80 BELL SYSTEM TECHNICAL JOURNAL 



common circuits or "markers" to control the operation of the switches. 

 This has permitted the use of large assemblies of switches and asso- 

 ciated relays on unit frames which can be wired and completely tested 

 for operation in the factory before the units are shipped. 



In the design of the switching frames and associated control circuits, 

 one of the objectives realized has been the standardization of a rela- 

 tively small number of different types of equipment units, thereby 

 simplifying manufacture and merchandizing. This also simplifies the 

 engineering of the equipment by the Telephone Companies in the 

 preparation of their specifications to meet the particular trafitic 

 requirements of the various central offices. 



The marker system used for controlling the switching operations 

 has many advantages, the more important of which will be disclosed 

 later in the general description of the operation of the equipment. 

 It might be mentioned here, however, that the marker is an equipment 

 unit consisting almost entirely of relays, which completes its functional 

 operations in the establishment of a call in a fraction of a second. 

 This short operating time permits a few markers to handle the entire 

 traffic in the largest office. The markers are connected momentarily 

 by means of multi-contact relays to the various switching units of 

 the office to control the establishment of the calls through the crossbar 

 switches. 



An outstanding advantage of the marker system of control is the 

 "second trial " feature, by means of which two or more attempts can be 

 made to establish a call over alternate switches and trunks when the 

 normally used paths are all busy. The markers are arranged to 

 detect short-circuited, crossed, grounded and open-circuit conditions 

 at all vital points, and before releasing from a connection they make 

 circuit checks to insure that the connection has been properly estab- 

 lished. When trouble conditions are detected, they make a second 

 attempt to complete the connection, after sounding an alarm and 

 recording the location and nature of the trouble encountered. The 

 marker system facilitates the introduction of new service features and 

 changes in operation, which may be found desirable from time to 

 time, due to the fact that the principal controlling features of the 

 entire system are vested in a small number of markers. 



Apparatus 

 Crossbar Switch 

 The crossbar switch from which the system derives its name is the 

 basic switching unit of the system. Figure 1 shows the front view of 

 a 200-point crossbar switch. 



