contacts in the outlet to complete the circuit. This term is The Telecom Handbook S 33 THE PBX still sometimes used to refer to the wires needed to complete the telecommunications circuit. It may also be used as a term for a POTS (plain old telephone service) line, which is a regular dial tone line. The cordboard was the point at which the control of the switching took place, connecting telephones to outside lines in varying combinations by plugging in cords. This controlled the backroom switching equipment (PBX). As the number of outside lines and telephones grew, along with the need for less reliance on the switchboard operator, newer types of switching systems evolved. These automated the basic operations of the cordboard. This took place in both business locations and telephone company central offices (where cordboards had also been used). Several generations of electro-mechanical switching meth- ods followed. The invention of the transistor permitted the use of electronics within the switch. This device, along with the integrated circuit, provided higher speed and lower cost, using less power. Telephone systems, in both businesses and in the telephone company central office, now resemble computers. The system logic is no longer electro-mechanical, as in the past. It is now controlled by stored instructions in the system memory. In some older sys- tems (1960's, 1970's and early 1980's) there is a physical path set up through the PBX for every call. This is called a space division system. In other systems, the spaces were time-shared by small encoded bits of conversation in what is called time division. Telephone System Design (PBX and Key Systems) The way in which a telephone system is set up is often called the system design. Up until the 1980's, the differences between Key Systems and PBXs were clear. Key systems were for smaller businesses, and their distinguishing characteristic was that each outside line (outside telephone number) appeared on everyone's telephone. 34 30 The Telecom Handbook CHAPTER 2 Key