hear when we raise the hook that it is ready to receive digits. After dialing it keeps us informed about whether the circuit establish- ment is successful by sending us a ringing tone when the telephone at the other end rings. When subscriber B answers, the exchange switches off both the ringing signal and the ringing tone and connects the circuit. At the end of the conversation, an on-hook condition is detected by the exchange and the speech circuit is released. In next sections we explain in more detail one of the subscriber signal- ing phases, the transmission of dialed digits from a subscriber's telephone to the local exchange. 2.3.2 Rotary Dialing The telephone set has a switch that is open in the on-hook condition and closed when the hook is off. This indicates to the telephone exchange when a call is to be initiated and when it has to prepare to receive dialed digits. In old telephones, which exchanges still have to support, this method of local- loop connection/disconnection is used to transmit dialed digits as well (Figure 2.4). We call this principle rotary or pulse dialing. Subscriber A Subscriber B Exchange Off hook Dial tone B-number Ringing tone Ringing signal B-Answer Conversation On hook On hook Release of speech circuit Figure 2.3 Subscriber signaling. Introduction to Telecommunications Network Engineering Current on the local loop Current One cycle Current detector 50 mA 60 ms Telephone set Local loop Telephone 40 ms exchange Figure 2.4 Rotary, or pulse, dialing. In rotary dialing a local loop is closed and opened according to the dialed digits, and the number of current pulses is detected by the exchange. This signaling method is also known as loop disconnect signaling. The main disadvantages of this method are that it is slow and expensive due to high- resolution mechanics and it does not support supplementary services such as call forwarding. The local-loop interfaces in telephone exchanges have to support this old technology though it has been gradually replaced by tone dialing. When a digit