1472 THE BELL SYSTEM TECHXIC'AL JOUK.VAL, XOVEMBER 1957 



ties most frequently found in the telephone plant, and some comments 

 are made on each. 



3.1.1 Voice Frequency Circuits 



There is a variety of open-wire facilities of this type. They are mostly 

 short, and two-wire. Thus, repeaters can to advantage be turned one-way 

 for data service. Delay correction is discussed later. 



Voice frequency cable facilities over more than a very short distance 

 are loaded. This gives appreciable delay distortion. The loading used 

 is indicated by a letter denoting the spacing, followed by a number 

 denoting the loading coil inductance. Thus "H-44" means G,000-foot 

 spacing, of 44-millihenry coils, and "B-88", 3,000-foot spacing of 88- 

 millihenry coils. Conductor capacities range from 0.62 microfarads per 

 mile for toll circuits, to 0.82 microfarads per mile or sometimes even 

 higher for local circuits. This affects the delay distortion. 



3.1.2 Type-C Carrier Circuits^^ 



This is an open-wire three-channel system operating at different 

 frequencies in opposite directions, over the same pair. Historically there 

 has been a variety of C systems developed, but only the C-5 system 

 exists in any extensive quantity. The upper frequency cutoff in the 

 voice channel is well under 3 kc. The delay distortion varies widely 

 with the specific channel and direction of transmission. There is a 

 variety of channel frequency allocations, and the distortion varies 

 with this also. The delay distortion over some channels increases rapidly 

 above 2,400 cycles. The frequency shift discussed before may be as 

 much as ±20 cycles. 



3.1.3 Type-N Carrier Circuits^ 



This is a short-haul twelve-channel system for use over cables. Be- 

 cause of its economy it has been extensively introduced. Its principal 

 characteristic, in the application of data circuits, is that it uses com- 

 pandors. It therefore presents a noise problem. The delay distortion, 

 introduced almost exclusively by the terminals is not excessive, and 

 depends very little upon circuit length between the terminals. The N 

 system uses double sideband transmission, and therefore exhibits no 

 frequency shift between input and output signals. , 



3.1.4 Broadband Carrier Systems Using A Channel Banks^'" 



There is a variety of carrier systems designed for paired cable, coaxial 

 cable, open wire, and radio, that use a standard grouping of twelve 

 channels with associated filters, known as an "A channel bank." The 

 delay distortion in these associated filters constitutes nearly all of the 

 distortion measurable over the complete system. These are single side- 

 band systems, and unless the local modulator and demodulator carrier 



