oaded or failed switch. STR, conversely, responds to trunk congestion in the outgoing trunking field and is triggered on a particular trunk group when less than a certain number of circuits are idle in that group. SDOC and STR are two-level control systems. The first level indicates less conges- tion than the second level. The first-level response is typically limited to control of traffic destined for HTR points, whereas the second level applies controls to both HTR points and other traffic, typically alternate-routed traffic. HTR traffic can also be manually enabled. HTR traffic is automatically detected by the AT&T 4ESS switch based on an analysis of destination-code completion statistics. This analysis is performed on a 3- and 6-digit basis every five minutes. In Nortel DMS-100 and DMS-200/500 switches, HTR codes can also be manually selected and enabled. Automatic controls, such as SDOC and STR, are intended to be activated by a switching system within a matter of seconds in response to a switch or trunk-group overload. These controls provide rapid protection for the network and, by their code-selective basis, attempt to restrict traffic that has a low probability of completion. When automatic controls trigger, network traffic managers monitor their operations and adjust system parameters to deal with the particular network condition, whether it is a general overload, a mass call-in, a natural disaster, or a major network-component failure. Among these parameters are call-completion determinations that designate a code "HTR" and control-response options that designate the amount of traffic to be controlled or trunks to be reserved at each triggering level. Since the optimum control response depends on the severity, geographical distribution, and type of overload, maximizing the calls carried by the network requires coordination and combination of automatic and manual control responses. 21.7 NETWORK MANAGEMENT SYSTEMS IN ENTERPRISE NETWORKS 21.7.1 What Are Network Management Systems? Ostensibly a network mana