become severely overloaded. Timeouts are designed into switches to release common- control components after excessively long delay periods and provide the customer with a signal indicating call failure. Switching congestion timeouts with short holding-time attempts on circuit groups replace normal holding-time calls. Switching delays spread rapidly throughout the network. A trunk-group overload usually occurs during general or focused overloads and/or atypical busy hours. Some of the overload causes not discussed above are facility outages, inadequate trunk provisioning, and routing errors. The results of trunk-group overload can be essentially the same as those previously discussed for general overloads. However, the adverse effects are usually confined to the particular trunk group or the apex area formed by the trunk group and those groups that alternate-route to the overloaded trunk-group. Trunk-group overload problems can often be minimized or handled completely by the use of temporary NTM reroute controls until a more permanent solution can be provided. 21.6.5 Network Traffic Management Controls 21.6.5.1 Circuit-Switched Network Controls. There are two broad categories of NTM controls: Protective Controls. These controls remove traffic from the network during overload conditions. This traffic is usually removed as close as possible to its origin, thus making more of the network available to other traffic with a higher probability of completion. Expansive Controls. These controls reroute traffic from routes experiencing over- flows or failures to other parts of the network that are lightly loaded with traffic because of noncoincident trunk and switching system busy hours. Implementation of either type of control can be accomplished on a manual or automatic basis. For example, manual controls are activated by network traffic managers, and auto- matic controls are activated by network components. In some switches, these controls are implemented on a planned control-response basis that is preprogrammed into the