824 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1952 



connected with another in the same town. It was a natural step to de- 

 velop the equipment so that operators in nearby towns could complete 

 toll calls through this local dial equipment. This was done first by using 

 the local equipment and then with progressive modifications making it 

 more and more suitable for toll. 



By these means through the decades of the 20's and 30's regional 

 networks were developed for operator toll dialing, using step-by-step 

 types of equipment, particularly in Southern California, Connecticut 

 and Ohio. Also many short haul toll calls in metropolitan areas were 

 handled in connection with the panel type dial equipment which was 

 developed for automatic switching in these areas. 



Also during this period the I'ange of customer dialing in large metro- 

 politan areas was extended, where local service is measured by message 

 registers, through arrangements for the multiple registration of calls 

 for which the charge was more than one local unit. 



An important feature of switching development in this period was 

 the perfecting of "common control" switching systems for large metro- 

 politan areas endowed with a high degree of intelligence and great 

 reliability. As will be shown, still more extensive and complicated func- 

 tions must be performed by the common control systems of a nationwide 

 automatic switching system. 



Also throughout this period great advance was made in the quality 

 and stability of long distance circuits. Telephone connections, some with 

 as many as five circuits in tandem, were being regularly established by 

 telephone operators with satisfactory overall transmission. The limita- 

 tion was in the speed and accuracy with which multiple switches could 

 be made by operators rather than in the overall transmission charac- 

 teristics. 



Several factors have worked together to bring about a big expansion 

 of long distance telephone service. These include the great growth in the 

 numbers of telephones in service, improvements in long distance trans- 

 mission, in switching, and in methods of traffic operation. Since auto- 

 matic switching becomes increasingly attractive as the traffic density 

 increases, this large growth pointed toward the desirability of further 

 mechanizing the switching operations. 



In 1943 there was cut into service in Philadelphia the first installation 

 of the No. 4 toll crossbar system. This system was designed to enable 

 general automatic switching of toll connections in and out of large metro- 

 politan areas and had many of the capabilities necessary for nation- 

 wide switching. 



The various considerations already mentioned, coupled with the sue- 



