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The past decade has seen the worldwide introduction of electronic switching with stored program control. Initially the application of this new technology was in local exchanges but today electronic systems are being introduced into trunk systems as well. One of the latest applications is No. 4 E.S.S., a very large trunk switch. This system, in combination with local systems such as No. 1 E.S.S. and new signalling arrangements, offers major innovations in telecommunication service. The introduction of stored program electronic switching at the local level in the Bell System has led to literally hundreds of new services and features being made available and many more being developed or planned. With stored program control and new signalling arrangements in the connecting trunk systems, the door is opened to both extending the local services and introducing totally new services. These trends are discussed along with examples of possible services. Finally, the effects on development, manufacture, administration, and the customer are reviewed.

The equalization and regulation problems of the L3 system are described and a theory of equalization of complex systems is outlined. The location and function of the various equalizers are explained, including the roles and design of the various fixed, dynamic, and manual equalizer networks. The analogue computer used in the regulation system is described, together with the cosine-equalizer adjusting technique used with manual equalizers. Finally the circuits and operation of the regulation system and its components are presented.