th a release guard (RLG) signal. The RLG message indicates to the receiving exchange that the connection has been cleared and the channel released by the other exchange. It also ensures that both exchanges have cleared the cir- cuit to make it available for a new call. 2.6.3 Switching Hierarchy During the early years of the telephone, the switching office or exchange was located at a central point in a service area and it provided switched connec- tions for all subscribers in that area. Hence, switching offices are still often referred to as central offices. As telephone density grew and subscribers desired longer distance con- nections, it became necessary to interconnect the individual service areas with trunks between the central offices. With further traffic growth, new switches were needed to interconnect central offices and a second level of switching, trunk or transit exchanges, evolved. Currently national networks have several switching levels. The actual implementation of the hierarchy and the number and names of switching levels differ from country to country. Figure 2.12 shows an example of a possible network hierarchy [1]. The hierarchical structure of the network helps operators manage the network and it makes the basic principle of telephone call routing Introduction to Telecommunications Network Engineering International International traffic connections exchange Regional center Area 1 Area 2 Sectional center Primary center Toll center or end office (Local exchange) Figure 2.12 An example of switching hierarchy. straightforward; the call is routed up in the hierarchy by each exchange if the destination subscriber is not located below this exchange. The structure of the telephone number, explained in Section 2.5, supports this simple basic principle of routing up and down in the switching hierarchy. 2.6.4 Telephone Call Routing Calls that are carried by the network are routed according to a plan, a set of rules. The routing plan includes the numbering plan and network configuration. 2.6.4.1 Numberin