exchange is bursty and non-uniform, circuit-switching does expend resources inefficiently. We will see that packet-switching overcomes this limitation (but only at the cost of slower switching). To connect terminals across a network, the switches need to know when to establish the path, when to tear it down, what resources it will need to command (that is, number of circuits to be used), and where to route the call. All this information is exchanged between the switches within the net- work, and also communicated between the terminals and the network, on every call. Collectively, the information related to the control of the connec- tion is referred to as signalling. Introduction to Signalling The purpose of signalling is to exchange control information between sys- tems of any kind. Familiar applications of signalling include, among others, the control of railway traffic and air traffic. When a pilot asks for permis- sion to land at an airport, he is exchanging signalling information with the control tower. The control tower then provides the airplane with a time slot and a runway on which to land. The presence of signalling in telephony implies that two kinds of traffic are exchanged in a telephone call: sig- nalling traffic-controlling the establishment and release of the voice path -and voice traffic. We commonly refer to these different kinds of traffic as planes. Thus, two planes can be found in a telephone call: the control plane and the user plane. The control plane handles the procedures controlling the user plane and the user plane handles the actual data or voice trans- mission (see Figure 1-5). The evolution of the signalling plane has been tightly tied to the evolu- tion of the user plane. New features in the switches could not be exploited without a signalling system capable of taking advantage of them. New sig- nalling systems are designed to make optimal use of the latest advances in the user plane. However, there have also been cases where new signalling systems were introduced without any progre