g Plan The global rules for the highest-level numbering, country codes, and overall numbering (maximum length and SO on) are given by ITU-T. The national telecommunications authority coordinates the national numbering plan. It defines, for example, trunk or area codes and operator prefixes used inside the country. It also defines nationwide service numbers (e.g., emergency numbers). These service numbers are defined to be the same wherever the call is originated and they require additional intelligence from switching sys- items. Their routing principle is explained later in Section 2.10.4. At the regional level, the numbering plan includes digits allocated to certain switching offices, exchanges, and the subscriber numbers for sub- scribers connected to a certain switch. The Telecommunications Network: An Overview 2.6.4.2 Switching Functionality for Routing From the received signaling information (dialed digits), a switching system must be able to interpret the address information, determine the route to or toward the destination, and manipulate the codes in order to advance the call properly. This includes the deletion of certain digits and automatic alternate routing. Number conversion may also be needed when, for example, the emer- gency call dialed with a nationwide short emergency number has to be routed to a regional center that has a different physical telephone number. Some of this intelligence for routing may be stored in a centralized control system from which the exchanges request routing information. This modern network struc- ture is called an intelligent network (IN) and is described in Section 2.10. 2.6.4.3 Route Selection Guidelines The basic routing principle is hierarchical: If the destination does not belong to the subscribers of the switch or of the switches under it, the call is routed upward; otherwise, it is routed to the port toward that destination (Figure 2.13). In the example of Figure 2.13, a Finnish subscriber makes a call to Stockholm, Sweden, and dials the international prefix "00,"