(i) Register marker: Strowger selectors perform counting and searching. However, the crossbar switch has no ‘intelligence’. Something external to the switch must decide which magnets to operate. This is called a Register Marker. Since it takes less than a second to operate the switch, a marker can control many switches and serve many registers. Thus, even a large exchange needs few markers. This is further stage of common control, which we shall call centralized control.
(ii) Conditional selection: When a marker is instructed to set up a connection from a given incoming trunk to a given outgoing trunk, this also defines the link to be used and the select and bridge magnets to be operated to make the connection. The maker does not make the connection until it interrogated the busy/free condition of the outgoing trunk and of the relevant link. Only if both are found to be free does it operate the switches. This is called Conditional Selection.