Integrated Services Digital Network

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ISDN telephone

Integrated Services Digital Network or Integrated Voice and Data Network ( ISDN ) is an international standard for a digital telecommunications network . The German-language term Integrated Language and Data Network was the original term, it stood on an equal footing (sometimes even preferred) alongside the English-language term, which was also introduced in the German-speaking area in order to take account of the international nature of the system. Alternatively, the international term can also be translated into German as a service -integrated digital network . This network, various services , such as telex ( telex ), Teletex , Datex-L ( circuit-switched data transmission ), Datex-P ( packet-switched data transmission) and telephony transmitted and communicated.

Before the introduction of ISDN, there were separate networks for the services mentioned, between which there were gateways , for example between the teletype network and Teletex or from the telephone network to the Datex networks. Since the telephone network was the best known of the networks mentioned and the telephony service is still the most widely used today, the term ISDN is often equated with telephone .

By replacing the analog exchanges with digital technology, the performance of the subscriber line could be doubled (two calls or connections at the same time ), while the operation of the end devices remained largely the same for the user. The remote data transfer (eg. As well as the dial into the Internet is) with ISDN faster and more convenient than using a telephone modem .

There are now other telephony technologies, such as GSM , UMTS and LTE for mobile communications , as well as IP telephony (VoIP). For a long time, ISDN formed the basis for all other telephone networks. In terms of network technology, all exchanges in Germany were converted to ISDN, although the subscriber connections did not have to be digitized. The channels of analog connected subscribers are converted into a digital signal by the exchanges and forwarded. In Germany, however, new connections are mostly implemented via the Next Generation Network (NGN).

Since the early 2000s, every member state of the European Union has had ISDN telecommunications structures. ISDN was available across the board in Germany .

historical development


In the 1970s, digital technology reached the telephone network and was intended to replace mechanical exchanges. This should achieve better utilization of the lines and more convenience for the user. The responsible organization, the Comité Consultatif International Téléphonique et Télégraphique (CCITT, today ITU Telecommunication Standardization Sector (ITU-T), an advisory technical committee of the International Telecommunication Union (ITU)), developed technical specifications (recommendations) for a digital telephone network, the first passed under the name ISDN in 1980.


In the mid-1980s, numerous strategists in the European electrical industry and the then EC Commission feared that Europe would fall significantly behind the USA and Japan in the field of telecommunications if the state monopoly anachronisms were not abolished and national special solutions were not able to compete break up.

In order to prevent this scenario, uniform standards and common markets should be created. For this purpose, the European Telecommunications Standards Institute (ETSI) was founded by the EC Commission in 1988 to develop standards for a common digital telephone network. On April 6, 1989, under his leadership, the DSS1 standard (also called Euro-ISDN ) was launched by 26 network operators from 20 European countries, which was supposed to standardize the national ISDN systems and bring some technical improvements. In December 1993, Euro-ISDN was introduced on the basis of the Memorandum of Understanding on the Implementation of a European ISDN .


ISDN telephone system Eumex 220PC

In 1979 the Deutsche Bundespost decided to digitize all local exchanges in the Federal Republic of Germany . Field tests in Berlin (under the name DIGON = "DIGitales OrtsNetz") had shown that two independent duplex channels could be transmitted simultaneously through the use of digital technology. In 1982 the decision to use ISDN technology was made and the plans were put into concrete terms. This was followed by the construction of a test track in West Berlin, and finally, in 1987, two pilot projects in Mannheim and Stuttgart . In 1989, the official operation of the national ISDN according to the 1TR6 standard began (at that time simply referred to as ISDN by the Deutsche Bundespost , today referred to as national ISDN for better differentiation ). The Deutsche Bundespost was thus the pioneer for ISDN in Europe.

The digitization of the analog telephone network for 100 years was considered a gigantic investment project, with which the Federal Republic and its telecommunications companies should be catapulted to the top in the promising telecommunications market. In this first major digitization project, the separation of the digital transport route from the services based on it (as an additional communication service) was in the foreground right from the start. Since the Internet was not yet widespread, people initially thought primarily of image transmission or image telephony. Since many consumers did not yet understand the purpose of digitization, the abbreviation “ISDN” was derisively changed to the backronym “Is something necessary?”. At the same time, warnings were given about the risks of ISDN. For example, some data protection experts have argued that ISDN is a “qualitative leap” in total recording, as it enables the recording and storage of all connection data (see data retention ).

After the necessary software changes in the exchanges had been completed by May 1994, Euro-ISDN was commercially available in Germany. Since September 1995, the telephone network has been digitized to such an extent that ISDN is available across the board (for exchanges that are not yet digital, through external connection ; full digitization was achieved at the end of 1997). Until mid-1996, the conversion to ISDN technology was supported by Deutsche Telekom with a large subsidy measure - up to DM 300 was credited for a new connection and up to DM 700 for a telephone system .

For a long time, the marketing of ISDN was of exceptional strategic importance in Deutsche Telekom's business strategy. Therefore, as the only established network operator in the world, they decided to introduce ADSL technology across the board in favor of ADSL-over-ISDN ( Annex B ) , which is afflicted with range and bandwidth disadvantages . Customers with an analog T-Net connection should not have any DSL availability or bandwidth advantages over T-ISDN customers. With the changeover to NGN , Deutsche Telekom will no longer market ISDN and the provider has terminated the ISDN connections in order to test the conversion to IP-based connections in the area. Deutsche Telekom uses Annex J for IP-based connections for splitterless DSL ("DSL without splitter") . According to the planning, this means the end of ISDN telephone connections (and thus for ADSL-over-ISDN, "Annex B") in the Deutsche Telekom network.

At the end of 2006 there were 12.65 million ISDN basic connections (exactly one third of the total number of telephone connections ) and 113,000 ISDN primary rate connections . In 2016 there were 8.23 ​​million analog, 4.57 million basic and 85,000 primary rate connections as well as 26,000 public telephone stations, and the trend is still falling.

In 2009, 32.1% of all households in Germany had ISDN connections. From 2007 to 2013 the number of ISDN basic connections in Germany fell from 12.86 million to 9.02 million.

According to Telekom, the shutdown of ISDN should be completed by the end of 2020. Other providers (e.g. Vodafone ) advertise that they Template: future / in 2 yearswant to enable their existing ISDN customers to have a "soft migration" by 2022 .


In Austria , digitization began in 1978 with the introduction of the OES (Austrian Electronic System) by the Post and Telegraph Administration (PTV). From 1986 the OES technology was used across the board. In February 1992 an ISDN pilot test was started in the area of ​​the Viennese local exchange “Dreihufeisengasse”, to which 200 basic connections had already been connected by the end of the year. The entire Austrian telephone network was digitized by 1999, this year there were a total of 247,000 ISDN connections. In 2002 the number rose to a total of 438,000. The Austrian implementation of ISDN differs from others. a. in that there is a "global number" that cannot be assigned to any device via MSN . Some ISDN devices (e.g. telephone systems) have to take this specific feature into account in order to function properly. With the Telekom Austria product AON-Complete - the first Austrian Internet flat rate - there was a boom in ISDN new registrations on November 15, 1999, while an ISDN B-channel served the Internet flat-rate, while telephony was also possible via the second B-channel . The ISDN Complete tariff was discontinued for new customers at the end of February 2000 due to massive protests from competitors.


In 1988, Swissnet 1 was the first ISDN to go into operation in Switzerland . A total of 250,000 customers had been acquired by 1996, and in 2004 there were over 900,000 connections. In 2008, however, the proportion of ISDN connections decreased again. Since at that time a VDSL modem achieved data transmission rates of 20,000 kbit / s in the receiving direction via an analog line, the importance of ISDN and the restrictions of ADSL-over-ISDN have decreased. In addition, the provider Swisscom (which is the only one to offer ISDN in the emerging market) oppose other solutions: Sunrise Communications and Salt Mobile with pure wireless solutions , as well as various cable network operators (such as UPC Switzerland , Quickline , NetPlus , ImporWare - a total of well over 1 million customers ) with an offer for data, telephone, fax and television signal for over 200 channels, including many in HDTV quality on broadband networks. Swisscom started the large-scale switch from ISDN to IP in 2017. All connections were converted by the end of September 2019.

United States

In 1992 the 5ESS system was introduced in the USA under the name NI-1 (US National ISDN Phase 1) , which differed greatly from DSS1. An improved version of this system was later introduced as NI-2. Due to the lack of funding and the pricing structure, this system has only remained a niche product in the USA.

At the same time, AT&T is offering its own NI-1-compatible system under the name "5ESS". The 5ESS-2000 data standard, which is based on Very Compact Digital Exchange (VCDX), provides NI-1 features for digital terminals at an analog exchange and is therefore a bridging technology between analog and digital telephony for a relatively small group of customers.

International distribution

The spread of ISDN developed very differently around the world.

ISDN connections per 1000 inhabitants in 2005:
Norway 401
Denmark 339
Germany 333
Switzerland 331
Japan 240
Great Britain 170
Finland 170
Sweden 140
Italy 105
France 90
Spain 58
United States 47

Differences to the analog connection

The main difference to the analog landline connection is the digital transmission to the end device. This makes it possible to transmit several channels simultaneously via one connection. With the ISDN basic connection , two channels are available that can be used completely independently of one another for telephone calls , faxes or data transmission ; For example, you can send a fax during a phone call or make a phone call at the same time and (without ADSL) surf the Internet.

For an ISDN multi-device connection, up to 10 phone numbers (called Multiple Subscriber Numbers , MSN ) can be assigned in Germany , which can be distributed to any ISDN terminal device. Differentiated by the service IDs , an MSN can be used for various applications (services), for example for telephony and ISDN data transmission , without these interfering with one another - in theory. In practice, conflicts arise when, for example, a fax from an analog connection (i.e. without a service ID) calls an MSN that only uses the service ID to differentiate between fax and telephone. In practice, therefore, this function was usually not relied on, but one of the ten MSNs was assigned to each device. In addition, ISDN provides numerous switching-related performance features , the control information of which - as well as the signaling for setting up and clearing connections - are transmitted over a separate data channel ( D channel ).

Digital transmission enables numerous quality improvements compared to analog technology: The signals can be transmitted loss-free with continuous digital transmission. In analog transmission, the signal is only amplified , not regenerated. Not only is the useful signal amplified, but also noise and external voltages . The longer the connection, the smaller the signal-to-noise ratio becomes in the case of analog transmission, which means that the quality of the transmission deteriorates. The voice quality of digital transmissions is therefore significantly better. In addition, data transfers are faster because no modem has to be interposed, but the data is transmitted directly over the network. In principle, when using effective coding and modulation methods, transmission via a connection line can be much faster than the ISDN speed of 2 × 64 kbit / s (e.g. with DSL ), the limitation to the frequency range of 300 Hz to 3400 Hz typical for speech in however, the transmission and switching systems limit the speed.

To connect analog end devices such as telephones, fax machines, answering machines or modems to an ISDN connection, you need an a / b converter, which is also known as a terminal adapter (TA for short), or an ISDN telephone system with analog extension connections .

The disadvantage of ISDN technology compared to analog connections is that a simple corded telephone without its own power supply is not intended to be operated during regular operation - at least either NTBA or the ISDN telephone must be supplied with power externally according to the ISDN specification. The exception to this is emergency operation, in which, if the NTBA is not supplied with mains voltage, the supply voltage on the S 0 bus is reversed and the (then only permitted) terminal device is signaled that it must limit its consumption.

Publicly available connector types

An ISDN connection is available in two versions: as a basic connection (on a U k0 interface) or as a primary rate connection (on a U k2 or U G2 interface ).

Basic rate interface (BRI)

A basic connection has two user channels ( B channels ) and one channel for control information ( D channel ). Each of the two user channels offers a data transmission rate of 64 kbit / s (USA and some other countries 56 kbit / s), the control channel (D-channel) of 16 kbit / s. A further 16 kbit / s are used for frame identification (synchronization) and for service purposes, so that the gross bit rate on the basic connection is 160 kbit / s.

Basic connections are available as

  • Multipoint connection (point-to-multipoint) for connecting up to eight ISDN terminals
  • System connection with base number (point-to-point) for connecting a single telecommunications device, for example a telephone system

Primary rate interface (PRI)

A primary rate connection has 30 user channels with 64 kbit / s each (USA and some other countries 56 kbit / s) and a control channel with 64 kbit / s and another channel for synchronization and maintenance with a further 64 kbit / s. It is only available as a system connection and is used to connect telephone systems or for 2 Mbit / s leased lines .

Provider in Germany

In Germany, since the third stage of the postal reform in 1998 , other network operators have been able to offer telephone connections in addition to Deutsche Telekom , whereby the alternative network operators usually rent the so-called last mile , i.e. the connection line from the local exchange to the subscriber's home, from Deutsche Telekom and use it to connect the participant to their own switching technology. Connection network operators and Internet providers can also offer ISDN connection services on the basis of call-by-call / Internet-by-call and preselection via existing T-ISDN connections from Telekom.

In recent times the phenomenon of the so-called “fake” ISDN connection has been increasing. A provider makes an S 0 interface available to the customer via an IAD , but without supporting all ISDN functions. These are then mostly connections based on NGN . There is then no longer any classic transmission via Uk 0 in the baseband. This technique is used e.g. B. offered by cable network operators with a Fritz! Box from the manufacturer AVM , since only Voice over Cable is possible over the cable television network .

Physical specifications

Cabling for multi-device connections (point-to-multipoint)

Connection diagram for the S0 bus

In a multi-device connection to connect to the local exchange takes place as with an analog connection over a copper wire pair . The old TAE socket has actually become superfluous, but mostly remains for cost reasons (for connection of an NTBA by the customer; NTBA with self-assembly ). As a rule, the NTBA is connected to the TAE socket with a special cable supplied. The NTBA converts the digital signal from the incoming two-wire U K0 to the four-wire S 0 interface .

Alternatively, the following terminal designations are possible in the connection diagram opposite when using UAE sockets : 1a = 4; 1b = 5; 2a = 3; 2b = 6

If the plug-in options available on the NTBA are insufficient or if the end devices are to be set up spatially separated, a passive S 0 bus up to 150 m long can be connected if necessary . For this, cables with a core diameter of at least 0.6 mm should be used; special shielding is usually not required; Category 3 cables are sufficient. Up to eight terminal devices can be connected to a maximum of twelve IAE or UAE sockets at the same time, a maximum of four devices can be supplied with power via the NTBA (12: 8: 4 rule). The end of the S 0 -BUS should two 100 Ω - termination resistors are terminated. These terminating resistors prevent the signal from being reflected at the open end of the bus system. Termination can only be neglected if the line is theoretically infinitely long. An installation with the NTBA in the middle of the bus requires terminating resistors at both ends of the bus. In this case, the resistors in the NTBA must be switched off.

The NTBA is not a terminal, but a network component: The transition from the public telephone network to the subscriber's own in-house network (with all rights and obligations) is not the so-called 1st TAE , as is the case with the analog connection , but the NTBA. If there are additional analog devices (for example additional alarm clocks or changeover switches) in the house (in terms of circuitry) in front of the NTBA, these must be removed before the ISDN connection is put into operation.

Cabling for the system connection (point-to-point)

With a system connection, only one ISDN device is connected to the NTBA or NTPM . This is usually a telephone system .

  • In the case of a basic connection, the cabling is basically as described under multi-device connection, with the difference that a maximum of one socket is used. It is not necessary to connect the NTBA to the house power supply (see power supply for S 0 ).
  • With a primary rate connection, the cabling is usually six-core; two pairs of wires for the S 2M interface and one pair of wires for the power supply of the NTPM , as this is usually supplied with power by the telephone system .
  • The cable length between the NTBA and the telephone system, as the only connected device, can be a maximum of 500 to 1000 m, depending on the type of cable used.
  • In addition to special ISDN cables ( Western plug , two wire pairs used), network cables can also be used to connect the devices.

Power supply

Control power supply

To be able to connected devices with power supply, which is connected to the domestic power supply NTBA generates a supply voltage of 40  V . This is sent to the end devices via the S 0 bus and may have a maximum load of 4.5  W. The power is supplied by coupling into the signal wires. In order not to obstruct the transmission and reception electronics, the voltage is built up between the wire pairs for the transmission and reception direction. So no voltage can be measured within a wire pair. This concept is also known as remote feed . With regular power supply, the positive pole of the supply is on pins 3 and 6 of the NTBA, the negative pole on pins 4 and 5.

The connection of the NTBA to the 230 V supply is only necessary if end devices without their own power supply (for example an ISDN telephone) are to be connected directly to the NTBA or to a connected S 0 bus . If all connected devices have their own power supply (for example a cordless telephone or a telephone system ), the NTBA does not need to be connected to the 230 V supply; the NTBA always receives the energy for its own operation from the exchange via the subscriber connection line . The latter type of installation saves energy and can have a positive effect on the service life of the NTBA, as the integrated power supply unit is then not in operation and less heat is generated.

Emergency power supply

So that an emergency call can still be made to the police or fire brigade in the event of a power failure in the house , suitable ISDN telephones can also be supplied with power from the local exchange independently of the local power supply (emergency power operation). However, the power that the NTBA delivers in the event of a power failure is limited to 380 mW. In the case of emergency power operation, only a single ISDN telephone (capable and authorized for emergency power supply) can be supplied, although several devices capable of emergency power supply can be connected to the S 0 bus, but they do not (may) have an emergency power supply authorization , as this would overload the emergency power supply or a protective shutdown entry. This option of activating emergency operation on a telephone is usually implemented as a mechanical switch. As a result, this setting can also be changed in the event of an already existing emergency operation, as often only the basic functions of the ISDN telephone are available here: Telephoning can be carried out normally, but convenience features on the device with high power consumption, such as hands-free talking and the display, work usually not in emergency power mode.

In contrast to the normal supply, the emergency supply voltage is applied to the lines of the bus with reversed polarity. This is how ISDN terminals recognize emergency power operation.

Not all types of telephones have an emergency power supply. For example, the base stations of cordless telephones definitely need a local power supply. B. can be done from a UPS .

Logical specifications


In Germany, ISDN was originally offered according to the national 1TR6 standard , but a Europe-wide uniform ISDN standard ( DSS1 ) has existed since 1991 ; ISDN with DSS1 protocol is also known as Euro-ISDN . Outside Europe and in telephone systems , other implementations are also used. The last ISDN connections that still supported the national 1TR6 protocol were finally converted to the DSS1 protocol in December 2006.

In the USA there is ISDN under the name NI-1 (US National ISDN Phase 1) and NI-2. The data transmission rate of the user channels ( B channels ) is only 56 kbit / s due to the PCM coding and voice compression ( μ-law ) used in North America .

In Japan and Hong Kong there are ISDN systems with the name INS-Net 64 , in Australia TPH 1962 .

Voice transmission

The speech signals for transmission in the Euro-ISDN with a sampling rate of 8  kHz digitized ( pulse-code modulation , PCM) and using a logarithmic curve that takes into account the peculiarities of human perception ( ITU -T standard G.711 , A-law method), coded at 8 bits per sample. This results in the typical ISDN transmission speed of 64 kbit / s (8000 times per second 8 bits). The frequency range from 300 to 3400 Hz is transmitted  .

However, it is also possible to transmit the G.722 codec via ISDN (see HD telephony ), as this also requires 64 kbit / s bandwidth. This transmits frequencies from 50 Hz to 7000 Hz. Both remote stations must be HD-capable.

Data transfer

The B channels are bit-transparent and synchronous so that any line codes can be used. In order to double the data transmission rate , the two B-channels of a basic connection can also be bundled . In order to use this option, end devices are required that are able to synchronize the two B channels (for example ISDN PC cards or video conference systems ).

With the help of suitable routers , several or all of the user channels of a primary rate connection can be bundled. This enables data transfer rates of up to 1920 kbit / s (net) to be achieved. This option is used in particular for networking remote locations within a company network or for dedicated lines to the Internet .


V.110 is an ITU-T protocol for the use of end devices with V-series interfaces ( e.g. V.24 interface) on integrated services networks. V.110 implements a bit rate adaptation to adapt the data transfer rate of slow end devices, e.g. B. modems to ISDN. The data transfer rates are standardized up to 19.2 kbit / s; Most V.110-compatible terminal adapters , however, have data transfer rates of up to 38.4 kbit / s. Each bit of the V interface is mapped into one bit of the 64 kbit / s stream of the B channel , the remaining capacity is filled with filler bits. In some implementations, lower multiplexing speeds can be used. The bit rate adaptation described in V.110 is also often used outside of ISDN.


V.120 is a further development of the V.110 protocol . The standardized data transfer rate is up to 56 kbit / s. V.120 provides possibilities for statistical multiplexing.


The signaling is done with ISDN out-of-band - it is transmitted on a separate channel, and not as in the analog network, for example, using the tone dialing process in the voice channel. As a result, the connection establishment and the control of the switching- related performance features work more reliably and quickly. Technically, the D-channel is used for signaling , which has a data rate of 16 kbit / s for basic access and 64 kbit / s for primary rate access.

In the core network, the Signaling System No 7 protocol is used for signaling between the exchanges on the so-called central signaling channels .

Reference points and interfaces

An ISDN connection consists of two parts: the subscriber line ( the U K0 interface for the basic rate connection ; the U K2 interface for the primary rate connection ) and the in-house cabling (the S 0 bus for the basic rate connection ; the S 2M interface for the primary rate connection ) . The subscriber line is terminated by a network termination (for the basic connection NTBA ; for the primary rate connection NTPM ).

ISDN reference points

Functional units:

  • ET: Exchange Termination ( local exchange )
    • Exchange (layers 1 to 3)
  • LT: Line Termination (local exchange)
    • Line transmission facility
    • Implementation between a relatively low-rate subscriber connection and a high-rate multiplex connection on the exchange side
  • NT1: Network Termination 1 ( NTBA )
    • Layer 1
  • NT2: Network Termination 2
    • Layer 1 to 3
    • optional, fulfills (if available) mediating or concentrating tasks (e.g. a telephone system)
  • TA Terminal Adapter ( Terminal Adapter , from converter)
    • adapts TE2 to the requirements of NT1 or NT2
  • TE1: Terminal Equipment Type 1 (ISDN terminal device)
    • Device that meets all ISDN interface recommendations
  • TE2: Terminal Equipment Type 2 (terminal device not compatible with ISDN)
    • Device that does not meet the ISDN interface recommendations

The interface to software is usually established by the CAPI . The Hisax drivers were also used in the past under Linux .

Addressing with ISDN

Structure of the ISDN addresses

ISDN addresses are specified in accordance with ITU -T guideline E.164 . The ISDN address consists of the ISDN number and sub-address. The ISDN number addresses, for example, a subscriber on a basic connection . The subaddress is a maximum of 32 characters long and is used, for example, to address a host in a LAN (this must be connected to the ISDN via a suitable gateway ). The subaddress is transparent for ISDN and only known to the subscribers using it.

ISDN emulation via NGN

Since the increasing migration of the circuit-switched fixed networks to the NGN network topology, some providers have been offering "ISDN" via IP telephony over existing DSL connections as so-called ISDN NGN connections . This takes place where the provider does not maintain its own local exchanges or no longer expands them and instead offers its telecommunications services exclusively via data connection - wholesale ( bitstream access , T-DSL resale ) or its own DSLAMs .

Instead of the NTBA, the network is terminated with the ISDN S 0 bus provided for ISDN terminals by an integrated access device and communication is IP- based via a SIP gateway. Typical features of ISDN are simulated or emulated, but this is not a full-fledged DSS1 ISDN connection; Due to the lack of a data channel, only voice services are usually supported - numerous ISDN service features are therefore not available (e.g. group 4 fax , B-channel bundling, Datex-P, parking / unparking).

Usually there is also no emergency power supply. If the normal power supply fails , the subscriber to such an emulated ISDN connection cannot be reached if there is no UPS and cannot make a call, unlike a connection with an emergency power supply. Even with an existing UPS, however, it must be ensured that an outdoor DSLAM that may be connected in between is not supplied with emergency power and thus communication fails - in contrast to ISDN, the switching center was supplied with emergency power here and outside the switching centers only passive technology ( cable splitter ) was normally installed . With ISDN over IP, there is a proprietary protocol that enables ISDN with all its features also via Voice-over-IP connections, but is rarely used because of the cheaper SIP that can be implemented.


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Web links

Commons : ISDN  - collection of pictures, videos and audio files

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This article was added to the list of excellent articles on March 7, 2005 in this version .