nts generated by the signal from far end (receive pair) Figure 2.7 Local loop and 2W/4W hybrid. The 2W/4W hybrid performs the following operations: Separates the transmitting and receiving signals. Matches the impedance of the 2W local loop to the network circuit. Provides a loss to signals arriving on the receiving path, preventing them from entering the transmitting path, which would cause echo. The ISDN basic rate interface uses bidirectional 160-Kbps data trans- mission on a 2W circuit (ordinary subscriber loop). There the transmission directions are separated with the help of digital signal processing technology. Many applications use the transformer circuit described earlier together with digital signal processing technology to improve performance. In every subscriber set quite the same principle as the 2W/4W hybrid is used to attenuate the subscriber's own voice from the microphone to the ear- phone (Figure 2.7). The reader can imagine what happens when the micro- phone generates an alternating current in the telephone set of the figure. 2.5 Telephone Numbering An international telephone connection from any telephone to any other tele- phone is made possible by unique identification of each subscriber socket in the world. In mobile telephone networks, each telephone set (or subscriber card) has a unique identification number. The numbering is hierarchical, and it has an internationally standard- ized country code at the highest level. This makes national numbering The Telecommunications Network: An Overview schemes independent from each other. E. .164 specifies the structure of inter- national telephone numbers and it is presented in Figure 2.8. In the follow- ing sections, we explain the fields of the telephone number shown in Figure 2.8. 2.5.1 International Prefix An international prefix or international access number is used for interna- tional calls. It tells the network that the connection is to be routed via an international telephone exchange to another country. The international pre- fix may differ