Home phone services

Introduction

When we want to contact a person located faraway from us we use a telephone, a telecommunications device which is used to transmit and receive sound. Most telephones operate through transmission of electric signals over a complex telephone network which allows almost any phone user to communicate with almost any other. Our  sound signals pass through a number of electronic systems before reaching the destination. The interposed electronic systems may be defined as the telephone exchange. Depending upon the location of the caller and the receiver  the sound signals may have to pass through a number of exchanges.

Definition of telephone exchange

In the field telecommunications the word “telephone exchange” is defined as the an  electronic system that connects telephone calls. It is denoted by the first three (3) numbers of a telephone number, when you do not consider the area code.  If a phone number .555-1234 is considered, the number “555” indicates the exchange. This number is known as prefix. The telephone exchange or telephone switch  is normally housed inside a building along with other connected equipments. The building is called as the central office.

Central office

A central office is a structure used by the telecommunications department  to locate the  equipments  needed to process telephone calls and direct the data traffic. These offices are also known as  telephone switching centres, wire centres or telephone exchanges. A typical central office houses a main distribution frame (MDF) along with the cabling, PABX or central office equipment, MDF heat coil protectors,  grounding systems, radio and radar systems along with  communications-electronics (C-E) etc.

A brief historical overview  of  telephone exchange or central office

            In United State the  first telephone exchange was opened  in 1878 at New Haven, Connecticut. Soon  after the introduction of the telephone in 1878, subscribers were required  to “ring” the office to  get the help of  an operator to complete the call. As the number of telephone connections  expanded from only a few dozen to a few thousand, human operators were required to be housed in a central office building to complete calls.

            During the 1910s a large number of operators were engaged in front of switchboard to route calls day and night. During these  early days any call one makes whether next door or across town had to go through  the operator sitting at his/her switchboard.

            With the invention of the stepping switch device during 1891 the automation in the telephone circuit switching was possible. A number of step switches were  arranged in banks and integrated with a line-finder that detected the subscriber’s entry in to the circuit and hooked to a “dial tone” bank to show that the line was ready. This method varied from country to country.          

            Human operators were replaced with electro-mechanical methods of switching as the usage of telephones exploded. Now the operator was required to operate machines located in a defined “central” area  from where all telephone circuits radiate out to users. At this juncture the name  “Central Office” was coined. The name still remains  although the operators have been replaced  by technicians who repair and monitor the electronic hallways.

            In London the first telephone exchange was opened on 1 March 1902 with just 200 subscribers at the Savings Bank building in Queen Victoria Street and was under the control of the General Post Office. The exchange covered .an area of about two-and-a-half miles of the capital. Notable subscribers included the Treasury, the War Office and Fleet Street.  By 1905 the exchange capacity was extended to 10,000. The  ‘City’ exchange was  officially opened in November 1907 with a capacity of 15,000 lines to meet the growing demand. 

            In 1933, Faraday became the telephone centre of the world with the opening of the international telephone exchange.

            In 1935, an automatic exchange was opened with more than 6,000 working lines. The new exchange employed 60 engineers for over 15 months  for completing the task of switching subscribers over to the new exchange.

 Manual service exchanges

In a manually controlled exchange the customer lifts the receiver and contacts the operator to connect the call to a requested number.           When a  subscriber  lifted the receiver, a signal  lamp near a  jack would light on the panel in front of the operator.

            The operator then  would plug one of the answering cords available in the panel into the subscriber’s jack provided on the  switch board and ask, “number please?” by switching ON a headset.

            On getting a reply from the subscriber the operator might plug the ringing cord in to the jack of the called party. A ringing cycle is then started.

Pre-digital automatic exchanges

In order to  eliminate the need for human telephone operators, automatic exchanges, or dial service were introduced in the early 1900. Operators were replaced by computerized exchanges progressively. The telephone switch ,a device for routing calls from one telephone to another,  was introduced in the automatic exchanges. The switch formed part of the public switched telephone network that makes the exchange to sense  an off hook telephone condition and to provide a dial tone to that phone, When a number is dialled by the subscriber, the same is sensed based on the pulses or DTMF tones generated by the phone and then the call is connected to the called number with in the same exchange or to another exchange. The connection is maintained  until  one of the parties hangs up the phone. In this type of tracking the call is called supervision. A billing equipment  incorporated   in the exchange  counts the call duration and records the same. Another feature called automatic number identification (ANI) was also implemented.Motors, shaft drives, rotating switches and relays were used in the initial stages of development. Such exchange is called as electromechanical switches.

Electromechanical Signalling

The word trunk is used to mean a circuit connecting two switches. Signals are used to transmit information from the transmitting end of a circuit to inform a user at the receiving end that a message is to be sent.

Dual-tone multi-frequency (DTMF) signalling is used for telephone signalling over the line in the voice-frequency band to the call switching centre. Electromechanical switches using a variety of DC voltages and signalling tones  were used for communication with one another over trunks before the adoption of  Signalling System 7.