1116 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1952 

 ADAPTABILITY TO NEW SERVICE FEATURES 



One of the major advantages of common controls, which has been 

 covered in part but which deserves further emphasis, is their adaptability 

 to new service features. Key sets and new dialing devices can be intro- 

 duced at customers' stations and operator positions by readily feasible 

 modifications of registers and senders. New pulsing schemes can also be 

 introduced as they are developed as evidenced by the introduction of 

 multi-frequency pulsing over the past few years. Nationwide customer 

 dialing, now under development, can be readily introduced in existing 

 common control systems by economical modifications without the use 

 of either directing codes or second dial tone. Step-by-step systems re- 

 quire at least partial senderization to provide equivalent service. In 

 short, the flexibility of common controls and the concentration of the 

 control elements in a relatively few circuits makes the addition of new 

 service features easier and more economical than in direct dial systems. 



MAINTENANCE ASPECTS 



Experience has shown that switches with a large amount of motion, 

 especially those with brushes which wipe over bank terminals, tend to 

 wear excessively and require considerable maintenance effort and even 

 replacement, at times. On the other hand, s^\itches with short motions 

 and relay-like action require little maintenance and tend to have long 

 life. Furthermore, the switches which employ wiping brushes mostly use 

 base metal contacts, whereas relay-like switches can readily be equipped 

 with precious metal contacts — and in most cases are so equipped — ^with 

 the elimination of the transmission noise to which base metal contacts 

 are subject. The crossbar switch is a relay type of switch Avith precious 

 metal contacts and considerations such as those mentioned influenced its 

 adoption. The advantages of relay type switches are not necessarily 

 limited to common control systems since such switches have been used 

 in direct dial control systems. The first use of the crossbar switch in 

 Sweden was in a step-by-step system, for example. However, economical 

 arrangements for using such switches in large systems require markers. 

 This is because economy must be achieved by having more than one call 

 ()(!cupv a switch at a time and marker control is necessary to attain this 



Important maintenance advantages have been introduced in systems 

 using decoders and markers. In this category are the self-checking fea- 

 tures, second trials with changed order of preference, and trouble report- 

 ing features. In No. 5 crossbar the al)ility to report the location of a line 



