ins and they operated as variable resistors supplied with battery voltage from an exchange site (see the subscriber loop on the left-hand side of Figure 2.2). When sound waves pressed the carbon grains more tightly, loop resistance decreased and current slightly increased. The variable air pressure generated a variable, alternating current to the subscriber loop. This variable current con- tained voice information. The basic operating principle of the subscriber loop is still the same today, although modern telephones include more sophisticated and better quality microphones. 2.2.2 Earphone Alternating current, generated by the microphone, is converted back into voice at the other end of the connection. The earphone has a diaphragm with a piece of magnet inside a coil. The coil is supplied by alternating current produced by the microphone at the remote end of the connection. The cur- rent generates a variable magnetic field that moves the diaphragm that pro- duces sound waves close to the original sound at the transmitting end (see the subscriber loop on the right-hand side of Figure 2.2). 2.2.3 Signaling Functions The microphone generates the electrical current that carries voice informa- tion, and the earphone produces the voice at the receiving end of the speech Introduction to Telecommunications Network Engineering circuit. The telephone network provides a dialed-up or circuit-switched serv- ice that enables the subscriber to initiate and terminate calls. The subscriber dials the number to which she wants to be connected. This requires addi- tional information transfer over the subscriber loop and from the exchange to other exchanges on the connection, and this transfer of additional informa- tion is called signaling. The basic subscriber signaling phases are described in the following section. 2.3 Signaling to the Exchange from the Telephone Telephone exchanges supply dc voltage to subscriber loops, and telephone sets use this supplied voltage for operation. The conventional telephone does not include any el