e intervals. After the expiration of the time interval, one call to the specified code or address is allowed access to the network, after which the call-gapping procedure is recycled for another time interval. Circuit-directionalization control changes 2-way circuits to 1-way operation. Circuit-turndown control removes 1- or 2-way circuits from service. Reroute controls serve in a variety of ways to redirect traffic from congested or failed routes to other circuit groups not normally included in the route advance chain but that have temporary idle capacity. Reroutes override the normal routing algorithms in a switch. Reroutes can be used on a planned basis, such as on a recurring peak- calling day or in response to unexpected overloads or failures. "Regular reroute" affects traffic overflowing a trunk group. "Immediate reroute" (IRR) affects traffic before hunting the trunk group for an idle circuit. Reroute controls may redirect traffic to a single or to multiple routes. The multiple option is referred to as a "spray reroute." 21.6.5.2 Automatic Controls in Modern Digital Switches. Current, computer- based switches may include the following types of automatic controls: Selective dynamic overload control (SDOC) Selective trunk reservation (STR) Dynamic overload control (DOC) Trunk reservation Selective incoming load control (SILC) SDOC and STR are considered "selective" protective controls because they can selec- tively control traffic to HTR points8 more severely than other traffic. If the probability of SHIRT (hard to reach) points have 3- or 6-digit destination codes. Calls to these points have a low probability of completion. 21.7 NETWORK MANAGEMENT SYSTEMS IN ENTERPRISE NETWORKS completing through the network is very low and the outgoing trunk groups or connected switches are overloaded, selective protective controls can prevent wasted usage of these overloaded network resources for traffic to HTR points. SDOC responds to switching congestion by dynamically controlling the amount and type offered to an overl