THE TYPE PI CARRIER SYSTEM 381 



Methods used generallj^ throughout the Bell System.* These facilities 

 consist of mixed gauges of high capacitance cable extended at their outer 

 ends by 109-mil steel, 104 mil-copper or copper steel open wire. 



In engineering the carrier line design or carrier layout, the Plant Engi- 

 neer will determine the carrier line laj^out necessary to meet the over-all 

 requirements for a suitable carrier transmission path on the available 

 physical facilities. To do so he need only be familiar with the general 

 capabilities of the carrier system, its basic "building blocks," and the 

 limitations that must be considered in applying the system to the physi- 

 cal line. The capabilities of the carrier system have been described in 

 earlier sections. From those descriptions it can be seen that the basic 

 "building blocks" for a PI carrier system are: 



1. Central office channel terminals 



2. Remote channel terminals 



3. Repeaters 



4. Ac or dc remote terminal and repeater power supplies 



5. Carrier line networks and filters 



A carrier application of these "building blocks" is shown in schematic 

 form in Fig. 26. 



The low-pass filters or carrier blocking networks shown are placed at 

 the junctions of the carrier line and side leads of customer drops served 

 by ph}' sical or derived voice frequency circuits on the base carrier facil- 

 ity. These filters are required to reduce the bridging loss of the side leads 

 at carrier frequencies and to keep carrier frequencies out of the customer 

 drops to prevent annoyance to the customers. High-pass filters are pro- 

 vided to make the carrier line continuous at carrier frequencies, but 

 divide it into isolated sections for voice frequency distribution. 



In addition to these blocks, an autotransformer may be required at 

 the junction of the open wire and cable. The autotransformer, either 

 alone or in conjunction with a junction line filter, is required to eliminate 

 reflection losses and reduce crosstalk at carrier frequencies due to im- 

 pedance mismatch between the cable and open wire. The junction line 

 filter is required to allow the carrier and physical voice frequency circuit 

 to be used on different pairs in the cable and on the same open-wire pair 

 beyond the cable-open-wire junction. This is necessary where the physi- 

 cal circuit is so long that load coils are required on the voice frequency 

 cable pair and non-loaded cable pairs are required for carrier. A pair 

 of junction line filters may also be used to provide a voice frequency 

 by-pass around a repeater. As illustrated in Fig. 26, this may be necessary 



*L. B. Bogan and K. D. Young, Simplified Transmission Engineering in E.\- 

 change Cable Plant Design, A.I.E.E. Communication and Electronics, No. 15 

 page 498, Nov. 1954. 



