Telephone Network
Introduction:
A telephone network is also known as a telecommunication network, which is used for making telephone calls between multiple parties.
How it works:
To make a call, a telephone subscriber begins by taking the telephone ‘off-hook’-in the process, signaling the local central office that service is requested. The central office, which has been monitoring the telephone line continuously, responds with a dial tone. After receiving the dial tone, the user enters the telephone number ofthird party.After that, central office stored the previously entered number. Then it translates the number into an equipment location and then gives a path to that location. After giving the path of the desired location, it checks whether the third party’s line is already busy or not.The called party’s line may be busy in the same central office or in another central office. If the call is in another central office, then the central office switch will handle the whole calling process.That is intraoffice call. Another the call is within the central office, then the call will be directed either to a closest central office or to a distant central office with the help of a long distance network.That is called interoffice call. In interoffice calls, a separate signaling network is used to combine the call progression through multiple switches and telephone switches.If the called party’s line is busy in intraoffice calls, it will not wait anymore and will directly show a signal that the call is busy until the calling party returns the ‘on-hook’ condition. Another if the call is not busy, it will start ringing. It will give the signal to the party that the call is taking place. If the call does not go well it will be disconnected and a voice path will be created through the system to both of the caller and the receiver.The voice will be continued until the party goes back again on the previous position. Lastly the call will be disconnected and call charge will recorded and will be shown
Network types:
There are a number of different types of telephone network:
1. Landline Network:
In landline network, telephones directly wired into a single telephone exchange.This is also calledpublic switched telephone network or PSTN.
2. Wireless Network:
In wireless network, all the telephones are mobile and can move around anywhere within the coverage area.
3. Private network:
A closed group of telephones are connected to each other in private network. It uses a gateway to reach the outside world. This is used inside companies and call centers and is called a private branch exchange (PBX).
4. Integrated services digital network or ISDN.
Public telephone operators (PTOs), owns the first two types network and provides services to the national government.
Virtual Network Operators (VNOs), leases capacity wholesale from the PTOs and sell on telephony service to the public directly.
Components:
Telephone network consists of some major components. As-
• Switching
• Signaling
• Transmission

Switching:
• Manual Switching:In 1878 the first telephone exchange was installed in New Haven where it was permitted for maximum 21 customers to reach each other manually by central switch board. In early days each instrument of the telephone was connected with the central switch board.The manual switchboard was quickly extended from 21 lines to hundreds of lines. Each line was terminated on the switchboard in a socket and a number of short, flexible circuits with a plug on both ends of each cord were also provided. Two lines could thus be interconnected by inserting the two ends of a cord in the appropriate jacks.

Fig:A manual central switchboard in an American city
• Electronic switching: When transistor invented in 1947 it helped to develop electronic switching. It helps to establish phone calls by the help of computerized system which can inter connect between telephone circuits and digital electronics. It can troubleshoot of its own and can detect problems. Later in 1977 semiconductor memory is also used to develop the communication system.
• Digital Switching: It is a device that handles generated digital signal that passed through telephone company central office and the responsibility of it is to pass them in company’s backbone network. After that it receives the digital signal from office channel bank and hence it transfers to wide area network.A “centrex” is a digital switch at the central office that manages switching for the private company from the central office
Signaling
• Rotary Dialing: It was designed to operate electromechanical or electronic switching so that the operation speed was limited by the operating speed of switches. Inthe Bell System, the dial pulse period is nominally 1/10th of a second long, permitting a rate of 10 pulses per second. Modern telephones are now wired for push-button dialing, but even they can usually generate pulse signals when the push-button pad is operated in conjunction with electronic timing circuits.

Fig: Rotary Dialing
• Push Button Dialing:In 1950 it was learnt that push button dialing was double efficient than rotary dialing hence people started to use push button dialing. The reason is that it has got dialing numbers(button) so that user easily can dial this buttons and can access to call. The possibility of error minimized. There is 10 dialing digits (0 through 9) are assigned to specific push buttons. The pad also has two more buttons, the star (*) and pound (#) symbols, to accommodate various data services and customer-controlled calling features.

Fig: Push Button Dialing

• In Band Signaling: In early days’ signal was given by the process of Direct Current between telephone instrument and operator. So for long distance it became a problem because DC can’t be carried to long distance. Hence after AC was used and the problem of long distance was solved.
• Out of Band Signaling: As in band signaling there was many problems so they discovered out of band signaling to overcome the problems of in band signaling. So to solve long distance communication problem they invented common channel signaling which is dedicated to serve as a data link, carrying address information and certain other information signals between the microprocessors employed in telephone switches.
Transmission:
• Analog to Digital Transmission: It is a device that converts analog signal into digital. Analog information is transmitted by modulating a continuous transmission signal by amplifying a signal’s strength or varying its frequency to add or take away data. Digital information describes discontinuous form of any data or events. After that a process found named frequency division multiplexing which describes the total bandwidth that is found for communication medium is divided into a series of non-overlapping band, where each band carries separate signal.
• Coaxial Cable: Coaxial cable is used as a transmission line for radio frequency signals. Its applications include feedlines connecting radio transmitters and receivers to their antennas, computer network connections, digital audio, and distributing cable television signals.

Fig: Coaxial Cable
• Optical Fiber Cable:Due to great bandwidth, low cost and reliability it becomes preferable for telecommunication network. It permits to transmits data in higher rate. Again non zero dispersion optical fibers invented where it permits multiple wavelength transmission at higher data rate.
• Overseas Transmission: For overseas transmission they started to think how can it be done. They got terrestrial radio signal which transfer signal through the land medium. It also broadcast from land based station. Another method that is satellite, it communicates through bouncing the signal from one side of the earth to another.The satellite boosts the signal and sends it back down to Earth from its transmitter dish to a receiving dish somewhere else on Earth.

Fig: Works of Satellite in Telephone Network

Conclusion: Finally, the evolution of telephone network made our communication life easier. Almost we can reach each side of the world. It’s a revolutionary change that makes the communication world as digital world.