ance monitoring of switch nodes and providing alerting mechanisms to indicate critical conditions. An- other level of insurance can be attained by providing the NCC op- erator with the ability to detect, diagnose, and localize network problems to a particular switch or trunk SO that corrective action may be taken. The NCC can also provide mechanisms for the co- ordination of maintenance activities and ensuring management con- trol through the use of reports. 5.6.1 Alarms In a centralized control center environment, the operator must be made aware of critical maintenance conditions rapidly. The network switches remote call maintenance information to the network control center. However, in a multi-switch network, the volume of mainte- nance information can be SO large that critical messages get over- looked. To ensure that critical conditions are not overlooked, an NCC alarm feature can identify messages requiring immediate op- erator attention. Every message from each switch in the network can be transmitted to the NCC and compared with pre-defined alarm conditions. If there is a match, an alert is immediately generated. Further, network alarm status information can be dynamically displayed via color graphics, giving the operator an overview, at a glance, of current network alarm status. This avoids the traditional deluge of rows and columns of numbers. A typical graphics display consists of a geographic rep- resentation of all nodes and trunks in the network. Immediate rec- ognition of alarm conditions can be assured by changing the node symbol of the affected node from green to flashing red and sounding 74 TELECOMMUNICATIONS an audible alert which can only be silenced by operator acknowl- edgment. The actual message which triggered the alarm can be printed on a specially designed alarm printer. 5.6.2 Automatic Trunk Testing The NCC provides a centralized capability to automatically or man- ually test each trunk in the network for noise, as well as gain, and report these results to the operator in an efficient, o