1452 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1957 



the nature of the transmission compromises that might be needed. As 

 a result, a variety of data transmission systems in different stages of 

 development have been investigated including some telegraph systems. 

 Certain conclusions have been reached regarding their suitability for 

 use over private line telephone facilities. (By "private line" facilities 

 are meant facilities leased to the subscriber on a more or less permanent 

 private basis, and that are not set up by operators at a telephone central ' 

 office switchboard.) 



These conclusions are summarized below. In the later text there is 

 included the background material for the conclusions which includes a 

 brief characterization of the facilities. Finally there is included a dis- 

 cussion of the line treatment which may be needed for the best trans- 

 mission. 



It should be emphasized that not all possible data systems for use 

 over telephone circuits are covered. The problem considered covers 

 particularly some recently proposed applications, for which the need of 

 a relatively high bit rate is important. Also, only the more promising 

 comparatively conventional systems, which have been relatively well 

 tested and can be readily applied, are considered. More radical designs 

 are conceivable but they would require more extensive investigation 

 before conclusions could be reached concerning them. It is clear also 

 that the designs involved in the choice of a system are determined by 

 the type of service it is to provide. 



1.1 Conclusions 



It is concluded that about the fastest transmission of data which can 

 be accomplished with the present art over message-type telephone 

 facilities is obtainable with an amplitude modulated vestigial sideband 

 system. Such a system will be presented in some greater detail below. 

 Its frequency spectrum is similar to that of a telephotograph signaP and 

 a number of the transmission problems involved are the same. 



This system will provide about 1600 binary digits of data (or "bits") 

 per second, but it requires some special selection and considerable treat- 

 ment of many types of circuits. This treatment is necessary to reduce 

 noise, particularly of the impulsive type, and to correct for delay dis- 

 tortion. 



Such a system is therefore considered suitable where a high bit rate 

 is essential. It will be developed in the text that beyond the matter of 

 the delay correction and other treatment of circuits required, the vestigial 

 sideband process imposes a certain signal-to-noise penalty. A further 



