COMMON CONTROL SWITCHING SYSTEMS 1095 



mercial panel offices. A semi-mechanical rotary system was installed in 

 Landskrona, Sweden, in 1915 but remained in service for only a short 

 time. A similar system was installed later in 1915 in Angiers, France. 

 The first full mechanical rotary installation was at Darlington, England, 

 in 1914. This sj'stem is still in service. 



A common control system using Strowger switches, the director sys- 

 tem, was developed in 1922. This development was prompted by the 

 desire to provide automatic equipment in the London, England, multi- 

 office exchange where the layout of the outside plant required consider- 

 able tandem trunking if a reasonably economical trunk network was to 

 be achieved. All of the outside plant in London for the manual system 

 was iniderground and it was required that this arrangement be retained 

 when dial equipment was installed. This tended to fix the routes of tele- 

 phone cables and to make it expensive to open new direct routes as new 

 offices were opened. The trunking economies of tandems were extremely 

 desirable under tliis condition and common controls with translation 

 were necessary for a practical scheme capable of operating with the 

 tandems. The director scheme, which in principle parallels the sender- 

 translator scheme of the panel system, was designed to meet this situ- 

 ation. The director system was first placed in operation in Havana, 

 Cuba, in 1924 and later in London in 1927. 



EVOLUTION OF THE MARKER PRINCIPLE 



In retrospect, it is obvious that the development thinking up to the 

 early 1920's was limited by the belief that it was necessary to have the 

 selectors do the testing for idle trunks even with common controls. This 

 arrangement had been successfully used in the step-by-step system and 

 it was natural to follow the same plan in the panel, rotary and director 

 systems. Subsequent development of the common-control idea, starting 

 with an experimental "coordinate" system in 1924, has resulted in 

 marker systems in which the trunk testing is done by the markers. 



The coordinate system derived its name from the method of operation 

 of its switch, the process resembling the method of marking a point by 

 the use of coordinates. The switch was essentially a large version of the 

 crossbar switch and selected and held a set of crosspoints by the opera- 

 tion of horizontal and vertical members. Translation of the called office 

 code, selection of a trunk, and operation of the switches to connect a 

 transmission circuit to the trunk were functions of a new circuit, the 

 marker, which the sender called into use for a fraction of a second after 

 it had received the office code digits. 



When the marker does the testing for idle trunks the trunk access from 



