t to be faulty equipment. Tests are made in order to establish conformance established, either to a remote tester or to the other channel of the to specification, in terms of correctly generated signals and the calling tester. If a speech call transmission test is required, the call ability to receive, and respond correctly to, error free and errored has to be to another tester or ISDN telephone, because it is not data. A terminal equipment (TE), for example, may be tested by permissible to have two simultaneous speech calls on one instru- connecting it to a simulated network, and network terminating units ment. In the case of data calls, the following logical call connection (NTs), by connecting them between a simulated terminal and a schemes are possible: simulated network. A loop test through the network, with loops at strategic points to One channel (B1 say) of a tester transmits, via the network, to identify problem areas, or an end to end test through the network, the other channel of the same tester (B2), and B2 transmits, via verifies conformance and quality, and can be achieved without the network, to B1, performing two full duplex tests using the taking the equipment out of service. Full quality of service testing full bandwidth of one Basic Rate circuit, as shown in Figure to CCITT Recommendation G.821 involves the recording of 61.17. periods of performance, within specified error ratio bands, for one 2. One channel (e.g. B1) of the tester transmits, via the network, month. This is normally only practical at the installation and com- to the other channel (B2) of the same tester, which loops the missioning phase. connection and retransmits, via the network, back to B1, per- ISDN test equipment 61/19 ISDN basic rate tester ISDN line Public switched telephone network 'U' Reference 'S' Reference point point Figure 61.17 Logical connections for ISDN Basic Rate subscriber loop testing forming one full duplex test using the full bandwidth of one frequency, to determine path loss and frequency response, signal