n is estab- lished. This signaling principle is known as dual-tone multifrequency (DTMF) signaling. Tones are detected at the subscriber interface of the telephone exchange and, if necessary, signaled further to the other exchanges through which the connection is to be established. All digital local exchanges have a capability to use either pulse or tone dialing on a subscriber loop. The subscriber is able to select with a switch on his telephone which type of dialing is to be used. Tone dialing should always be selected if the local exchange is a modern digi- tal one. Advantages of tone dialing are as follows: It is quicker and dialing of all digits takes the same time. Fewer dialing errors result. End-to-end signaling is possible. Additional push buttons are available (*, #, A, B, C, D) for activation of supplementary services. Upper frequency band/Hz 1,209 1,336 1,477 1,633 Push buttons A, B, C, and D are not available in ordinary Lower frequency telephones band/Hz DTMF = Dual tone multifrequency DTMF telephone keypad Figure 2.5 Tone dialing. Introduction to Telecommunications Network Engineering Supplementary services enable subscribers to influence the routing of their telephone calls. These services, for example, call transfer, are not avail- able with telephones that use pulse dialing. To control these services we need control buttons * and #, which are available only in push-button telephones that use tone dialing. We use tone dialing also to control value-added services. Value-added services are services that we can use via the telephone network but that are usually provided by another service provider, not the telecommunications network operator. One example of value added services is telebanking. Tones are transmitted on the same frequency band as voice, and during a call we are able to dial digits to transmit, for example, our discount number and security codes to the telebanking machine. The worst disadvantage of a fixed subscriber telephone is still the poor man-machine interface that makes new serv