ome to the called telephone. Also, the company the functionality of that part of the system. In modern day terms, this would have to pay the rental charge for every telephone line used. would be referred to as distributed processing. With a key system or PABX, incoming calls can be answered by As technology changed, the architecture changed to consolidate one person and then the call transferred to the correct person. The the major control elements in one part of the system. The technology number of telephone lines from the public exchange is now deter- used in this changed from relays to wired logic to microprocessors, mined by the expected maximum number of simultaneous calls (i.e. but the fundamental principle has remained the same. traffic) rather than the required number of telephones. Key systems As microprocessor technology has evolved and become more cost and PABXs pay for themselves by the reduction in exchange line effective, there is now a trend to move back towards more of a rental costs, the increased convenience to callers, and the increased distributed processing environment. Processors in various parts of productivity of employees. the overall system off load tasks from the central processing unit, Early key systems and PABXs were distinguished mainly by how and hence improve the overall performance of the system. Individ- they answered and made external calls. Key systems were (and still ual products have different architectures in terms of what tasks to are) mainly used in smaller locations without a centralised answer- perform locally and what tasks to perform centrally, but the fun- ing position. Incoming calls ring several telephones and employees damental principle of a hierarchical structure of processors com- have a collective responsibility to answer calls and transfer calls to municating with each other, and with the rest of the system, is now the correct person. PABXs usually do have a centralised answering common. position where the prime responsibility is to answer incoming calls and 