d control capabilities of the Integrated Services Digital Network. network. All three of these areas in the Public Switched Telephone Network have evolved in different stages and at different rates. However, they have evolved in such a manner as to enable them to 44.2 International standards for ISDN converge towards an Integrated Services Digital Network (ISDN) at minimal cost. The principles guiding the evolution of the public telephone net- Until fairly recently the whole of the Public Switched Telephone work from the analogue to the digital environment were established Network (PSTN) was based upon analogue transmission and within CCITT in the 1970's. However towards the end of this switching techniques. The telephone converts the acoustic waves of decade CCITT recognised the high level of interest in evolving this the speaker into an electrical signal occupying a bandwidth of the network further to establish the concept of an Integrated Services order of 4kHz (300Hz to 3400Hz). This is transmitted over the Digital Network (ISDN). An initial series of recommendations, the telephone network in this form to a remote telephone which then I-Series, were first agreed and published at the end of the 1980-84 reconverts the electrical signals back to acoustic waves. The electri- plenary period. These were further enhanced during the 1984-88 cal signals are carried over a copper pair in the local network to a plenary period resulting in a full set of recommendations being switching node. published in the Blue Book and during this plenary period further Switching at the local exchange used space division techniques, enhancements have been made and published in 1992. originally Strowger and later crossbar and reed relay, on the base- The CCITT I series recommendations are structured SO that new band signal in order to concentrate traffic onto junction routes to recommendations can easily be included in the appropriate sections. main network switching centres which were again of the space The structure is shown in Figure