With neat diagrams explain the configuration of a step-by-step switching system.

Schematic diagram for such an exchange is given in Figure.  Every subscriber is connected to a single rotary pre-selector switch at the exchange, outputs from this switch being connected to a bank of two-motion switches called "group selectors". The  out-puts  from  pre-selector  switches  of  a  whole  group  of subscribers are connected together in parallel as that group of subscribers share a single bank of group selectors. When a subscriber lifts his telephone, cradle switch causes a circuit to be completed back to telephone exchange, signalling that subscriber wishes to make a call. This causes pre-selector switch to step around until it finds a free group selector. Pre-selector switch then stop in this position and group selector is 'seized' by subscriber wishing to make a call.

On  seizing  group  selector,  the  subscriber  dials his  first  digit  and  selector switch  moves  up  to  the  appropriate  row  on  switch  contact  array.  Every final selector has the possibility of connection to 100 lines. 1000 lines are thus divided into 10 groups of 100 each, group being identified by the first digit in the subscriber's number. Vertical motion of the group selector hence selects a final selector in the group associated with first digit dialled. Every individual row  of  contacts,  or  levels,  of  group  selector  is  connected  to  a  bank  of  final selectors associated with a specific group of 100line numbers. Having dialled first digit to select appropriate group, group selector arm then automatically rotates  in  vertical  direction  until  it  finds  a  free  final  selector.  In  final selector, both directions of motion are under the control of subscriber's dial and after  dealing  two  further  digits,  connection  is  established,  providing  called subscriber's to answer his telephone.