utomatic SO switching was done manually using a switchboard. Strowger developed the first automatic switch (exchange) in 1887. At that time, switching had to be controlled by the telephone user with the help of pulses generated by a dial. For many decades exchanges were a complex series of electromechanical selectors, but during the last few decades they have developed into software-controlled digital exchanges. Modern exchanges usually have quite a large capacity-tens of thousands subscrib- ers-and thousands of them may have calls ongoing at the same time. 2.1.3 Signaling Signaling is the mechanism that allows network entities (customer premises or network switches) to establish, maintain, and terminate sessions in a net- work. Signaling is carried out with the help of specific signals or messages that indicate to the other end what is requested of it by this connection. Some examples of signaling examples on subscriber lines are as follows: Off-hook condition: The exchange notices that the subscriber has raised the telephone hook (dc loop is connected) and gives a dial tone to the subscriber. Introduction to Telecommunications Network Engineering Dial: The subscriber dials digits and they are received by the exchange. On-hook condition: The exchange notices that the subscriber has fin- ished the call (subscriber loop is disconnected), clears the connection, and stops billing. Signaling is naturally needed between exchanges as well because most calls have to be connected via more than just one exchange. Many different signaling systems are used for the interconnection of different exchanges. Sig- naling is an extremely complex matter in a telecommunications network. Imagine, for example, a foreign GSM subscriber switching his telephone on in Hong Kong. In approximately 10 seconds he is able to receive calls directed to him. Information transferred for this function is carried in hun- dreds of signaling messages between exchanges in international and national networks. Signaling in a subscriber loop is discussed 