Modifying SDLC link level 2 test

To test NCP Synchronous Data Link Control (SDLC) links, use the SDLC link level 2 (LL2) test.

If you run this test over an extended period of time, you can increase the possibility of repeating an intermittent error that is hard to re-create.

Rule: If you want to test connectivity to a physical unit only, activate the physical unit instead of using an LL2 test.

Depending on the SDLC link you want to test, do one of the following tasks:

To test an SDLC link between an NCP and a physical unit that is attached on a multipoint line:
1. Ensure that the physical unit name in the ID operand is inactive and dedicated to the test. Other physical units on the same link can remain active.
2. Issue the MODIFY LL2 command.
To test an SDLC link between two NCPs:
1. Ensure that the primary link station is inactive. The primary link station is the link named in the LL2 command.
2. Ensure that the secondary link station is active. The secondary link station is the link that will respond to the test.
3. Issue the MODIFY LL2 command. Start the test from the primary link station.

The MODIFY LL2 command causes the SSCP to send a test RU to the NCP to which the test terminal is connected. The NCP returns test results to the requesting SSCP in a Record Test Results RU.

The test results include the following information:

The number of test frames transmitted by the node specified by the nodename parameter
The number of test frames received by the node specified by the nodename parameter
The number of test frames received without error by the node specified by the nodename parameter (test frames that were successfully returned by the test station)

If these three numbers are not the same, an error in the link or a physical unit has occurred. Use a line trace to further isolate the problem.

To test the SDLC link, VTAM® sends test data over the link from the controlling NCP to the remote station, which is an NCP or a peripheral physical unit. The data is then echoed back to the sending NCP. This NCP then compares the data received with the data sent and forwards the results to VTAM. When VTAM receives the test results, it sends message IST549I to the initiating console to indicate how the link level 2 test ended: data returned without errors, data returned with errors, or no data returned because of an inoperative link or initialization error. See message IST549I in z/OS Communications Server: SNA Messages for a complete message description.