TOLL OFFICES AND TRUNKS
Figure A-5 depicts several lines and types of equipment and types of "offices" found in the public network. Most of the terms in this figure are self-explanatory, but it should prove useful to amplify some of them:
Trunk: A communication channel between two switching systems.
Tandem office: A broad category of office that represents systems that connect trunks to trunks. Local tandem offices connect trunks within a metropolitan area. Toll offices connect trunks in the toll part of the network. With some exceptions, the end customer is not connected to a tandem.
Toll connecting trunk: A trunk between an end office (local office) and a toll office.
Figure A-5. Offices and trunks