f the circuit. After the provision of a dial Chapter 1 tone by the local exchange, terminals forwarded the digits of the callee's telephone number. With this information in hand, the local exchange routed the call towards the proper destination. In these first automated systems, the only signalling direction was for- ward, for example, from the caller's terminal to the switch. Any information that had to be transmitted backwards to the user was sent through the user plane. So if the callee was busy, for instance, a busy tone was sent through the user plane, and it was up to the user to react by hanging up. The control plane never knew if the user hung up because he or she decided to abort the call before the callee answered or because the callee was busy, which proved to be a disadvantage down the line. DC and AC Analog Systems The next signalling systems to be devel- oped were the DC and AC analog systems, so-named according to the type of current they used. The gain was that DC and AC systems allowed sig- nalling backwards. Having signalling in the callee-to-caller direction rep- resents an important saving in some situations, as Figure 1-6 illustrates. The caller dials a phone number, and each switch in the path between caller and callee reserves a circuit to transport the voice content of this call. When the signal arrives at the callee's local exchange, the final switch in the path, it locates the connection to the callee's terminal. The switch checks the availability of this connection and notices that it is being used for another call at that moment. In a system that does not enable signalling backwards, the local exchange then generates a busy tone. This tone is transmitted to the caller SO he or she knows to give up on the call. The caller is expected to hang up after hearing this tone but may not immediately do SO. Once he does, signalling indicating the caller has gone is sent forward towards the callee's local exchange, which at last triggers the release of all the circuits along the path. It works, b