9.4 Carrier Testing Services

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In addition to the diagnostic and loopback test capabilities built into many of today’s CSU-DSUs, users rely on telephone company-provided test systems like AT&T’s Automated Bit Access Test System (ABATS) for DDS. For nationwide AT&T ACCUNET DDS customers, ABATS can actually command each and every CSU-DSU termination into a line loopback (LL) and remote terminal loopback (RTL) test to isolate network problems.

ABATS is activated when a user places a call to the serving test center (STC) to report a problem. The STC interacts with the ABATS test center where technicians troubleshoot the customer’s entire network, right down to the DTE interface of the remote CSU-DSUs. The test center verifies test data after they are looped through the CSU-DSU, where the data are decoded, reclocked, and retransmitted. Many smaller organizations, with neither the diagnostic test equipment nor the technical expertise required to run sophisticated diagnostic systems, have come to rely on this service for maintaining their networks.

In addition, users can subscribe to AT&T’s CTS, which is available over inter-LATA DDS circuits only. From an asynchronous terminal and a leased or dial-up data set on the customer’s premises, the user can access the network test system. The user can use the service provider’s computer resources to identify a problem on the network and isolate its cause by initiating loopback tests in both directions, as well as tests on the DSU. This, coupled with the built-in CSU-DSU diagnostics, provides one means of maintaining the customer’s circuits.

Tests initiated through ABATS and CTS are disruptive, however, because they are conducted on an “in-band” basis; that is, the test signals replace the user’s production data. To lessen the effect, users may schedule circuit testing for off-peak hours. Because these services utilize standard LL and RTL testing, users can take advantage of them as long as they have a CSU-DSU that is completely compatible with the old Western Electric Company’s (WECO) 500A type of DSU.

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