Integrated Services Digital Network (ISDN)

Integrated Services Digital Network (ISDN): ISDN means Integrated Services Digital Network. ISDN was developed by ITU-T in the 1980s. These are digital telephony and data transport services offered by regional telephone operators. ISDN includes the physical, data link, and network layers of the OSI model.

The main objective of ISDN is to provide fully integrated digital services to the users. ISDN is actually a set of communication protocols that were offered by telephone corporations. These include the digital services of the telephone network. ISDN allows the data to be sent over existing telephone cables, and these data are video, audio, text, and file in the form. The main goal of the ISDN is provided to the end-to-end connectivity in the worldwide from of the WANs. Another goal of ISDN is also to provide a combination of voice and non-voice services.

Narrowband ISDN is the first generation of the ISDN. Narrowband ISDN is based on the basic unit of the switching, and this unit is 64 kbps. The frame relay is the primary device of the Narrowband ISDN. Broadband ISDN (B-ISDN) is a second generation of the ISDN. The second generation of ISDN supports very high data rates, usually hundreds of Mbps. Asynchronous Transfer Mode (ATM) is the main technical contribution of the B-ISDN, which is also known as the cell relay.

The Architecture of ISDN shown in below:

Advantages of ISDN

1. ISDN can operate the video, voice, text, and data simultaneously.

2. The call setup of the ISDN is faster than the analog modem.

3. It provides higher data rates.

4. ISDN provides fully digital services, so the probability of error is very low.

ISDN provides three types of digital services:

1. Bearer Services: Bearer services offer the path for transfer data between the users without the network manipulating the content of that data. Bearer services are related to the Physical, Data-link, and Network layer of the OSI model. Bearer services can be used with the help of the packet switched, cell switched, circuit switched, and frame switched networks.
2. Tele Services: Tele services depend on the facilities of the bearer services. It is designed for the requirements of complex users. Tele Services provides various types of services such as Telephony, telex, telefax, and video conferencing.
3. Supplementary Service: Supplementary services provide extra features to Bearer services and Tele services. Examples of the Supplementary services are call waiting, message handling, reverse charging, etc.

There are two types of interfaces associated with the ISDN:

1. BRI (Basic Rate Interface)
2. PRI (Primary Rate Interface)

BRI: BRI stands for Basic Rate Interface. In the ISDN, BRI provides two B-channels and one D-channel to initiate connections, i.e. (2B + D). The B-channels work at the 64 Kbps for each channel, while the D-channel works at the 16 Kbps.

PRI: PRI stands for Primary Rate Interface. In the ISDN, PRI provides 23 B-channels and 1 D-channel. The PRI offers an aggregate data rate of 1.544 Mbps. The PRI services are very expensive, and it is used to connect many computers and devices on a large scale. 

The ISDN channels table shown below with each transmission data rates:

Channels	Transmission Data Rates
A	A-channel operates the 4 KHz analog telephone data rate.
B	B-channel operates the 64 Kbps digital PCM channel data rate for voice and data.
C	C-channel operates the 8 or 16 Kbps digital channel data rate.
D	D-channel operates the 16 Kbps digital channel for out of the band signaling.
E	E-channel operates the 64 Kbps digital channel data rate for internet ISDN signaling.
H	H-channel operates the 384, 1536, or 1920 Kbps digital channel data rate.