9.5 User-Controlled Diagnostics

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Telephone company diagnostics such as ABATS and CTS provide only the most rudimentary test capabilities. Because these methods are based strictly on loopback testing, which is always interfering, other network control and diagnostic systems are frequently chosen instead. Of special importance for DDS is the newer DDS with secondary channel previously mentioned and illustrated in Figure 9.2.

By providing a completely independent, low-speed (auxiliary) data channel, network surveillance can be performed on a continuous, non-disruptive basis. In addition, non-disruptive loopback testing may be performed over the secondary channel without interfering with the production data, which continues to flow over the primary channel. Overall network reliability is also improved because a failure in the data channel will not inhibit passage of alarms and, consequently, prevent corrective action from being taken.

While some CSU-DSUs provide a “virtual” secondary channel on conventional DDS circuits (by multiplexing test data along with user data), these devices do not provide the testing flexibility of DDS/SC. Because the primary and secondary channels derived with this approach are not truly independent, continuous real-time tests cannot be performed on the secondary channel without affecting production data. Also, because this technique steals bandwidth from the primary channel, equipment that requires precise clocks (such as channel-splitting multiplexers) may not run properly.

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