On Distributed Communications Series

V. History, Alternative Approaches, and Comparisons

Contents
Preface
Summary
I. Introduction
II. The Distributed Network Concept
III. Early History
IV. Specific Hardware Proposals
V. Conclusions
Appendix A. Summary Charts

Appendix B

The DDD System

In the Bell System Direct Distance Dialing (DDD) network (Fig. 15), telephone number dial pulses are relayed to a Toll Center; if there are no lines available to the desired end destination, the call is routed to the next higher hierarchical level, the Primary Center. If again there are no free circuits to the end destination, the call is relayed via a Sectional Center, and thence to a Regional Center. (All Regional Centers in the United States are connected to each other.)

Thus, a telephone call starting from any place in the country will work its way up the hierarchical network, seeking an end link to the called telephone via a Toll, Primary, Sectional, or Regional Center, then back down the chain to the end telephone. Destruction of a small number of the upper echelon targets is magnified in the remainder of the communication network. The hierarchical procedure increases the target value of upper echelons of the switching network.

An ever increasing number of calls in the Direct Distance Dialing System are being transmitted from one Toll Center to an end Toll Center by direct trunk lines without requiring transmission through the upper echelons. In telephone language, these direct paths are called "high usage groups." The feasibility of being able to use such point-to-point non-switched high-usage circuit groups increases as the number of subscribers using the long distance telephone plant increases.

Each of the links shown in Fig. 16 is made up of channels following separate paths, or using "diversity of assignment" (see ODC-I).

While determination of the vulnerability of switching will depend upon the actual numbers of options tested at each switching center, it is found in practice that very few of the large set of all possible connections existing are ever tried, as alternate routing is designed primarily to effect more economical handling of traffic.[1]

Figure 16 shows that in practice most connections are established by the use of only a few tandemly connected links. Therefore, it appears reasonable to assume that the survivability curves for diversity of assignment may be indicative of network performance.


[1]Truitt, C. J., "Traffic Engineering Techniques for Determining Trunk Requirements In Alternate Routing Trunk Networks," The Bell System Technical Journal, Vol. 33, March 1954, pp. 277-304.


List of Publications in the Series