848 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1952 



end a further loss of two db is added. This loss which is four db greater 

 than the via net loss is known as "terminal net loss." (TXL) 



The data and methods used in the derivation of the via net loss are 

 rather complex and not within the scope of this paper. 



Assignment of Facilities Among Trunks 



The definite routing patterns established for the toll machine switching 

 operation impose more severe transmission conditions on certain classes 

 of circuits than on others. For example, a trunk in a ''final" group be- 

 tween a TC and a PO can become involved in an eight-link connection, 

 whereas a trunk in a "high usage" group, say, between a PO and another 

 PO will not be involved in more than a three-link connection. 



This creates a need and provides an opportunity for allocation of the 

 available facilities among the various tnmk groups in a way that will 

 provide the best overall service. For example, to the extent practicable 

 it is desirable to assign carrier grade facilities to trunks in "final" groups 

 that may be involved in connections with the maximum number of links. 

 Facilities with less favorable transmission characteristics ma}" then be 

 reserved for trunks in groups that are used for connections involving 

 fewer links. 



TRANSMLSSION PERFORMANCE 



Table I shows the approximate range of transmission losses between 

 toll centers under the manual plan compared to ranges that appear 

 practicable under the proposed fundamental plan, which, of course, per- 

 mits more links in tandem. 



Trunk Transmission Stability 



It is as important that the transmission loss of a trunk used in the 

 contemplated toll dialing network be maintained at or close to its 

 assigned value at all times as that the assigned value be right. On multi- 

 switched connections even a relati^'ely small consistent excess or defi- 

 ciency in the loss in the indi\idual trunks can accumulate to overall 

 excesses or deficiencies in loss large enough to cause difl^culty - by making 

 it hard for people to hear if the attenuation becomes too great or by 

 creating excessive echo, crosstalk or noise if the loss becomes appreciably 

 less than normal. 



This subject has been extensiveh' studied for the past several years 

 and it appears that some changes in practices and the introduction of 



