Explain Dial Tone and Ringing Tone in strowger telephony.

Dial Tone: it is used to indicate that the exchange is ready to recognize dialed digits by the subscriber. The subscriber must start dialing simply after hearing the dialing tone. However, sometimes, a few seconds may elapse before the dial tone is heard. It happens particularly in general control exchanges that use shared resources for user interfaces. The dial tone is a 50 Hz or 33 Hz or 400 Hz continuous tone as demonstrated in Fig. The 400 Hz signal is typically modulated with 50 Hz or 25 Hz.

Figure: Dial Tone

When the called party line is acquired, the exchange control equipment sends out the ringing current to the telephone set of the called party. Such ringing current has the familiar double-ring pattern. At once, the control equipment sends out a ringing tone to the calling subscriber that has a pattern similar, which of the ringing current as demonstrated in Fig. The two rings in the double ring pattern are separated through a time gap of 0.2 s and two double ring patterns with a gap of 2 s. The ring burst has period of 0.4 s. The frequency of the ringing tone is 133 Hz or 400 Hz.

Figure: Ringing Tone