ut it entails that many circuits in the network remain engaged by this connection even after the exchange reports that the caller is unavailable. DC and AC analogue systems make better use of network resources. Sending busy status through the control plane allows the user plane, circuit-switched path for the call to be released. The busy signal that the caller hears is instead generated by his or her local exchange, and no resources are reserved unnecessarily while the user is informed about the status of the call and prepares to react (see Figure 1-7). Thus, having signalling in both directions helps manage the network more efficiently. The control plane receives more, and more accurate, infor- mation about the status of the call. Signalling in the Circuit-Switched Network Figure 1-6 Callee's The callee is busy local exchange and the only signalling direction is forward. BUSY TONE Caller's local exchange FDM and In-band Signalling Circuit-switching continued to develop in the next decades. Where each cir- cuit used for telephony consisted of a copper wire, SO a switch handling 500 circuits needed 500 cables. Frequency Division Multiplexing (FDM) made it possible to use a single wire for several simultaneous calls. In FDM sys- items, every voice path occupies a different bit of the frequency spectrum (see Figure 1-8), in effect redefining a circuit as a frequency within a phys- ical cable instead of the cable itself. FDM was developed around 1910, but wasn't actually implemented until 1950. At that point a coaxial cable was capable of providing 1,000 circuits-a solid gain. FDM systems demanded new signalling procedures; the signals within a cable between two switches were needed to control many circuits instead of one. Switches now needed a way to relate the control plane of a specific call to its user plane. The solution would prove to be in-band signalling systems, meaning that signalling is carried on the same frequency as speech-they travel together in the same circuit. In simplest terms, seizure of the circu