THE N-1 CARRIER SYSTEM 3 



such as crosstalk balancing, is eliminated; band filter discrimination can be 

 reduced; and signal levels can be raised without undue interference. 



The N system employs a cable pair in each direction. In order to operate 

 in a single cable the two directions are further separated by the use of 

 different frequency bands; 44-140 kc for one direction on one pair; and 

 164-260 kc for the other direction on the other pair. Double-sideband, 

 carrier transmitted operation, very similar to that of a radio system, is 

 used, with channels spaced 8 kc apart. The voice channel bandwidth is 

 250-3100 cycles. The dialing and supervisory control frequency is at 3700 

 cycles. 



Frequency frogging, involving interchange and inversion of frequency 

 bands at each repeater, is accomplished by modulation with a 304 kc carrier, 

 and serves two important purposes: Circulating crosstalk paths around the 

 repeater are blocked; and the system is made self -equalizing for as many as 

 ten repeater sections, having a gross loss of between 400 and 500 db. 



Either paired or quadded, 16, 19, 22, or 24-gauge cable conductors can 

 be employed, with suitable variation in repeater spacing. The nominal 

 spacing of repeaters is 8 miles for 19-gauge and 6 miles for 22-gauge con- 

 ductors. No limitation is placed on the percentage of cable conductors on 

 which N carrier can be applied in a toll cable. Accordingly, as many as 1800 

 channels can be obtained from a 300-pair cable. For built up connections, 

 two N systems can operate in tandem to make up a toll trunk. At the most, 

 not more than 4 to 6 links of N are expected in tandem in a long multilink 

 connection. 



Many additional transmission features are listed and briefly described 

 in Table I. 



Frequency Allocation 



The frequency allocation of the system is shown in Fig. 1. In order to 

 coordinate system frequencies in the same cable some with odd numbers of 

 repeaters, some with even numbers, and some circuits starting or stopping 

 at intermediate repeater points of other systems, it is necessary to arrange 

 the terminals to transmit and receive either high or low group frequency 

 bands. The channel modulators and demodulators in the terminals, how- 

 ever, use carriers only in the high group band at 8 kc intervals between 168 

 and 256 kc. Thus, when transmitting high group frequencies to the line 

 and receiving low group frequencies, the high group transmitting unit 

 (HOT) merely amplifies the channel frequencies. The associated low group 

 receiving unit (LGR) however, employs a group modulator with 304 kc 

 carrier that inverts the received low group of line frequencies into the upper 

 band for channel separation in the receiving channel band filters. When 



