How telephone set is connected to the Telephone Central Office or Exchange Office | applied electronics engineering


How telephone set is connected to the Telephone Central Office or Exchange Office

By Applied Electronics - Friday, April 14, 2017 2 Comments
Telephone is the oldest form of communication system used today. Still today, with invent of wireless communication, telephone system are used for home and offices. The telephone network is called PSTN(Public Switched Telephone Network). To understand telephone system one has to understand the connection between the Telecom system in the Telephone carrier company and the telephone set used at home. Here some of the basic things about the connection between the telephone carrier office switch and the telephone set in the home or office are explained.

Below is illustrated a basic diagram that shows how a telephone exchange office is connected to home telephone office via local loop.

The local loop is just the name given to the section of communication channel between the telephone exchange office and the home/office telephone set. It is a simply a two wire connection. The ends of the wire has connector called called the RJ11 which stands for Registered Jack 11.

The telephone office passes DC and AC signal to and from the Telephone set. The telephone office in the on-hook condition sets up a DC voltage in the telephone set using the local loop. This is illustrated below.

Note that the above figure is highly simplified. It merely indicates that the telephone exchange office sets up a DC voltage of 48V between the two wires in the local loop. The loop loss recommendation or standard is 8dB. This voltage is setup in the user telephone set.

The user telephone set has distinguishing kinds of circuits inside it performing different function. The different circuits are the ringer circuit, the speech circuit, the voltage regulator, the on-hook and off-hook main switch, the keypad, the dialer circuit etc.

When there is a call to the user. The central office sents out ringing signal which is an AC signal of magnitude of 90V RMS and frequency of 20Hz. Once the telephone handset is picked up, that is in the off-hock condition, a loop current flows which is about 20mA to 80mA.


2 comments to '' How telephone set is connected to the Telephone Central Office or Exchange Office "

  1. Thank you for your article. I just have one question, does the telephone exchange has the same meaning as the central telephone or it is just the connection between telephone set in office and the central telephone?

  2. central office and exchange office are both telco(telephone company) offices and if you want to be specific exchange office is more specialized than central office. exchange office contains all equipments that central office has but not vice versa.

    home telephone is connected to central office. the central offices are connected to exchange office. exchange offices are connected to regional exchange offices(eg cities) and to the international gateway.

    so it depends on where the called telephone is located. if the called user telephone is connected to the same central office as your telephone then only the central office is involved. but if the called user is in another city or country for example then exchange office is involved.

    so effectively, central office and exchange office are the same in appropriate discussion