1470 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1957 



good operation'' over toll circuits of comparatively limited length (350 

 miles for four-wire voice frequency, 1,900 miles for K carrier). The higher 

 speed system was designed only for local plant. 



It is clear that this system has too low a bit rate for the application 

 contemplated, even in the faster form. It is also not generally adaptable 

 to the variety of circuit lengths which are expected to be encountered. 



III. UTILIZATION OF THE TELEPHONE CHANNEL 



The discussion herewith covers a broad examination of some major 

 characteristics of telephone communications facilities, to evaluate their 

 bearing on the choice of a system for the data transmission service 

 outlined before. It is of course clear that different conclusions might be 

 reached for other types of service. 



The first item is an outline review of the different types of message 

 telephone facilities in the plant. This is followed by an analysis of the 

 different possibilities in the use of the frequency spectrum, of noise, and 

 of delay distortion, in the application of the data signals. 



3.1 Telephone Facilities 



There is a rather wide variety of facilities to be found in the tele- 

 phone plant, to be examined with respect to the factors that are ger- 

 mane to the present question. 



The first of these factors is the frequency bandwidth capability of 

 the facility. For message-type voice circuits, this is generally character- 

 ized as being three kilocycles (with the exception of "emergency banks," 

 which are substantially narrower, and are not to be considered as 

 useable for data transmission).^^ However, some of the telephone circuits 

 in the plant, aside from the emergency banks, are also somewhat nar- 

 rower than 3-kc, and in any case, not all the band is effectively usable 

 for data transmission. As will be seen, the net a^^aiIable band is, in 

 practice, about half of the 3kc. 



Part of the reason that not all of the frequency band is effectively 

 usable is that the circuit shows delay distortion. This tends to become 

 large at both the lower and the upper edges of the band. Some details 

 of the delay correction are discussed further below. 



Another impairing factor in telephone facilities is the nonlinear dis- 

 tortion encountered. In voice frequency facilities this comes from ampli- 

 fiers and loading coils, and increases progressively with circuit length. 

 In carrier facilities the nonlinear distortion arises almost exclusivelv at 



