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Tandem Trunking

In a very small community, limited telephone service could be given without a central office for connecting the lines. Each telephone user, however, would require a telephone line to each of the other subscribers. To use a simple illustration, it is easily shownref.16 that without a central-office switchboard a total of 28 connecting lines and 56 line jacks (seven at each location) would be required to give service between only eight telephone users. With a simple switchboard, however, only eight lines and eight jacks are required.

From the comparison just given, and from the discussions of telephone trunking in multioffice exchanges (page 366), it is evident that connecting the different central offices in an exchange with trunk circuits offers a problem similar to connecting telephone sets. Just as a number of lines and jacks can be saved by using a switchboard to connect telephone sets, so a tandem office and a tandem switchboard can be used to connect telephone offices and thus reduce the number of trunks and the amount of central-office equipment required.ref.17 It will be recalled that, in a large exchange having many offices, a very large percentage of the calls originating in one office must be trunked to another office for completion.

Figure 22. Between each two offices in a multi-office area two groups of trunks are required, each group connecting the "A" board of one office with the "B" board of the other. (Reference 16.)

The saving in trunks and equipment is indicated by Figs. 22 and 23. The tandem office may be considered a central office for switching central offices. This principle is used in both manual and dial systems.

Figure 23. When the tandem plan is used, only two groups of trunks to the tandem office are required. (Reference 16.)



Last Update: 2011-05-30