A "Community Dial Office" (CDO) was a small Class 5

telephone exchange A telephone exchange, telephone switch, or central office is a central component of a telecommunications system in the public switched telephone network (PSTN) or in large enterprises. It facilitates the establishment of communication circuits ...

in a rural area. These most often provided capacity for 1,000 or fewer customers and were designed for unattended operation. CDOs could be step by step, all relay or

crossbar Crossbar may refer to: Structures * Latch (hardware), a post barring a door * Top tube of a bicycle frame * Crossbar, the horizontal member of various sports goals * Crossbar, a horizontal member of an electricity pylon Other * In electroni ...

. Many offices provided four-digit local dialling to small rural communities where a call outside the local exchange was long distance. Initial deployments were based on step-by-step equipment such as the

Western Electric Western Electric Co., Inc. was an American electrical engineering and manufacturing company that operated from 1869 to 1996. A subsidiary of the AT&T Corporation for most of its lifespan, Western Electric was the primary manufacturer, supplier, ...

350A (first deployed on May 27, 1928, in Arcadia, California) and its successor models 360A, 355A and 356A. These switches had some design similarities to No. 1 step-by-step systems already in use in large offices (10,000 subscribers or more) but were of a simpler design (line finders would only need to scan hundreds of lines instead of the thousands in use in the cities) and not designed for expandability. Any operator assistance or toll call billing for these unattended stations was handled remotely at a larger exchange in the city. In the 1960s,

Northern Electric Northern Electric was an electricity supply and distribution company serving north east England. History It had its origins as the North Eastern Electricity Board, formed as part of the nationalisation of the electricity industry by the Ele ...

developed the SA-1 community dial office as a small Class 5

crossbar switch In electronics and telecommunications, a crossbar switch (cross-point switch, matrix switch) is a collection of switches arranged in a Matrix (mathematics), matrix configuration. A crossbar switch has multiple input and output lines that form a ...

for local use in rural areas. In the late 1970s, Western Electric's number 3 Electronic Switching System (ESS) was installed in small communities with up to 4,500 telephone lines; it also served as a workhorse in some large

private branch exchange A business telephone system is a telephone system typically used in business environments, encompassing the range of technology from the key telephone system (KTS) to the private branch exchange (PBX). A business telephone system differs from ...

installations. Late in the 20th century the smallest offices were replaced by

remote concentrator In modern telephony a remote concentrator, remote concentrator unit (RCU), or remote line concentrator (RLC) is a concentrator at the lowest level in the telephone switch hierarchy. Subscribers' analogue telephone/PSTN lines are terminated on con ...

s connected to a host office in an adjacent exchange area, which may be several miles away. Others were replaced with

distributed switching Distribution may refer to: Mathematics *Distribution (mathematics), generalized functions used to formulate solutions of partial differential equations * Probability distribution, the probability of a particular value or value range of a vari ...

hardware (a remote switching centre controlled by a digital switch in the city) or with small digital switches such as Nortel's DMS-10 (a self-contained exchange for up to 8,000 subscribers).

References

{{telecommunications Telephone exchange equipment