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

telephone exchange telephone exchange, telephone switch, or central office is a telecommunications system used in the public switched telephone network (PSTN) or in large enterprises. It interconnects telephone subscriber lines or virtual circuits of digital syste ...

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. 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 The Western Electric Company was an American electrical engineering and manufacturing company officially founded in 1869. A wholly owned subsidiary of American Telephone & Telegraph for most of its lifespan, it served as the primary equipment ma ...

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 Elec ...

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 configuration. A crossbar switch has multiple input and output lines that form a crossed pattern of ...

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 workhorse in some large

private branch exchange A business telephone system is a multiline telephone system typically used in business environments, encompassing systems ranging in technology from the key telephone system (KTS) to the private branch exchange (PBX). A business telephone syst ...

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/Public switched telephone networ ...

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 varia ...

hardware (a remote switching centre controlled by a digital switch in the city) or with small digital switches such as Nortel's

DMS-10 Digital Multiplex System (DMS) is the name shared among several different telephony product lines from Nortel Networks for wireline and wireless operators. Among them are the DMS-1 (originally named the DMS-256) Rural/Urban digital loop carrie ...

(a self-contained exchange for up to 8,000 subscribers).

References

{{telecommunications Telephone exchange equipment