This test will query the far-end switch or terminal adapter to determine whether the signaling connection is functioning properly at Layer 3. It will select a B-channel in the in-service or maintenance service state and send an ISDN layer-3 SERVice message, which requires a response from the far end. The test will not be performed if there are no B-channels in an appropriate ISDN service state (as when none are administered or they are all out of service).

A PASS indicates only that a message was successfully sent to the far-end switch or terminal adapter. The ISDN PRI Specification allows up to 2 minutes for a response. Tests #639 and #636 check the health of the D-channels and DS1 Interface Media Modules. This test goes one step further by checking the communication path from the processor, through the TDM/packet bus and through the Media Module, and on to the far-end switch or terminal adapter. A special ISDN message is sent to the far-end switch or terminal adapter, which must respond within a specified amount of time. This test is designed to ensure that the communication path between the switch and the far end is up and operational, and that the two endpoints can properly exchange ISDN control messages.

This test runs on ISDN-SGR (ISDN-PRI Signaling Group).