A line test system must perform inward as well as outward tests (see Figure 5). Specifically, it must be able to use the test-access port into the switch or DLC to look inward toward the electronics of the line circuit. This type of test allows the line test system to verify that a POTS line circuit is functioning and that the circuit can originate a call. The most important benefit of an inward test is fault localization. With the ability to localize faults, it is easier to dispatch the correct technician, improve MTTR, and reduce overtime, three crucial business objectives. For example, if accurate inward testing is available and there is a problem with the line circuit of a DLC, the right technician can be sent with new boards. If the problem is in the copper, a different technician can be sent to clear the fault. The inward test is also valuable in testing ISDN, an increasingly popular technology for Internet access.

Figure 5. Test at Conversion Point

If a service provider uses a digitized or copper bypass pair, it can only look outward and may not have sufficient information to dispatch properly or test for ISDN prequalification. Switch-based test equipment cannot generally look inward either, because this function is not normally built into it.