system of microphone, wire, and earpiece. Such a telephone circuit, although simple, lacks flexibility of
operation. As soon as additional telephones are connected to this simple circuit, the users of the system interfere
with each other. To prevent this interference and to achieve flexibility of operation, switchboards are inserted in
the lines between the telephones. Dc voltage is needed to furnish transmission power in most telephone systems.
This dc voltage may be furnished either by the switchboard or by the individual telephones. When power is
furnished by the switchboard, the system is called a common-battery system. When power, usually batteries, is
furnished by the individual telephones, it is called a local-battery system.
b. A Multistation Telephone System. A switchboard or other switching device is usually located at a
telephone central office. A telephone system with one central office consists of a number of telephone stations
connected by lines to the central office, so that any two telephones of the system may be connected for two-way
conversation. Such a system may serve a few stations or thousands of stations. A complete multistation
telephone system includes the individual telephone stations, outside plant equipment, central offices, and
(1) The outside plant equipment consists of the interconnecting transmission lines and support facilities
such as poles or underground conduits.
(2) Central offices include all the equipment required for making connections between the telephones and
the switchboards, and for making connections between associated switchboards.
(3) Transmission lines used to connect telephones to switchboards are called local lines, while the
transmission lines interconnecting switchboards are called trunks.
2-3. THE TELEPHONE SWITCHBOARD
a. Telephone System Without a Switchboard. A telephone system frequently consists of hundreds, even
thousands, of telephone stations. In operation, the system permits voice communication between any of the
telephone stations which are part of it. Without a switchboard, all of the stations in the system would be tied
together with a maze of wires, as shown in figure 2-1. It is apparent from this figure that whenever one person is
talking, everyone else must listen. If a second user of the system desires to direct his message to a different
telephone, confusion due to interference is bound to result. In spite of the handicap of this type of system, it still
finds use in a conference network, wherein all parties can hear any one speaker, and each speaker in turn may
b. Telephone System Having a Switchboard. An important saving in wire is obtained by including in the
system a centrally located switchboard, as shown in figure 2-2. Each telephone station is then connected directly
to the switchboard, not to any of the other telephone stations. The connecting wires and their attachments
constitute a transmission line. Conversation between any two stations without interference with the others is
made possible by connecting their transmission lines at the switchboard. The connections are made by a
switchboard operator or attendant by means of connecting (switchboard) cords.