COMMOX CONTROL SWITCHING SYSTEMS 1113 



ciatc Ihciu ill \;iri()us lypc^s and combinalioiis of tyjx's ol' jjiilsing. Types 

 of outpulsing found today in \aiious systems include revertive, panel 

 call indicator, dial pulsing, dc key pulsing, and multi-froquoncy pulsing. 

 Panel sender tandem and No. 4 toll can also sentl digital information 

 ahead to operators by the call announcer method which uses voice an- 

 nouncements derived from recordings on film. Provision for receiving 

 and sending several types of pulsing in one system makes it more flexible 

 since it can then comiect to a variety of cciuipments. Regenerating the 

 jiulses adds to the range without the need of adding pulse repeaters. 



Some of the advantages which common control systems dei'ivc from 

 the al)ility to operate with a modern type of pulsing can be brought out 

 by a ])rief description of multi-frequency pulsing which is a relatively 

 recent development. Digital information is transmitted over any facility 

 capable of handling voice by sending spurts of alternating current which 

 consist of pairs of frequencies in the voice range selected out of five 

 freciuencies. There are ten such pairs. At the receiving end a check is 

 ma(l(^ to insure that exactly two frequencies are received for each digit. 

 When onh' one or more than two frequencies per digit are detected the 

 call is not set up but a reorder signal is returned to the originating end. 

 In addition to the advantages of being capable of transmission over voice 

 facilities, including repeaters and carrier systems, and of providing checks 

 for accuracy, this type of pulsing can be transmitted at the rate of seven 

 digits per second at present. Operators can be provided with keysets 

 capable of sending IMF pulses into either local or distant SAAdtching equip- 

 ment with improA'ed operating resulting from the higher speed and otlier 

 advantages of MF pulsing. 



It is quite feasible to add new types of pulsing to common control 

 systems. Multi-freciuency pulsing has only recently been added to cross- 

 })ar tandem, for example, although it has been in use with other crossbar 

 systems for some time. In this case it required the development of new 

 senders capable of receiving and sending the MF pulses. The addition of 

 these senders, even in existing offices, is not a difficult job. 



IMPROVED STATION APPAK\TUS 



The stations in most exchanges are pro\'idcd with dials whic^h operate 

 at approximately 10 pulses per second. In stcp-liy-stej) exchanges this 

 pulsing speed is the maximum permitted by the capabilities of the 

 .switches. In panel and crossbar areas the common equipment is capable 

 of operating with higher speed dial pulsing, and PBX and central office 

 operators in these areas are usually given dials that operate at about 

 18 pulses per second. 



