300 THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1953 



Common Control Circuits — Markers 



The switching of all connections in the office is performed by a group 

 of common control circuits known as markers, any one of which may be 

 utilized on a particular call. The principal functions of the marker are 



(1) to determine or receive the specific location of a calling circuit; 



(2) to translate input signals into the specific location of a called circuit 

 or group of circuits; (3) to test for availability and seize a called circuit 

 or one of a group of circuits; (4) to locate, test and seize a s\vitching path 

 between calling and called circuits; (5) to set up the connection; and 

 (6) to take alternative action in case of trouble or busy conditions. A 

 marker performs these functions in a very short period of time so that a 

 few circuits can handle the requirements of an office. In the original 

 design of No. 5 crossbar, a single type of marker handled all connections. 

 This was the arrangement specifically handled by the throwdown ma- 

 chine. Later design has introduced three types of markers; dial tone, 

 completing, and combined. 



As an example of the function of the control circuits, when a sub- 

 scriber originates a call, a connection is automatically established from 

 the subscriber fine circuit on a line link frame via a marker connector to 

 an available marker. The marker identifies the location of the line and 

 establishes the fact that it is a new call requiring a register. It tests all 

 registers and trunk link frames and chooses an idle frame with idle 

 registers. The marker then gains access to the correct line link and trunk 

 link frames via the frame connectors, chooses an idle register, tests all 

 usable channels, picks a particular channel and operates the crossbar 

 switch magnets to close the connection between line and register. After 

 storing the line location in the register for later use, the marker discon- 

 nects itself. 



When the subscriber completes diaUng, the register connects itself to 

 an idle marker via the marker connector. It transfers to the marker the 

 location of the originating line and the called number. If the call is 

 local to the office, the marker determines the location of the called line 

 from the number group circuit (a translating device) and tests and 

 chooses an intraoffice trunk. The marker then gains access to the link 

 frames through the frame connector, tests the called line for busy, picks 

 a channel, and establishes the connection, thereafter removing itself 

 and the register from the connection. 



During the course of the foregoing events, a call may encounter var- 

 ious delays beyond the minimum circuit operating time in setting up the 

 connection. Delay may be caused by meeting a temporary busy condi- 



