nally if necessary, it can reduce or entirely cut off traffic originating from overflows is called a final route. Only a small proportion of calls use switching centres to prevent the overload from spreading. the complete backbone of final routes, since transverse routes are Network management can also provide economies if the busy used whenever these are free. hours in different parts of a network do not coincide. Calls which The traffic levels at which direct routes, tandem routing and would normally be routed through part of the network which is busy automatic alternative routing should be used depend on the relative can be routed instead through another part of the network which is costs of direct and tandem routes (including associated switching less busy. Thus, overall, fewer circuits are required to carry the total and signalling equipment). Low traffic and high-cost direct routes traffic. In the USA, there is a time difference of three hours between indicate tandem working. High traffic and low-cost direct routes the East and West coasts with a corresponding difference between the busy hours. Thus, it is economic to route long-distance calls between North and South either via the East or the West of the country at different times in the day. 29.9 Dimensioning It is uneconomic for a telecommunication network to have sufficient circuits for all subscribers to make calls simultaneously. Instead, sufficient circuits are provided to make the probability of a sub- scriber finding congestion (i.e. all circuits are busy) on the required route to be acceptably small in the busy hour. This probability is called the grade of service (GOS). Thus, a high grade of service Traffic (erlangs) corresponds to a poor service and vice versa. In a circuit-switched network, if a call encounters congestion, the call is lost. In a Figure 29.14 Domains for employment of tandem (T), message-switching or packet-switching network, if a call en- high usage (H), and direct circuits (D) counters congestion, it enters a queue and SO is d