 to make nearly real-time decisions for routing. This brought about dynamic routing such as AT&T DNHR (dynamic nonhierarchical routing). The advent of CCITT Signal- ing System No. 7 (Chapter 7) working with high-speed computers made it possible for optimum routing based on real-time information on the availability of route capacity and shortest routes. Thus the complex network hierarchy started to become obsolete. Nearly all reference to routing hierarchy disappeared from CCITT in the 1988 Plenary Session (Melbourne) documents. International connectivity is by means of direct/high- usage routes. In fact, CCITT Rec. E. 172 (Geneva 10/92) states that "In the ISDN 12 era, it is suggested that the network structure be non-hierarchical Of course, reference is being made here to the international network. 1.3.8 Variations in Traffic Flow In networks covering large geographic expanses and even in cases of certain local net- works, there may be a variation of the time of day of the BH or in a certain direction of traffic flow. It should be pointed out that the busy hour is tied up with a country's culture. Countries have different working habits and standard business hours vary. In Mexico, for SPC stands for stored program control. This simply means a switch that is computer-controlled. SPC switches started appearing in 1975. 12 ISDN stands for Integrated Services Digital Network(s). This is discussed in Section 12.4. INTRODUCTORY CONCEPTS instance, the BH is more skewed toward noon because Mexicans eat lunch later than do people in the United States. In the United States, business traffic peaks during several hours before and several hours after the noon lunch period on weekdays, and social calls peak in early evening. Traffic flow tends to be from suburban living areas to an urban center in the morning, and the reverse occurs in the evening. In national networks covering several time zones where the difference in local time may be appreciable, long-distance traffic tends to be concentrated in a few hours common to BH p