Recent developments in 4-wire switching of long-distance telephone circuits

WITH the rapid expansion of dial operation on toll telephone calls since the end of the last war, automatic switching machines have taken over many of the functions previously performed by operators on convenient toll switchboards. One of the results of this mechanization, or perhaps one of the prerequisites for its economic justification, was that a varying number of toll circuit links, from one to eight, may be used to build up a connection between two terminal points. It should be understood that the number of links may vary within these limits on different occasions, even between the same two end points and on the same type of calls, at the discretion of the machines and governed automatically by the prevailing traffic conditions. Furthermore, in addition to the maximum of eight links in a toll connection, there will usually be at least one more link at each end, connecting the toll offices with the local offices serving the calling and called subscribers, and there will always be one more link at each end connecting the subscriber´s telephone with its local office. This situation differs considerably from that which existed with previously used methods of toll switching; it introduces new transmission problems requiring new engineering solutions.