and Push-Button Codes (North America) Table 7.7 shows the audible call progress tones commonly used in North America as presented to a subscriber. Subscriber subsets are either dial or push-button, and they will probably be all push-button in the next 10 years. A push-button actuates two audio tones simultaneously, similar to the multifrequency systems described previously with interregister signaling. However, the tone library used by the subscriber is different than the tone library used with interregister signaling. Table 7.8 compares digital dialed, dial pulses (breaks), and multifrequency (MF) push-button tones. 7.4 COMPELLED SIGNALING In many of the signaling systems discussed thus far, signal element duration is an impor- tant parameter. For instance, in a call setup an initiating exchange sends a 100-msec seizure signal. Once this signal is received at the distant end, the distant exchange sends "proceed to send" signal back to the originating exchange; in the case of the R-1 system, 7.4 COMPELLED SIGNALING this signal is 140 msec or more in duration. Then, on receipt of "proceed to send" the initiating exchange spills all digits forward. In the case of R-1, each digit is an MF pulse of 68-msec duration with 68 msec between each pulse. After the last address digit an ST (end-of-pulsing) signal is sent. In the case of R-1 the incoming (far-end) switch register knows the number of digits to expect. Consequently there is an explicit acknowledg- ment that the call setup has proceeded satisfactorily. Thus R-1 is a good example of noncompelled signaling. A fully compelled signaling system is one in which each signal continues to be sent until an acknowledgment is received. Thus signal duration is not significant and bears no meaning. The R-2 and SOCOTEL are examples of fully compelled signaling systems. 3 Figure 7.4 illustrates a fully compelled signaling sequence. Note the small overlap of signals, causing the acknowledging (reverse) signal to start after a fixed time on receipt of the forward signal. This is 