956 THE BELL SYSTEM TECHNICAL JOURNAL, JLUY 1956 



INTRODUCTION 



Currently there are over 230,000,000 long distance calls made in the 

 Bell System per month. They range from relatively simple connections 

 involving a single intercity trunk to complex connections involving sev- 

 eral intercity trunks in tandem, perhaps totaling 4,000 miles in length. 

 In each case there is a toll connecting trunk at each end. Almost half of 

 this traffic involves distances over 30 miles. The transmission engineer's 

 problem is how to provide uniformly good and dependable transmission 

 so that every one of these calls will be satisfactory to the customers in- 

 volved. To accomplish this requires among other things that: 



1. The design loss of every trunk must be the lowest permissible from 

 the standpoint of echo, singing, crosstalk and noise. 



2. The actual loss of every trunk must be kept close to the design loss 

 at all times. 



Meeting the first requirement is a matter of system design and circuit 

 layout engineering. The factors involved have been covered in a previous 

 article.^ Meeting the second requirement is an important function of the 

 maintenance forces and is discussed in this article. 



THE PROBLEM OF NET LOSS MAINTENANCE 



The transition from manual operation under the ''general toll switch- 

 ing plan" to dial operation under the "nationwide dialing plan"^- ^ is re- 

 quiring material changes in intertoll trunk design and also in techniques 

 for maintaining these trunks. While precise maintenance is becoming in- j 

 creasingly necessary, it is also becoming more difficult to achieve. There 

 are three important reasons for this. 



First, the nationwide dialing plan increases both the possible number 

 of trunks used in tandem for a given call and the variety of the connec- 

 tions in which any particular trunk may be used. This increases' 

 the chances of impairment due to deviations from assigned loss in indi- 

 vidual trunks since these deviations may combine unfavorably in multi- 

 switched connections. To minimize this, the transmission stability of the 

 individual trunk links must be better than under the old plan. 



Second, more and more of the trunks are being put on carrier because 



* H. R. Huntley, Transmission Design of Intertoll Telephone Trunks, B. S.T.J. , 

 Sept. 1953. 



2 H. S. Osborne, A General Switching Plan for Telephone Toll Service, B. S.T.J. , 

 July, 1930. 



' A. B. Clark and H. S. Osborne, Automatic Switching for Nationwide Tele- 

 phone Service, B.S.T.J., Sept., 1952. 



* J. J. Pilliod, Fundamental Plans for Toll Telephone Plant, B.S.T.J., Sept. 

 1952. 



