PRIVATE LINE DATA TRANSMISSION 1457 



hits), over telephotograph type circuits, which employ delay distortion 

 correction. This is a "normal" condition error rate, and does not in- 

 clude abnormal circuit conditions, such as static, trouble conditions, 

 etc. It also assumes a more or less even error distribution. 



One of these systems gives very desirable word start indications by 

 using an additional level of signal. "Words" are groups of signal elements 

 of fixed total number each. A distinctive separation signal greatly 

 simplifies their recognition at the recei^'er, particularly after short line 

 interruptions. 



As the price, however, of both the speed and the third level signal 

 indication, this system is more vulnerable than the others to impulse 

 noise, of the kind encountered in hitherto installed Nl carrier and open- 

 wire circuits, which have not been treated for impulse noise. It is also 

 more vulnerable than the other systems to sudden level changes in the 

 received signal. 



The multiple channel group is generally characterized by a lower bit 

 rate. Of all of them only one, the frecjuency-shift carrier telegraph, 

 shows capability of consistent operation over untreated Nl carrier. On 

 this type of carrier the frequency-shift telegraph permits a bit rate of 

 the order of only half that obtainable with the vestigial-sideband S3^s- 

 tems. This telegraph system performs much better over other types of 

 telephone facilities but even then its bit rate is only about three fourths 

 that desired. 



2.1 Digital Data Transmission Service 



2.1.1 Some Desirable Major Requirements 



1. Transmission of a maximum of 1,600 bits per second with an 

 error rate not to exceed one in every 100,000 bits (or once per minute). 



2. Applicability to most telephone facilities for private line use. 

 Some selection of circuits and some treatment of those selected will be 

 acceptable. The circuits carrying data are to be one-way terminal 

 circuits only (i.e., not to be linked in tandem). It is expected that for 

 the bulk of the service the circuits would not be likely to run over some 

 200 to 300 miles in length, but a small number of 3,000-mile circuits 

 is considered possible. 



2.1.2 Characteristics of the System 



The system which has been considered and recommended as promising 

 for the service outlined above has the following essential characteristics: 



1. Carrier at 2,000 cycles, with vestigial band extending up to some 

 2,400 cycles. Lower nominal effective band (half the bit rate in width) 



