Editing Strowger switch (section)

== Development ==
[[File:Télégraphie, Systeme Strowger.jpg|thumb|right|Télégraphie, Systeme Strowger]]

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 [[line-finder]] which searches "backward" for the calling line; so requiring only a few relays (in most cases two, a Line, and a Cut-off relay), for the equipment required for each customer line.

Later Strowger (SXS) exchanges often use a subscriber [[uniselector]] as part of the line equipment individual to each line, which searches "forward" for a first selector. This is more economical for higher calling-rate domestic or business customers, and has the advantage that access to additional switches can readily be added if the traffic increases (the number of linefinders serving a group is limited by the wiring multiple installed). Hence exchanges with subscriber uniselectors were usually used at British exchanges with a high proportion of business customers, e.g. [[director telephone system|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 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 [[scalability|scalable]].
{{Listen
|filename=Sxs intra to busy.ogg
|title=Strowger switch in use
|description=The sound of a step-by-step call reaching a busy circuit.
}}