Types of Telephone System

Telephone System

Telephone systems were primarily designed to deliver a human speech (voice). They are also utilized for data transmission. A modem is used to transfer data. The telephone network that connects the two consumers is known as the public telephone network (PTN). Because PTN links consumers via one or more switches, it is also known as a public telephone network (PSTN).

Public switched telephone network (PSTN)

The switching centers in the public switched telephone network are classified into five types, as shown in Figure. They are namely:

1. Regional offices (class1)

2. Sectional offices (class2)

3. Primary offices (class3)

4. Toll offices (class4)

5. End offices (class5)

Figure:   PSTN Hierarchy

The customer's phone is linked to the terminal (or central office) through a local loop. The terminal office is linked to one toll office. Several toll offices are linked to the headquarters. Several major offices are linked to divisional offices. Several division offices are linked to a single district office.

Phones are used to utilize rotary or pulse dialing. In the system, a digital signal is delivered to the terminal office for each number dialed. This sort of selection results in a selection mistake due to human error. The touch-tone method is being used to make calls. Instead of digital transmission, the user transmits two short bursts of the analog signal in this approach. The sent signal's frequency is determined by the row and column of the pad pushed. A 12-tone dialing system with push buttons is shown in the image. When a user dials the number 5, for example, the terminal receives two bursts of analog signals with frequencies of 770 and 1336Hz. The Touch-Tone dialing system is,


1209 Hz


1336 Hz


1477 Hz


1663 Hz



697 Hz






770 Hz






852 Hz






941 Hz







Electronic switching system

The figure shows a simplified schematic demonstrating how two telephone sets (subscribers) are interconnected via a central office dial switch. Each subscriber is linked to the switch through a local loop. The switch is an electronic switching mechanism (ESS machine). Local loops are terminated at the calling and called stations in telephone sets, as well as at the central office ends to switch machines.

Figure: Electronic Switching System

When the calling party's telephone set releases the hook (i.e. pulls the mobile phone out of the holder), the switch hook on the telephone set is released, resolving the DC channel between the tip and the loop ring through the microphone. The ESS machine detects DC current in the loop and recognizes it as a state of the hook. Because the loop is completed through the telephone, this technique is known as the loop start operation.

Completing a local phone call between two customers connected to the same switchboard follows a set of conventional processes, which are outlined below.

Step 1: Turn off the calling station.

Step 2 When the switching machine detects a dc current flow on the loop, it returns an audible dial tone to the calling station, indicating that the caller has access to the Switching machine.

Step 3 The caller calls the destination number using one of two methods: mechanical dial pulsing or dual-tone multifrequency touch-tone signals.

Step 4: After detecting the first called number, the switching machine removes the dial tone from the loop.

Step 5 The switch decodes the phone number and finds the local loop for the destination phone number.

Step 6: Before ringing the destination phone, the switching machine checks the destination Loop for dc current to determine if it is idle (on-hook) or in use (off-hook). Simultaneously, the Switching machine locates the signal path between the two local Loops through the switch.

Step 7 If the destination phone is not in use, the switching machine returns a station busy signal to the caller station.

Step 8 If the destination telephone is on-hook, the switching machine delivers a ringing signal to the destination telephone on the local loop while also sending a ring back signal to the calling station to provide some confidence to the caller that something is occurring.

Step 9 When the destination answers the phone, the loop is completed, allowing dc current to enter.

Step 10 The dc current is recognised by the switch as the station answering the phone. At this point, the switch eliminates the ringing and ring-back signals and completes the path through the switch, enabling the calling and called parties to start talking.

Step 11 When either end of the loop goes on hook, the switching machine identifies an open circuit and drops the connections through the switch.


Videophones work on the same concept as television transmission and reception. Voice communication takes place via a radio connection in the UHF band. A (camera) pick-up tube detects the scene. The video signal from the camera pick-up tube is amplitude modulated. The voice signal from the phone is frequency modulated. The relative position of picture and sound carrier frequencies stays the same as in a traditional TV system. If the channel bandwidth ranges from a to b MHz, the image carrier = (a+1.25)MHz and the sound carrier = (b-0.25)MHz. Solid-state image sensors are utilized to capture the figure/scene. An LCD screen is used to show information.

The communication takes place through via:

1. Coaxial cable links

2. Microwave space communication

3. Satellite communication

Figure: Video Phone

The figure shows a schematic block diagram of a videophone system. The picture signal from the camera is amplified and sent to a modulation amplifier, where it is amplitude modulated and mixed with the modulated frequency, the audio signal, and sent to the antenna transmission. When a user dials a number for an outbound call, the dial pulse creates a tone, and the call is routed through the switched telephone network. Modulated tones are transmitted. When a call is received at the destination, the called party will 'release' their hand. At that point, the transmitter is ACTIVE, and the acknowledgment signal is returned to the call control terminal with a 2150 Hz tone. However, after the video recognition signal is received, the speech merging network is activated at both ends, and the amplitude modulated picture is merged with the frequency-modulated speech and delivered down a common axis.

This establishes an audio/video link between the person calling and the person being called. After the image, the voice is separated on the video detector at the end of the discussion, and when the conversation is concluded, the combined network is turned off. These video phones are commonly utilized as a way of communication between industries, businesses, research institutes, and huge organizations.

Sreejith Hrishikesan

Sreejith Hrishikesan is a ME post graduate and has been worked as an Assistant Professor in Electronics Department in KMP College of Engineering, Ernakulam. For Assignments and Projects, Whatsapp on 8289838099.

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