Some Design Features of the N-1 Carrier Telephone System 



By W. E. KAHL and L. PEDERSEN 



Introduction 



The economies which result from sharing the cost of line facilities among 

 a number of channels, and the transmission advantages of carrier circuits 

 (in the form of high speed transmission which minimizes delay and echo 

 effects, low net loss and high quality), have combined to bring about a 

 revolution in long distance telephony. Whereas fifteen years ago only 8% 

 of the toll circuit mileage of the Bell System was furnished by carrier, today 

 carrier circuits comprise about two-thirds of the total mileage. The mini- 

 mum distance, however, for which carrier can economically replace voice 

 frequency transmission has been limited by the cost of the carrier equip- 

 ment, the cost of line treatment, the expense of installation and associated 

 job engineering, and the maintenance effort required. As a result, the shorter 

 toll circuits, relatively large in number though not in circuit mileage, have 

 continued to operate at voice frequency. The newly developed Type N-1 

 Carrier Telephone System is aimed primarily at expanding the application 

 for carrier into this field of short haul service. As explained elsewhere^, it 

 is designed to obtain the advantages of carrier for toll and exchange cable 

 circuits for lengths a small fraction of the previous economic minimum. 



Many system and circuit features contribute to this end. There are 12 

 channels per system with 8 kc spacing between carriers. The carrier and 

 both sidebands are transmitted. AU of the pairs in a single cable can be 

 used for Type N without special cable treatment. Repeaters are spaced 

 6 to 8 miles apart depending upon the gauge of the cable conductors. Power 

 is fed over the cable pairs to two out of every three repeaters, which can be 

 pole mounted. Different frequency bands on different pairs in the same 

 cable are used for opposite directions of transmission, 44-140 kc in one 

 direction, and 164-260 kc in the other. The frequency bands are interchanged 

 and inverted at each repeater to avoid important types of crosstalk, and to 

 provide automatic equalization of attenuation slope. Compandors, built 

 into the channel terminals, raise the lower speech volumes prior to transmis- 

 sion and restore them after reception, thereby minimizing the severity of 

 crosstalk and noise problems on the line and in the terminals. An out-of- 

 band signaling channel immediately above the speech band is provided by 

 built-in equipment. 



* "The Type N-1 Carrier Telephone System: Objectives and Transmission Features," 

 R. S. Caruthers, January 1951 issue of this Journal. 



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