fered load exceeds the engineered capacity and congestion is present, and it also shows the increased number of calls encountering switching delay. 2. A focused overload is generally directed toward a particular location and may result from media stimulation (e.g., news programs, advertisements, call-in contests, telethons) or events that cause mass calling to government or public service agencies, weather bureaus, or public utilities. Without the application of appropriate network controls, the effects of these types of overloads could spread throughout the network. Focused overloads are normally managed using code controls or, if anticipated, trunk-limited or choke networks. 3. A switching system overload occurs because each individual switch is engineered to handle a specific load that is known as engineered capacity. The engineered capacity No. of switching delays Congestion Overload Offered load (erlangs) Figure 21.4 Network performance during overload conditions. 21.6 NETWORK MANAGEMENT FROM A PSTN PERSPECTIVE is usually less than the total switching capacity. When the load is at or below engineered capacity, the switch handles calls in an efficient and reliable manner. However, when the load increases beyond the engineered capacity, delays can occur internal to the switch. This resulting internal congestion can spread, causing connected systems to wait for start-dial indications. This can also cause internal congestion in connected switching systems. At the onset of overload, also known as circuit shortage, the dominant cause for customer blockage is the failure to find an idle circuit. Circuit blockage alone limits the number of extra calls that can be completed but does not cause a significant loss in call- carrying capacity of the network below its maximum. As the overload persists and the network enters a congested state, regeneration-calling pressure changes customer blockage from circuit shortage to switching delays. Switching delays cause timeout conditions during call setup and occur when switches 