1456 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1957 



1.4 Outline of Paper 



As a result of the study, some recommendations were made. The dis- 

 cussion of this material is presented in Section II. It covers first the sort 

 of service, with comparatively high bit rate, for which there has been 

 user demand, and which can be furnished over private line facilities. 

 Secondly, the recommendations cover the broad features of the signal 

 characteristics which appear promising for use over such facilities. These 

 recommendations are at the present time evolving into an exploratory 

 system. The recommendations themselves, together with some general 

 remarks, are presented in Sections 2.0 and 2.1. The background material 

 on which these were based, and which covers consideration of the five 

 systems which have been listed, is presented m Sections 2.2 to 2.5. 



The discussion on the nature of the problems involved in transmission 

 of the signals over the telephone plant to be used (which above has 

 been listed in five categories) is finally covered in Section III. 



II. SYSTEMS OF BINARY SIGNAL TRANSMISSION 



2.0 Geyicral Remarks 



There are a number of arrangements in the present art, both experi- 

 mental and commercial, which are capable of sending digital data infor- 

 mation under something like the conditions required for the service 

 con.sidered. Study of these has led to a set of recommendations which 

 are outlined below. The specific arrangements are then discussed in 

 more detail. The description covers only the essential transmission 

 characteristics of the arrangments. 



The arrangments generally divide into tAvo groups, those which are 

 essentially short-pulse single channel (though some of these may include 

 a slow auxiliary channel) , and those which use frequency division multi- 

 plex channels and therefore employ longer individual pulses. One impor- 

 tant advantage of the multiple channel systems is noted in Section 3.3 

 as consisting of an increased immunity against impulse noise. 



The single channel group shows much similarity among the systems. 

 The principal difference is that of bit rate. The faster systems use ves- 

 tigial sideband transmission, at the expense of a certain increase in 

 vulnerability to noise as compared with those which use double side- 

 band transmission, and also at the expense of a more general need for 

 delay distortion correction because of the speed. 



The systems in this group which use vestigial sideband are expected 

 to be able to operate, with a very low error rate (some 1 error per 100,000 



