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Monday, 30 August 2021

Types of 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.


The data and telephone communications business is continuously evolving to suit the demands of telephone, video, and computer communications systems. More individuals are communicating with one another than ever before. To satisfy this need, previous standards are updated daily, while new standards are established and deployed regularly.  A hypothetical network developed by major telephone companies to offer worldwide telecommunications support for voice, data, video, and fax information inside the same network is known as an Integrated Services Digital Network (ISDN). ISDN is the combination of several services into a single multipurpose network. ISDN is a network that allows an unlimited number of independent users to be connected over the same communication network.

Principles of ISDN

The capacity to handle various voice (telephone) and non-speech (digital data) applications in the same network utilizing a variety of standard facilities is an essential aspect of the ISDN concept. ISDN is compatible with a wide range of applications, including switched and unmodified (dedicated) connections. ISDN's fundamental concept is a digital connection at 64 kbps. Customers connect to the ISDN system via a local interface that is linked to a digital transmission medium known as a digital pipeline. Depending on the customer's needs, several tube diameters are available with varying capacity (i.e. bit rates). A home client, for example, may just want a minimal capacity to contain a phone and a personal computer. Office complexes, on the other hand, may necessitate conduits with adequate capacity to handle a big number of interconnected digital telephones via private branch exchanges (PBXs) or a high number of computers on a local area network (LAN).

ISDN Architecture

The figure shows the block diagram of the ISDN function architecture. The ISDN network is intended to accommodate the customer's completely new physical connection. There are several protocols available that allow control information to be sent between the customer's device and the ISDN network. ISDN channels may be divided into three kinds. They are namely:

1) B channel: 64 Kbps

2) D channel: 16 Kbps or 64 Kbps                       

3) H channel: 384 Kbps, 1536 Kbps, 1920 Kbps

Figure: ISDN Architecture

According to ISDN standards, home network customers are given three full-duplex, time-division multiplexed digital channels, two running at 64 Kbps (called B channels, forbearer) and one at 16 Kbps (designated as D channel, for data). The D channel is used for signaling and sharing network control information. One B channel is utilized for digitally encoded audio and the other for data transfer.

The 2B+D service is known as the basic rate interface (BRI). BRI systems require bandwidth that can support two 64 Kbps B channels, one 16 Kbps D channel, and additional special bits. As a result, the overall bit rate of BRI is 192 Kbps.

Features of ISDN

The features of ISDN are given below

1) ISDN can handle a large range of voice (telephone) and non-voice (digital data) applications on the same network by utilizing a small number of defined facilities.

2) ISDN connections can be switched or non-switched (dedicated).

3) An ISDN will have intelligence for delivering service features, maintenance, and network management tasks.

4) The 64-Kbps digital link is the foundation of ISDN. New ISDN services should be compatible with 64 Kbps switched digital connections.

5) OSI (open system interconnection) standards can be utilized for ISDN.

6) Depending on national regulations, ISDN can be deployed in a variety of ways.


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.

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