Integrated Services Digital Network | ISDN

What is Integrated Services Digital Network (ISDN)?

Integrated Services Digital Network (ISDN):

The ISDN is an abbreviation of the Integrated Services Digital Network. The current communications networks vary with the type of services, such as telephone networks, telex networks, and digital data transmission networks. On the other hand, the ISDN is an integrated network for various types of communications services handling digitized voice (telephone) and nonvoice (data) information.

Fig.1 shows the current network configuration with individual networks, such as telephone network and a data network existing independently, and telephone sets, data terminals, etc. connected individually to each network


ISDN Definition

The CCITT defines the ISDN as follows :

  • A complete, terminal-to-terminal digital network. Fig.3 shows the end-to-end digital connectivity.


  • A network that provides both telephone and non-telephone services in the same network. Fig.4 shows the voice and non-voice services in the same network.

  • A network based on a digital telephone network.
  • A network that utilizes Signaling System No. 7 (SS7) for signaling between switching systems. Fig. 5 shows the signaling connection between Switching Systems.

  • A network offers standard user network interface. Fig.6 shows the standard user network interface.

ISDN Services

1) A wide range of services

The ISDN provides the following functions, as shown in Fig.7.

  1. Packet switching service
  2. Circuit switching service
  3. Leased circuit service

Circuit switching service includes both telephone and data circuit switching.

  • As shown in the figure, ISDN can interface with various terminals, such as a telephone set, FAX, Video terminal or personal computer to provide a wide range of services.
  • The ISDN concept can be summarized in two statements :
  • ISDN offers a variety of services, such as telephone, data and image transmission through one network.
  • ISDN handles all information digitally.

2) Standard user-network interface. Fig.8 shows the user-terminal/network interface.

  • The subscriber line is connected with an NT (Network Termination) installed at the customer premises.
  • Various terminals are connected to the NT. These terminals can include digital telephones, multi media terminal, digital facsimile machines, personal computers, etc. as shown in the figure.
  • The NT and terminals are connected by S or T interface (S/T interface), as recommended by the CCITT. Up to 8 terminals are connected to one S/T interface. The NT and terminals are connected using an 8-pin connector, which is also recommended by the CCITT.
  • As shown in this figure, the personal computer uses the RS232C interface that is different from the ISDN S/T interfaces, so a TA (Terminal Adapter) is provided to adapt the RS232C interface for use with the ISDN interfaces.

Fig. 9 shows the operation of various terminals in the home.

  • Each terminal is connected to the NT through the S/T interface which, in turn, is connected to the switching system through the subscriber line.
  • At the upper left of the figure a person is using a television telephone called a Video Phone, at the lower left, a person is watching a picture on a Videotext terminal.
  • At the upper right of the figure, a person is operating a personal computer, which requires the use of a TA to convert the computer’s RS232C interface to the S/T interfaces used by ISDN. At the lower right, a person is doing catalog shopping using a Videotex terminal.

3) Home Shopping and Home Banking

  • 10 shows home shopping and home banking services.
  • 10 shows a typical service made possible by ISDN. It shows something is being ordered to a department store and then delivered.

  • The goods are ordered using the Videotex terminal, and instruction is output to the bank to transfer the amount of the bill from your account.
  • The department store delivers the ordered goods.

4) Home Medical System

  • 11 shows the home medical system.
  • 11 shows another service provided by ISDN: the receiving of medical care at home.

  • The upper left shows the measuring of blood pressure, with the result shown on the videotex screen both at home and at a medical facility (show at the bottom right of the figure).
  • The lower left shows a consultation for medication using a TV telephone.

User Network Interface

ISDN User Network Interface Configuration

  • 12 shows the interface between the user and the network. Telephone service makes use of two wires for the subscriber line between the switching system and the customer’s premises. These same two wires can be ued by ISDN to receive ISDN services.
  • An NT (Network Termination) is installed at the subscriber’s home and connected to the subscriber line.

  • The Interface between the NT and the ISDN exchange (switching system) is called the U interface. This interface has not been defined in the CCITT Recommendations because circumstances are different in each country. The point between the NT and the on-premises terminals is called the S or T reference point. The ISDN user/network interface refers to these S/T points and is defined in the CCITT Recommendations.
  • The S/T interface uses four wires, two for sending and two for receiving. Since the U interface uses two wires, the NT provides a two-wire/four-wire conversion function.
  • CCITT recommends the use of AMI (Alternative Mark Inversion) code at the S/T point. AMI code is a bipolar waveform.
  • As shown in the figure, the ISDN Terminal provides an S/T interface that follows the CCITT Recommendations and can be connected directly to the NT. Since the personal computer and the analog FAX utilize a different interface from the S/T interface, they require protocol conversion by a TA (Terminal Adapter).

Service Access Points (Reference Points)

  • In the existing telephone network, a point at which a service is provided for a user, that is, a service access point is located at a rossete between the user’s telephone set and the subscriber line.

Since the ISDN provides various types of service other than telephone service through a plural number of terminals, various service access points are provided. Thus, service access points would have to be defined corresponding to the ISDN Services.

  • Fig 13 shows the user-network interface reference points which are based on the CCITT reference model and identifies the important reference points of the model.

  • The following describes the user-access points and the function of each for the basic user-network interface.

Network Termination (NT) :

  • The NT can be split into NT1 and NT2. NT1 and NT2 are terminating equipment for the network.
  • In this case, NT1 provides the Layer 1 functions, such as circuit termination, timing, and supply of electricity, while NT2 provides layer 2 functions, such as protocol, control, and concentration functions.

Terminal Equipment (TE) :

  • The TE can be split into TE1 and TE2. TE1 is an ISDN terminal which is connected to ISDN via the S/T interface. TE2 is a non-ISDN terminal which is connected to ISDN via a Terminal Adapter (TA) such as a personal computer or analog FAX as described in Fig. 12.

Terminal Adapter (TA) :

  • A TA is a physical device that is connected to a non-ISDN terminal (TE2) to permit access to ISDN.

S-Interface :

  • A 4-wire physical interface used for a single customer termination between a TA and NT2 or between TE1 and NT2.

T-Interface :

  • A 4-wire physical interface between NT1 and NT2.

R-Interface :

  • A physical interface used for a single customer terminator between TE2 and TA.

U-Interface :

  • The subscriber line is called U-Interface and utilizes 2-wires.

ISDN User Network Interface Points

1) Requirements of User-Network Interface

For us to utilize “integrated services” including voice and non-voice communications and the use of some new media, such as facsimile in offices and home, the following features must be provided for user-network interfaces :

  • Different services for each call
  • A switching mode (packet switched/circuit switched function) can be selected.
  • The data transmission speed can be selected.
  • A plural number of terminals can be concurrently connected.
  • The portability of terminals can be ensured.

2) Basic Structure of User-Network Interface.

The basic conditions for structuring the user-network interface that satisfy the preceding requirements can be summarized into the following three points :

  • Multi services
  • Common use of various services telephone/non-telephone and existing/new services. As shown in Fig.12, ISDN termianls, personal computers, FAX machines, etc. are connected to S/T points to offer various services.
  • Multi points
  • Up to eight (8) terminals can be connected to one (1) NT as well as point to point connection.
  • Fig 14 shows the multi points connection.


  • Terminals can be carried from place to place and connected to different sockets for use, just as home electrical appliances can be carried around and plugged into AC outlets.

3) Channel Classification

Various channels can be used to transmit information between a terminal and the switching system. These include B, D and H channels. Each channel has a different bit rate and information carrying attributes.

(a) B-channel

  • The B-channel carries user information such as voice and packet data at a rate of 64 kbps. However, the B-channel does not carry signaling information.

(b) D-channel

  • The D-channel interface carries mainly signaling information such as originating or terminating subscriber number, call origination and disconnect signals for circuit switching and packet switched user data at 16 kbps or 64 kbps.
  • The D-channel also permits multiple logical channels to be established for use in packet communications.
  • H-channel
  • The H-channel carries high-speed user information such as high-speed facsimile, video, high-speed data, etc. H channels do not carry signaling information for circuit switching by the ISDN.
  • Table 1 outlines these three channel types and characteristics.

Table 1: Channel Types and Characteristics

Channel Type Bit Rate Function
B 64 kbps ·                  To carry user information

·                  Circuit switchingmode and packet switching mode

D 16 kbps

64 kbps

·                  To carry signaling information for circuit switching
H H0: 384 kbps

H11: 1536 kbps

H12: 1920 kbps

·                  To carry high-speed packet data such as facsimile and video

·                  An H channel does not carry signaling information for circuit switching by the ISDN

Note : ·                  H0   :  64K X  6 =   384 kbps

·                  H11 : 64K X 24 = 1536 kbps

·                  H12 : 64K X 30 = 1920 kbps

3) Typical Interface Structures

  • Basic Interface
  • This interface is primarily for home use.
  • The basic interface is set at a transmission speed of 144 kbps. This provides two (2) 64 kbps B-channels for user information exchange and a 16 kbps D-channel for signaling and control. The interface is thus referred to as 2B+D.

Fig.15 shows the basic interface structure.

  • Primary Group Interface
  • These interfaces are primarily for business use. The primary group interface for the ATT system consists of a 1.544 Mbps line. This line can thus provide up to 23 B-channels at 64 kbps and a single D-channel at 64 kbps.
  • In Europe and other countries using CEPT system standards, the primary group is 2.048 Mbps and the interface is 30B-channels and single 64 kbps D-channel. This line is used for PABX etc.
  • Fig 16 shows the primary group interface structure.

  • Table 2 shows the typical user network interface structure.

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