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Let’s Go Trunking

The materials in this section are drawn directly from an excellent article by Charles Chatsworth in the Bell System Technical Journal [Chatsworth].

First, what is a trunk? It’s usually a 2 or 4-wire cable connected between two offices. It is used to carry the talking path and often for call signaling during call completion and after it ends. In the 4-wire configuration, voice is usually carried via two separate paths: to and from the endpoints.


 No single exchange operates in isolation; it needs connections to other offices to form a vast network. Trunks are the connective tissue to make this happen. The figure illustrates the range of trunking layouts which might be used. With the 10 offices assumed and direct trunks (2-way) between each office and every other office, 90 groups of trunks would be required.

Trunking plans diagram for NYC comparing 3 routing methods

In the 'full tandem' arrangement (middle image), each office connects to every other office through a central point; a Toll or Tandem Office. For this kind of office, there were no subscribers directly attached only other offices. Twenty trunk groups would be needed.  Between these two extremes with some offices reaching certain other offices through the tandem center and certain others by direct trunks, a great many combinations would be possible. In the case assumed (right image), 50 groups appeared to be the best combination.

The data at the bottom of the figure pertain to the NYC area trunking plan.  If only direct trunks were employed in the metropolitan area, some 43,000 groups would be required. On the other hand, if we followed only the strictly tandem plan, 850 groups would be required. However, in this case, unwarranted switching costs would be involved. By establishing a plan, however, involving both tandem and direct trunks, the most economical plan can be determined upon and in this case about 9,000 groups of trunks are required.

On extended trunk lines, voice attenuation posed challenges. Additionally, rapid signaling was challenging due to the capacitive and inductive components on the lines.  Nonetheless, trunking between offices was essential for building a holistic telephone network. See the section dedicated to Traffic Engineering to learn more about this important and fascinating topic. 


Charles Chatsworth,  General Engineering Problems of the Bell System, Bell System Technical Journal, October 1925 

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