What is a Telephone Central Office??

A central office is a structure used by telecommunications companies to house equipment needed to direct and process telephone calls and data traffic.  Telephone Central Offices (CO's) are also known as telephone switching centers, wire centers or telephone exchanges.  Central offices are analogues to electrical substations in the power industry.

The Telephone CO is typically a multi-story building constructed of concrete or masonry and is usually windowless.  The buildings are designed to withstand major climatic events (hurricanes, tornados, thunderstorms) and unauthorized access necessary to ensure the continuity of telephone service and protection of the sensitive electronic equipment.

Rather then plunking a faceless concrete monolith in the center of a city or suburban region telephone companies have had some ingenious methods to design an exterior facade that is compatible yet unobtrusive the the neighbors and surrounding community.

The telephone central office is the foundation for communication in modern society.  Without the quite, efficient operation of a central office, telephone calls and data streams (fax, Internet) would be non-existent.



When you think of an office, usually images of many people, file cabinets, computers and work stations fill our minds.  A central office could be thought of as the corporate headquarters of some mega company where thousands of people work busily throughout the day.  How did a telephone building filled with electronics come to be known universally as a "Central Office"?  

The term "central office" was coined from the early days of telephony.  Right after the introduction of the telephone in 1878, subscribers had to "ring" the office to solicit an operator to complete the call.  As the number of telephone subscribers grew from only a few dozen to a few thousand, human operators were then housed in a central office building to complete calls.  During the 1910s numerous people (operators) would sit at the "switchboard" and route calls day and night.  Back in the early days any call you wanted to make whether next door or across town almost always had to go through the operator sitting at his/her switchboard to be placed.

Soon after as telephone use exploded the human operator was replaced with more efficient and quicker electro-mechanical methods of switching.  So as the operators left their "desks"  to machines, it soon became the "central" hub for all telephone circuits radiating out to users in a defined area.  Hence the term Central Office.  This term remained and is still used today even though the people that now work in the offices are technicians whom repair and monitor the electronic hallways.



A telephone exchange by definition is the first three (3) numbers of a telephone number (not including the area code).  So if your phone number is 555-1234 the exchange would be "555".  This is also known as the number prefix.  In the early days of telephone service, telephone companies thought that remembering an entire 7 digit number would be to difficult and hence replaced the first two (2) numbers in the prefix with letters. ( This is why there are letters on your telephone keypad, three for each key with some exceptions).

So if we take our original number 555-1234 back before about the 1960s the two first numbers "55" would have been replaced with corresponding letters to the key.  In the case of "55" we have the letters: J, K, L to work with and come up with a word.  Lets use KL and the word KLeenix.  Our telephone number would really be KLeenix 5-1234.  So you would tell someone my phone number is Kleenix 51234 and when they dialed your number they would dial the corresponding first two letters of the word on their telephone.  The letters also correlated to the "exchange" or office the telephone was connected to and today some offices are still know after their old "exchange" name.  This seems more confusing then having to memorize now a 10 digit number (including the area code) which we are all use to.