How Telephone System Works?

Pre-requisites: Telephone Network

Telephone Network is used to provide voice communication. Telephone Network uses Circuit Switching. Originally, the entire network was referred to as a plain old telephone system (POTS).

In this article, We will see how telephone system works and some terminologies related to it. Let’s start with some terminologies related to telephone system.

Central Office

It is a point where all the trunk lines are converged which are coming from your homes, offices, etc. Suppose, You have a telephone at your home and the trunk line of this telephone is firstly hit at your area’s central office (local central office). Now, Your local central office will make further routing of the phone call. It’s like when you make a request on internet it hits to default gateway (which is your home router) and then gateway forwards it to ISP network (Internet Service Provider). In telephone system central office is similar to ISP network. If you have multiple trunk lines at your home and some trunk lines are busy than central office (Local Central Office) can route the phone call to the available phone number in your home (do routing based on availability).

Trunk Line

The main line between a home telephone network (more specifically to PBX of home) and central office is trunk line. It is similar to the fiber optic cable present in our home router. The number of simultaneous phone calls is depends on number of trunk lines. If we have two trunks lines in our home then we can able to take or receive two phone calls at a time. It is a connection line between central office to your PBX. You can refer the above image for better understanding. Trunk line typically used to connect switching center in telephone system. Each trunk line has a separate phone number so, If you have three trunk lines than you have three phone numbers.

PSTN – Public Switched Telephone Network

In simple words, It is a network of central offices. It is interconnection of telephone networks in the world. It is similar to the internet.

PBX – Private Branch Exchange

It is similar to your home router which manages all the devices connected to it. It do switching of phone calls on local line. Suppose, We have three trunk lines therefore we are able to make or receive three calls at a time. If you make a phone call then PBX will choose any one number which is available out of three numbers we have to make this call and the other user will receive the call from that phone number. It is similar to public IP addresses we have and PBX choose one of them but in telephone system it is phone number not public IP address. If someone calling and all the lines (phone numbers) are busy than it will forward the phone call to voice mail box for dropping the message who is calling and it’s gives instruction to voice mail box for auto attendant (Computer voice – Press 1, Press 2 is feature of voice mail box and we call this feature auto attendant) when someone calling.

Working

Suppose, Someone is making a phone call to you than how it is reach to you? It is through the PSTN and each central office will make a routing decision based on some routing protocols. It is similar to forwarding in routers on internet. Now, This phone call reaches to your local central office and it routes the call to your PBX using one of trunk line you have. PBX will deliver this call to your local phone network (it is depends on how PBX is configured and which phone call is to deliver to which device or devices in local telephone network).