my subscriber" but the des- tination subscriber is located "below me" and routes the call to the corresponding lower-level exchange, in this example, the local exchange. The local exchange selects the subscriber loop of the telephone number 13115 and connects a ringing signal to the subscriber. However, modern exchanges can do more than the simple strictly hier- archical routing just introduced. If there is a sufficient volume of traffic, calls may pass by a hierarchy level or may be connected directly to another low- level switch, as illustrated in Figure 2.12. This may be reasonable, for exam- ple, if the local exchanges of subscribers A and B are on the opposite sides of the regional border. The telecommunications operator is free to define the detailed actual routing to optimize the usage of the network. In this section we have described the switching hierarchy of the tele- phone network and the telephone call routing principle through the exchanges in this hierarchy. In modern networks the actual implementation may be different from this strictly hierarchical routing principle we described. Local telephone exchanges may analyze the whole telephone number, bypass the switching hierarchy, and route the call directly if the des- tination is a subscriber of a neighbor local exchange. Also, some sets of the telephone numbers have no fixed connection to the physical location of a subscriber loop. The IN technology, which we discuss later in this chapter, connects a dialed logical number and a certain physical telephone number (i.e., subscriber loop). Deregulation of the fixed telephone business has created another need for increased intelligence in the network. Local network operators have to be able to connect calls to other parallel networks belonging to competing The Telecommunications Network: An Overview network operators when subscriber A requires that. The need for this is indicated by the operator prefix dialed by the subscriber as discussed in Section 2.5. In the next section, we divide the global telecom