SXS - Development of The Strowger System

Development of The Strowger System

The commercial version of the Strowger switch, as developed by the Strowger Automatic Telephone Exchange Company, used a rotary dial for signalling to the exchange. The original final selector (connector) switch which connected to 100 customers was supplemented by preceding group selector stages, as the "cascading" enabled connection to many more customers, and to customers at other exchanges. Another requirement for commercial systems was a circuit to detect a busy connection (line) and return a busy signal to the calling subscriber.

Instead of dedicating an expensive first-stage selector switch to each customer as in the first exchange, the customer was given access to the first-stage switch of a telephone network, often by a linefinder which searched "backward" for the calling line; so requiring only a few relays for the equipment required for each customer line.

Later Strowger (SXS) exchanges often used a subscriber uniselector as part of the line equipment individual to each line, which searched "forward" for a first selector. This was more economic for higher calling-rate domestic or business customers, and had the advantage that access to additional switches could readily be added if the traffic increased (the number of linefinders serving a group of was limited by the wiring multiple installed). Hence exchanges with subscriber uniselectors were usually used at British exchanges with a high proportion of business customers eg director exchanges, or in New Zealand where the provision of local free calling meant that residential customers had a relatively high calling rate.

The fundamental modularity of the system combined with its step-by-step (hence the alternative name) selection process and an almost unlimited potential for expansion that gives the Strowger system its technical advantage. Previous systems had all been designed for a fixed number of subscribers to be switched directly to each other in a mesh arrangement. This became quadratically more complex as each new customer was added, as each new customer needed a switch to connect to every other customer. In modern terminology, the previous systems were not "scalable".