TELEl'HOSE TKASSMISSIOX M.MSIF.XASCr. 35 



standpHnt, are listed in Talili- I. Tlir oprralinj; features of inatniai 

 tt'lcphoiif systems are j;eiierall\' well known as are also the features 

 of step-l)>-stcp niarhine switehinn s\ stems, lioth ha\inn l)een in use 

 for man\- years. The panel machine swilchini; s\slem which is a 

 relatively recent <le\elopment was described in a pajier \^\ Messrs. 

 Craft, Morehouse and C'harlesworth.'' 



lAMIi: I 



Clitssifitalion of Cirmits in tin' EMhnnf^e .Area Plant ImporUtnl from u Transmission 

 Ma inlena n ce Sta nd point 



Masc.vl Officks 



Subscriliers' loops and sets 

 Operators' telephone sets 



M.vcHiNE Switching Offices 



Panel 



District selectors 

 Incoming selectors 

 Trunk circuits 

 Misil. circuits 



Step by Step 



Connectors 

 Toll selectors 

 Trunk circuits 

 Miscl. circuits 



Subscribers' loops and sets 

 Operators' telephone sets for 

 Special service positions 



General classes of exchange area circuits involving equi|)ment 

 other than contacts and wiring which affect telephone trans- 

 mission. 



While it may appear at first hand from the above discussion that 

 transmission testing in the exchange plant is a complicated and 

 expensive matter, this has not proven to be the case. It has been 

 found by experience that the systematic use of transmission measur- 

 ing sets, following the testing methods which have been developed 

 provides a means for peri(xlically checking transmission conditions 

 with a relatively small amount of testing apparatus and with a small 

 maintenance force. .\\\ of the transmission circuits exclusive of sub- 

 scriliers' lines in a 10,000-line central ofihce, either manual or machine 

 switching, can, for example, be completely tested by two men in a 



'Craft, .Morehouse and Charlesworth, "Machine Switching Telephone System 

 for Large Metropolitan .Areas," Journal of the A. I. E. £., .April, 1923. 



