es and Signalling in the Circuit-Switched Network Figure 1-12 TDM system. TDM link TDM table: Voice channel A = Time slot 1 Voice channel B = Time slot 2 Voice channel C = Time slot 3 pulses for signalling. A pulse occupying a certain frequency means seizure of circuit and tones are usually employed for transmitting addressing infor- mation. The amount of signalling information that can be transmitted this way is quite low. Therefore, analog signalling systems were not only expen- sive but also inefficient exchange-to-exchange. Digital transmission, of course, increased the amount of information to be carried by the control plane. Digital signalling enabled switches to exchange bit streams in place of discrete pulses. Just as combining letters according to certain syntactic rules can create first words and then sentences, defining rules for bit streaming can create signalling messages. A signalling message contains much richer information than a pulse or a tone can. Signalling protocols codify the rules to form messages from bit streams and their semantics. It is protocols that spell out the actions to be taken under any circumstances known to prevail within exchanges. These actions are usually described by means of protocol state machines, SO state machines can be thought of as the set of actions triggered by a certain event in a certain state. Access and Trunk Signalling The first digital signalling protocols were implemented between exchanges. But the subscriber line remained analog, and thus so did the signalling procedures, leading to a disjunction between Chapter 1 access signalling-signalling that passes between terminals and the network-and network signalling, also known as trunk signalling (see Figure 1-13). Later on, when the subscriber line became digital, this dis- tinction between access and trunk persisted, and different protocols were used for access and for trunk signalling. The motivation was to permit com- plex exchanges but keep terminals relatively simple, enabling the network operator to cont