Integrated Services Digital Network ISDN

ISDN Meanings

According to AbbreviationFinder, ISDN stands for Integrated Services Digital Network. From the point of view of the OSI standard, an ISDN stack consists of three protocols:

  • Physical layer
  • Link layer or data link layer (DLL)
  • Network layer, or network layer (the ISDN protocol, proper)

From the point of view of the interface with the user, protocols for User-Network Interaction and protocols for User-User interaction are included on the network layer. In the context of the OSI model, the protocols that are defined or referenced in ISDN. As ISDN it is essentially indifferent to user layers 4 to 7. Access concerns only layers 1 to 3. Layer 1, defined in I.430 and I.431, specifies the physical interface both for basic access as the primary. The differences with the OSI model are:

  • Multiple interrelated protocols.
  • Multimedia calls.
  • Multipoint connections.

For the D channel, a new data link layer standardization has been defined, LAPD (ISDN link layer protocol that comes from LAP-B (Link access procedure, balanced), Link Access Procedure on the D channel). This normalization is based on HDLC, modified to meet ISDN requirements. All transmission on the D channel is in the form of LAPD frames that are incremented between the subscriber equipment and an ISDN switching element. Three applications are considered: control signaling, packet switching, and telemetry. Channel B can be used for circuit switching, semi-permanent circuits, and packet switching. For circuit switching, a circuit is built on channel B on demand. A semi-permanent circuit is a B-channel circuit that has been established by agreement between the connected users and the network. Both the switched circuit and the semi-permanent circuit connection, the connected stations exchange information as if a direct full duplex link had been established. In the case of packet switching,

ISDN connections

ISDN provides three types of services for end-to-end communications. Switched circuits on channel B: the network configuration and protocols for circuit switching involve the user and the network for establishing and closing calls, and for access to network facilities Permanent connections on channel B: an indefinite period of time after of the subscription. There is no call setup and release over D-channel. ISDN-provided packet switching.

The pin indicated above is the connection used between the NT1 and the ISDN device that is going to communicate. A pair of wires arrives at the TR1 from the telephone exchange, which is where the digital communication signal travels and the power supply to the TR1 itself.


An ISDN address can be used to: Identify a specific Terminal within an ISDN digital line. Identify a network service access point in an OSIenvironment. Identify a network service access point in a non-OSI environment.

Numbering (services)

Multiple subscriber numbers. It allows Terminals connected to existing networks to reach compatible Terminals connected to a basic access in a passive bus-type configuration. Minimum requirements:

  • A number will be assigned to all Terminals belonging to the same service.
  • A different number will be assigned to the Terminals of the following services:

-Phone -Facsimile -V series data -Packet mode data The installation of a user with basic access to the ISDN is characterized by the existence of a network transmission equipment (TR or TR1), which separates the transmission to two wires from TR1 to telephone exchange, four-wire transmission between TR1 and Terminal equipment (ET or TR2) Wiring Configurations

  • Point to Point (1 ET)
  • Short passive bus (up to 8 ET’s)
  • Extended passive bus (up to 4 ET’s)

Advantages provided by ISDN

ISDN offers a number of advantages, including the following:


Currently the speed limit in communications through a telephone line using analog signals between the central and the user through the use of modems is around 56Kbps. In practice the speeds are limited to about 45Kbps due to the quality of the line. ISDN offers multiple digital channels that can operate simultaneously through the same telephone connection between central and user; digital technology is in the provider’s headquarters and in the user’s equipment, which now communicates with digital signals. This scheme allows a much higher speed data transfer. Thus, with a basic access service, and using a channel aggregation protocol, an uncompressed data rate of about 128 Kbps can be achieved. In addition,

Multiple device connection

With analog lines it is necessary to have one line for each user device, if they are to be used simultaneously. It is very expensive to send data (files or video) while having a spoken conversation. On the other hand, different interfaces are required to use different devices as there are no standards in this regard. With ISDN it is possible to combine different sources of digital data and get the information to the right destination. Since the line is digital, it is easy to control the noise and interference produced by combining the signals. Furthermore, the ISDN standards specify a set of services provided through standardized interfaces.


The way to make a call through an analog line is by sending a voltage signal that rings the “bell” at the destination phone. This signal is sent on the same channel as the analog audio signals. Setting up the call in this way takes a long time. For example, between 30 and 60 seconds with the V.34 standard for modems. In an ISDN connection, the call is established by sending a special data packet through a channel independent of the data channels. This call method is included within a series of ISDN control options known as signaling, and allows the call to be established in a couple of seconds. It also informs the recipient of the type of connection (voice or data) and from which number it has been called,


ISDN is not limited to offering voice communications. It offers many other services, such as computer data transmission (bearer services), telex, facsimile, videoconference, Internet connection…, and options such as call waiting, identity of the origin… Bearer services allow data to be sent via circuit switching (with a call procedure a fixed and exclusive path is established to transmit the data in the network, in the style of classic telephone networks) or by means of packet switching (the information to be sent is divided into packets of maximum size that are sent individually through the net).

Integrated Services Digital Network ISDN