What is ISDN? | Integrated Services Digital Network


ISDN stands for Integrated Services Digital Network. A not long time ago, the PSTN (Public Switched Telephone Network) was all analog from customer premises to the central office/local loop and from the central office to the backbone of the network.

PSTN was simply designed for analog telephone calls. With the advent of personal computers, digital data transmission was needed.

For voice calls everything remained the same, for computers, modems were developed to allow digital exchanges over existing analog PSTN networks.

Digital signals are modulated to analog signals and transmitted over the PSTN network. Only at the receiving end, analogy signals are demodulated back to original digital signals.

But analog signals have many disadvantages compared to digital signals;

  1. Analog signals cannot maintain high quality for the long-distance
  2. Analog signals carry less information compared to digital signals
  3. Analogy signals are not so flexible as digital signals in terms of data rate services and support


For these reasons, ISDN came to the rescue.

ISDN is the set of international communication standards in the 1980s and improved in the 1990s. It is a digital network to transmit voice, image, text, and video over existing circuit-switched PSTN.

Integrated Services Digital Network (ISDN) provides a single interface for hooking up your telephone, fax machine, and computers. Different types of devices such as analog phones, fax machines, or a computer are connected to an ISDN terminals adapter which converts different types of data into ISDN format so that they can share the same digital PSTN network.

Integrated Services Digital Network (ISDN) is intended to be a worldwide telecommunications network to replace existing public telecommunications networks.

Now, let’s see, what is ISDN?

ISDN means;

- ISDN is a communication line that allows digital communication, i.e. Data, voice, video, and graphics at very high speed.

- Support a wide range of services over a channel

- Provides a single common interface with which to access digital communication services that are required by various devices while remaining transparent to a user.


Services that ISDN supports

All services that ISDN supports are;

  1. ISDN supports voice calls
  2. Video conferencing
  3. Fax transmission
  4. Linking to the internet


ISDN is not restricted to Public Switched Telephone Network (PSTN) alone; it may be transmitted via packet switch network.

Types of ISDN network

There are two types of ISDN which are;

  1. ISDN BA / BRI

- Basic Access / Basic Rate Interface

- Consisting of B channel and D channel based on 64 kbps used for transmitting and controlling information

- Formula: 2B + 1D

- For KOBI’s and Home Users.

  1. ISDN PA / PRI

- Primary Access / Primary Rate Interface

- It consists of 23B channels and a 1D channel (used in the US) based on 64 kbps

- It consists of 30B channels and a 1D channel (used in Europe) based on 64 kbps

- Formula: 30B + 1D

- For Internet Service Providers (ISP) and Big Companies


Broadband ISDN

Broadband ISDN is a Broadband counterpart to integrate services digital networks which provides digital transmission over ordinal telephone company copper wires on the narrow band.

The main purpose of using Broadband ISDN is to provide transmission over ordinary telephone company copper wires on the narrowband and having a speed of a maximum 2MBits/Sec to 1GigaBits/Sec. Also related to Asynchronous Transfer Mode (ATM).

Advantages of ISDN

So the advantage of using the Integrated Services Digital Network (ISDN) are;

  1. Digital services (digital services having fewer errors)
  2. Connection is fast
  3. Higher bandwidth
  4. Voice, data, and video can all be sent over a single ISDN line.


Disadvantages of ISDN

  1. ISDN is very costly than other typical telephone systems
  2. ISDN requires specialized digital devices just like a Telephone Company.

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