conditions:		1. Configure User A and User B with Bearer Capability set to "Speech, 64kbit/s" 2. Configure all B-channels except two in the QSIG Link as out of service
Step	Test description			Verdict
				Pass	Fail
1	Establish a new call at User B to the address of a free user also connected to the Reference Implementation (RI)
2	Initiate a new call at User A to the address of User B
3	Check A: Is Busy indication received (audible or visual)?			Yes	No
4	Establish a new call at a user connected to the IUT to the address of any free user connected to the RI
5	Check: Is conversation possible on this call?			Yes	No
6	Clear the call at User A
7	Clear the call at User B
8	Clear the call established at Step 4
Observations:

Building on our experience in corporate network signalling systems, PQM Consultants have developed a set of interoperability tests for the QSIG basic service and for a subset of the published supplementary services. These have been used by our clients for the evaluation and acceptance of QSIG equipment from a number of suppliers. The tests cover basic call procedures and the resolution of both call collisions and simultaneous call attempts for end nodes and transit exchanges.

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