ISDN – What is the ISDN?

The ISDN technology (Integrated Services Digital Network, or ISDN) means complete digitization so that all communication established in digital form, providing a wide range of services.

Integrated Services Digital Network is a telephone system network. It is a wide area network becoming widely available. Prior to the ISDN, the phone system was viewed as a way to transport voice, with some special services available for data. The key feature of the ISDN is that it integrates speech and data on the same lines, adding features that were not available in the classic telephone system.

ISDN is a circuit -switched telephone network system, that also provides access to packet switched networks, designed to allow digital transmission of voice and data over ordinary telephone copper wires, resulting in better voice quality than· an analog phone. It offers circuit-switched connections (for either voice or data), and packet-switched connections (for data), in increments of 64 Kbit/s. 

Another major market application is Internet access, where ISDN typically provides a maximum of 128 Kbit/s in both upstream and downstream directions (which can be considered to be broadband speed, since it exceeds the narrowband speeds of standard analog 56k telephone lines). ISDN B-channels can be bonded to achieve a greater data rate; typically 3 or 4 BRIs (6 to 8 64 Kbit/s channels) are bonded.

ISDN should not be mistaken for its use with a specific protocol, such as Q.931 whereby ISDN is employed as the network, data-link and physical layers in the context of the OSI model. In a broad sense ISDN can be considered a suite of digital services existing on layers 1, 2 and 3 of the OSI model. ISDN is designed to provide access to voice and data services simultaneously.

However, common use has reduced ISDN to be limited to Q.931 and related protocols, which are a set of protocols for establishing and breaking circuit switched connections, and for advanced call features for the user. They were introduced in 1986. In a videoconference, ISDN provides simultaneous voice, video, and text transmission between individual desktop videoconferencing systems and group (room) videoconferencing systems.

The first generation of ISDN is called as a narrowband ISDN and it is based on the use of 64 kbps channel as the basic unit of switching and has a circuit switching orientation. The main device in the narrowband ISDN is the frame relay. The second generation of ISDN is referred to as the broadband ISDN (B-ISDN).

It supports very high data rates (typically hundreds of Mbps). It has a packet switching orientation. The main important technical contribution of B-ISDN is the asynchronous transfer mode (ATM), which is also called as cell relay.

The ability to transfer information between the user and ISDN structured in the form of information transfer channels:

• Channel A: It is an analog channel of 4 Khz.
• Channel B: It is a 64 Kbps digital channel that intended for the transport of user information.
• Channel C: It is an 8 or 16 Kbps digital channel.
• Channel D: It is a digital channel of 16 or 64 Kbps intended primarily for the transmission of user-network signaling information for communication control, although it can also use under certain conditions for the transfer of user information in services selection (telealarm, telecontrol, and Telemedia) and low capacity data transmission.
• Channel E: It is a 64 Kbps digital channel (used for internal ISDN signals).
• Channel H: It is a digital channel of 384, 1,536 or 1,920 kbps that provides the user with an information transfer capability.

These channels can be combined differently, giving rise to two types of access:
• Basic access.
• Primary access.

Basic access

Basic access, known as 2B + D access, BRA (Basic Rate Access) or BRI (Basic Rate Interface), provides the user with two B channels and a 16 Kbps D channel.

It allows to establish up to two simultaneous communications at 64 Kbps, being able to use the capacity of the D channel for low-speed data transmission.

The main application of this type of access occurs in small local network facilities with a small number of terminals (up to eight) that require digital transmission or small capacity digital exchanges.

Primary Access

Primary access, also called 30B + D access, PARA (Primary Rate Access) or PRI (Primary Rate Interface), offers the user 30 B channels and a 64 kbps D channel, thus providing a bandwidth of up to 2,048 Kbps (in the US it consists of 23 B channels and a 64 Kbps D channel, so it provides a bandwidth of up to 1,544 Kbps).

It allows to establish up to thirty simultaneous communications at 64Kbps without currently planning to use the capacity of the D-channel for data transmission.

You can also use other combinations of channels B, H0, H11 and H12, but always respecting the speed limit of 2,048 Kbps.

The main application of this type of access is the connection to ISDN of small digital exchanges, multi-line systems, and local area networks of medium and large capacity.

ISDN Interfaces

There are several kinds of access interfaces to the ISDN dermed :

 Basic Rate Interface (BRI)

Basic Rate Interface service consists of two data-bearing channels (‘B’ channels) and one signaling channel (‘D’ channel) to initiate connections. The B channels operate at 64 Kbps maximum; however, (in the U.S. it can be limited to 56 Kbps.

The D channel operates at a maximum of 16 Kbps. The two channels can operate independently. For example, one channel can be used to send a fax to a remote location, while the other channel is used as a TCP/IP connection to a different location. ISDN service on the iSeries supports basic rate interface (BRl).

The basic rate interface (BRl) specifies a digital pipe consisting of two B channels and 16 Kbps D channel. Two B channels of 64 Kbps each, plus one D channel of 16 Kbps, equal 144 Kbps. In addition, the BRl service itself requires 48 Kbps of operating overhead. BRl therefore requires a digital pipe of 192 Kbps. Conceptually, the BRl service is like a large pipe that contains three smaller pipes, two for the B channels and one for the D channel.

The remainder of the space inside the large pipe carries the overhead bits required for its operation. In the following figure shaded portion of the circle surrounds the Band D channels shows the overhead.

Primary Rate Interface (PRI) 

Primary Rate Interface service consists of a D channel and either 23 (depending on the country you are in). PRI is not supported on the iSeries. Or 30 B channels

The usual Primary Rate Interface (PRI) specifies a digital pipe with 23 B channels and one 64 Kbps D channel. Twenty-three B channels of 64 Kbps each, plus one D channel of 64 Kbps equals 1.536 Mbps. In addition, the PRI service itself uses 8 Kbps of overhead.

PRI therefore requires a digital pipe of 1.544 Mbps. Conceptually; the PRI service is like a large pipe containing 24 smaller pipes, 23 for the B channels and 1 for the D channel. The rest of the pipe carries the overhead bits required for its operation. In figure, the shaded portion of the circle surrounding the B and D channels shows the overhead.

Broadband-ISDN (B-ISDN)

Narrowband ISDN has been designed to operate over the current communications infrastructure, which is heavily dependent on the copper cable. B-ISDN however, relies mainly on the evolution of fiber optics. According to CCITT B-ISDN is best described as ‘a service requiring transmission channels capable of supporting rates greater than the primary rate.

Principle of ISDN

The ISDN works based on the standards defined by ITU-T (formerly CCITT). (The Telecommunication Standardization Sector (ITU-T) coordinates standards for telecommunications on behalf of the International Telecommunication Union (ITU) and is based in Geneva, Switzerland. The standardization work of ITU dates back to 1865, with the birth of the International Telegraph Union.

It became a United Nations specialized agency in 1947, and the International Telegraph and Telephone Consultative Committee (CCITT), (from the French name “Comite Consultatif International Telephonique et Telegraphique”) was created in 1956. It was renamed ITU-T in 1993.

Principle of ISDN according to ITU –T

The ISDN is supported by a wide range of voice and non-voice applications of the same network. It provides a range of services· using a limited set of connections and multipurpose user-network interface arrangements.

ISDN supports a variety of applications that include both switched and non-switched connections. The switched connections. Include both circuit and packet switched connections.

As far as possible, new services introduced into an ISDN should be arranged to be compatible with the 64 Kbps switched digital connections.

A layered protocol structure should be used for the specification of access to an ISDN.

This is the same as the OSI reference model. The standards which have already been developed for OSI applications such as X.25 can be used for ISDN.

ISDNs may be implemented in a variety of configurations.

ISDN Services 

The purpose of the ISDN is to provide fully integrated digital services to users. These services fall into categories- better services, teleservices and supplementary services.

1. Bearer Services: Bearer services provide the means to transfer information (voice, data and video) between users without the network manipulating the content of that information. The network does not need to process the information and therefore does not change the content.

Bearer services belong to the first three layers of the OSI model and are well defined in the ISDN standard. They can be provided using circuit-switched, packet-switched, frame-switched, or cell-switched networks.

2. Teleservices: In teleservices, the network may change or process the contents of the data. These services correspond to layers 4-7 of the OSI model. Teleservices relay on the facilities of the bearer services and are designed to accommodate complex user needs, without the user having to be aware of the details of the process. Teleservices include telephony, teletex, telefax, videotex, telex and teleconferencing. Although the ISDN defines these services by name, they have not yet become standards.

3. Supplementary Service: Supplementary services are those services that provide additional functionality to the bearer services and teleservices. Examples of these services are reverse charging, call waiting, and message handling, all familiar from today’s telephone company services.

Principles of ISDN

The various principles of ISDN as per ITU-T recommendation are:

I. To support voice and non-voice applications

The main feature of the ISDN concept is the support of a wide range of voice (for e.g. Telephone calls) & non-voice (for e.g. digital data exchange) applications in the same network.

2. To support switched and non-switched applications

ISDN supports both circuit switching and packet switching. In addition ISDN supports non-switched services in the form of dedicated lines.

3. Reliance on 64-kbps connections

ISDN provides circuit switched and packet switched connections at 64 kbps. This is the fundamental building block of ISDN. This rate was chosen because at the time, it was standard rate for digitized voice.

4. Intelligence in the network

An ISDN is expected to provide sophisticated services beyond the simple setup of circuit switched calls. These services include maintenance and network management functions.

5. Layered protocol architecture

A layered protocol structure should be used for the specification of the access to an ISDN. Such a structure can be mapped into OSI model.

6. Variety of configurations

Several configurations are possible for implementing ISDN. This allows for differences in national policy, in state of technology and in the needs and existing equipment of the customer base.