Simple Telephony - how does it work?

Simple Telephony - How Does It Work?

Simple Telephony - have you ever wondered what happens when you pick up the phone and make a call to a friend or colleague? This section describes how your voice gets from one end of the country to the other.

Making A Simple Telephony Call

The two traditional methods of making a simple phone call are via either a fixed line or a mobile (cellular) phone. For a fixed line phone, when you pick up the phone you hear a dialling tone, which tells you the line is free to make a call. After dialling your number, a 'circuit' is established between you and the person you are calling.

Just imagine a single telephone line running between you and that person, and you're pretty much there. This fibre optic cable runs from your home phone out to a local junction box and then on to the 'Local Exchange' of the service provider. This part is known as the Local Loop.

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From here the call enters the national network and is routed through to the Local Exchange near the person you are calling, on through their local loop and into their home. Now you may be thinking "that's all very well, but in simple telephony terms, how does my voice actually travel the length of the country in almost instant time?" Well, depending on both your telephone handset and the nature of the equipment used by your service provider, there were two ways this could be done :

1. Analogue Transmission

Every sound you hear is in analogue form. For this, think of your voice as a wave, which is a mix of time and amplitude. These waves travel down the phone line all the way to the person you are calling. At their end, their equipment will convert this back into your voice so the person you're calling hears what you're saying.

Analogue was the traditional method of transmitting a voice call. The main problem was that of voice clarity - if your call was travelling a long way, this radio wave was open to distortion and fading. This was particularly a problem when service providers were offering cheap phone call packages, often using poor quality network connections.
For that reason, amplifiers were used to increase the signal, though they had the effect of increasing the noise on the line as well.

This is why, for example, if you had called Australia a few years ago, you may have received not only a delay on hearing each other's voice, but also a lot of crackling or background noise, which was somewhat frustrating! In later sections, we'll explain how cheap VoIP service has contributed to this developing market.

2. Digital transmission

In modern telephone systems, most transmission is via digital signalling. In simple telephony terms, your voice is broken down into a binary code of series of '0's' and '1's'. It's not easy to imagine, but think of your voice being broken down into lots of small parts (8,000 per second). Each of these 8,000 parts contain 8 'bits' of information. You may have heard the speed of your phone line referred to as 64k bits per second (8,000 parts * 8 bits per part).

At the other end, the equipment receives this code, translates it back into your voice, and the person receiving the call hears your voice perfectly. Digital transmission is less exposed to 'noise' or disturbance on the line. Equipment known as repeaters are used, which not only amplify the signal, but also remove any disturbance or noise that has been picked up on the way. The 64 kbit line runs from your house to the local junction box (in the UK these are the green boxes found on many streets). This brings together the local lines from all houses serving that box.

From here, a bigger cable, containing many circuits, runs to the local exchange, where it joins the national network.

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