that an equipment failure has occurred. The expense of installing diagnostic restoral hardware must be weighed against the urgency of getting the remote site back online. It is clearly inadvisable to install backup modems and terminals at sites that are used only for sending monthly billings or inventory statements. On the other hand, sites involved with the transfer of large amounts of money are critical; downtime can cost thousands of dollars. Most networks, however, can safely withstand a few hours of downtime. Since most modems and terminals are not self-repairing, many central diagnostic schemes choose the less expensive alternative of isolating faulty equipment to prevent interference with further network operation. Equipment servicing then takes place at a more convenient time. More comprehensive schemes-involving diagnostics all the way to the remote terminal-assume a certain uniformity. That is, many vendors offer a complete diagnostic software package that runs on the user's mainframe. This is often contingent on the user buying all network components, including software, from the same vendor; however, one vendor often cannot supply all the necessary equip- ment for a complex network. The telecommunications manager is responsible for the operations and maintenance of network switches and trunks. This responsibility includes responding to complaints from network users, administrat- ing network features, managing switch data base modifications as well as monitoring and controlling traffic in real-time, and ensuring that the network is providing the desired grade of service in the most cost-effective fashion. These tasks are further complicated by mul- NETWORK MANAGEMENT 73 tiple network nodes and numerous linking trunks, all of which are geographically dispersed. 5.6 NETWORK CONTROL CENTER The network control center (NCC) provides network performance monitoring and centralized maintenance to ensure maximum avail- ability of network facilities. This is accomplished by providing con- tinuous on-line perform