Integrated Services Digital Network

Integrated Services Digital Network (ISDN) is an international standard for a digital telecommunications network, and can be translated as Integrated Services Digital Network. Through this network, different services such as telex ( Telex ), teletext, Datex -L ( circuit-switched data transmission), Datex -P ( packet-switched data transmission) and telephony are transmitted and communicated.

Before the introduction of ISDN there was for the identified services each have their own networks, between which it transitions (gateways ) were, for example between telex network and teletex or from the telephone network to the Datex- networks. Since the telephone network was the most famous of these networks and even today the telephony service is the most used, the term ISDN is often equated with a telephone.

By replacement of analogue exchanges with digital technology, the performance of the local loop could be doubled ( two simultaneous calls or connections ), that was all the operation of the terminals for the user largely the same. The data transmission ( for example, the connection to the internet ) with ISDN faster and more convenient than using a telephone modem.

Meanwhile, there are other techniques for making calls, as GSM and UMTS mobile communications and IP telephony ( VOIP). For a long time formed the basis for all other ISDN telephone networks. Technically, all network switching centers were converted to ISDN in Germany, but the subscribers did not have to be digitized. The channels of analog bridged participants are converted by the exchanges into a digital signal and passed. New connections are realized in Germany but mostly by Next Generation Network ( NGN).

Since the early 2000s, has each member country of the European Union ISDN telecommunication structures. In Germany ISDN was widely available. For new connections, some network operators offer only NGN. (see also section " in Germany ". )

  • 3.1 Basic connection
  • 3.2 Primary Rate Interface
  • 5.1 Connection with the multi- device connection ( point-to- multipoint)
  • 5.2 Wiring of the system connection (Point-to - Point)
  • 5.3 Power Supply 5.3.1 control power supply
  • 5.3.2 Emergency Power Supply
  • 6.1 implementations
  • 6.2 Voice transmission
  • 6.3 Data transmission 6.3.1 V.110
  • 6.3.2 V.120

Historical Development


In the 1970s, digital technology reached the telephone network, and to replace the mechanical switches. For a better utilization of the lines and more comfortable for the user should be achieved. The competent organization, the Consultative Committee International Telegraph and Téléphonique et (CCITT, now the International Telecommunications Union ( ITU) ), developed to technical specifications ( Recommendations) for a digital telephone network that have been adopted for the first time under the name of ISDN 1980.

In Europe

The mid-1980s feared many strategists in the European consumer electronics industry and the then EC Commission that Europe geriete in the field of telecommunications with the USA and Japan clearly falling behind if it does not succeed in abolishing the state monopoly anachronisms and competition of national special solutions to finish.

To prevent this scenario, common standards and common markets should be created. 1988, so by the EC Commission, the European Telecommunications Standards Institute (ETSI ) established, which should work out standards for a common digital telephone network. On April 6, 1989 (also called Euro -ISDN) under their direction by 26 operators from 20 European countries in the DSS1 standard launched, which should unify the national ISDN systems and some technical improvements brought. In December 1993 the introduction of Euro -ISDN was made on the basis of the Memorandum of Understanding on the implementation of a European ISDN.

In Germany

In the Federal Republic of Germany, the German Federal Post Office in 1979 decided to digitize all local exchanges. In field trials in Berlin (under the name Digón = " digital local loop " ) had shown that two independent duplex channels could be transmitted simultaneously through the use of digital technology. 1982 the decision was made for the ISDN technology and the plans were specified. This was followed in 1987 two pilot projects in Mannheim and Stuttgart. In 1989, the official operation of the national ISDN after 1TR6 standard (at that time by the German Federal Post Office just as ISDN, today, to better differentiate as a national ISDN is used ). The German Federal Post Office was thus the pioneer of 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 of the promising telecom market. At the same time warned of risks of ISDN. For example, data protection experts argued the Greens that ISDN is a " qualitative leap " in the total coverage, as it is the gathering and storing all connection data enable (see retention ).

After up to May 1994 necessary software modifications were completed in the exchanges, Euro -ISDN in Germany was commercially available. Since September 1995, the telephone network is digitized so far that ISDN is widely available (at a non digital exchanges by foreign intrusion, the complete digitalisation was achieved in late 1997 ). By mid- 1996, the transition to ISDN technology from the German Telekom was supported with a large funding measure - for a new connection up to 700 DM up to 300 DM and acquisition of a telephone system paid.

In the business strategy of Deutsche Telekom had the marketing of ISDN extraordinary strategic weight. Therefore chose the world's only incumbent carrier in the introduction of ADSL technology coverage for the afflicted with coverage and bandwidth disadvantages ADSL-over- ISDN (Annex B ). Customers with analog T- Net connection should have no broadband availability and bandwidth advantages over T- ISDN customers. From Deutsche Telekom but is used in the new so-called IP-based connections for DSL splitter-less ( "DSL without a splitter " ) Annex J. This then means the end for analog telephone lines and digital telephone ports of ISDN (and ADSL-over - ISDN, "Annex B") in the network of Deutsche Telekom.

The end of 2006, there were 12.65 million ISDN basic accesses ( exactly one-third of telephone lines total) and 113,000 ISDN primary rate interfaces. The end of 2011 there were 17.4 million analog, 10.93 million basic and 99,000 primary rate interfaces, the trend is decreasing.

In 2009, ISDN lines were connected in the Federal Republic with 32.1 % of all households.

In Austria

In Austria, the digitization began in 1978 with the introduction of the OES ( Oesterreichisches electronic system ) by the Post and Telegraph (PTV ). From 1986, the OES technique was widely adopted. In February 1992, an ISDN pilot project was started in the area of ​​Wiener local exchange " Dreihufeisengasse " at the end of the year already 200 base terminals were connected. By 1999, the entire Austrian telephone network has been digitized, this year there were a total of 247,000 ISDN connections. 2002, the number rose to a total of 438,000. The Austrian implementation of ISDN differs from other, inter alia, by the fact that there is a "global number" that can be assigned to any device via MSN. Some ISDN devices (for example, telephone systems ) must take into account this specific feature to work fine. With the Telekom Austria Product AON Complete - the first Austrian internet flatrate - it came on 15 November 1999 to a boom in ISDN registrations, while an ISDN B channel of the Internet was flat rate, telephony was the second B channel at the same time possible, the ISDN Complete tariff was already set end of February 2000 for new customers again due to massive protests of competitors.

In Switzerland

In Switzerland, the first was in 1988 with ISDN Swissnet 1 put into operation. By 1996, a total of 250,000 customers were won, in 2004, there were over 900,000 connections. In 2008, the share of ISDN ports but shrunk again. Since at this time reached a VDSL modem over an analog line data transfer rates of 20,000 kbit / s in the receive direction, the meaning of ISDN and the limitation of ADSL-over- ISDN has decreased. In addition, the provider Swisscom set (which is the only one in the opening market ISDN offers ) other solutions meet: Sunrise and Orange with pure wireless solutions, as well as UPC Cablecom (over 1 million customers ) with an offer for data, telephone, fax and television signal for about 200 channels, including much in HDTV quality on broadband networks.

In the U.S.

In the U.S. in 1992 under the name of NI -1 ( National ISDN U.S. Phase 1) was introduced in an ISDN system of DSS1 was very different. An improved version of this system was later introduced as NI -2. Parallel offers AT & T under the name 5ESS a private ISDN system. Due to lack of promotion and pricing design ISDN in the U.S. has remained a niche product.

International dissemination

Very different the distribution of ISDN developed world.

Differences for analog connection

The main difference to the analog landline connection is made in the digital transmission up to the terminal. This makes it possible to transmit a plurality of connection channels simultaneously. When ISDN Basic Rate Interface Two channels are available which can be used completely independently of telephone calls, fax and data transmissions; so you can, for example, during a telephone conversation surf the Internet send a fax or a phone call while (not ADSL).

For an ISDN multipoint connection may be awarded in Germany up to 10 phone numbers ( called multiple subscriber number, MSN ), which can be distributed to the ISDN terminal devices. Distinguished by the service identifiers, an MSN can be used for various applications ( services ), for example for telephony and ISDN data transmission, used without this interfering with each other - in theory. In practice, conflicts will arise if, for example a fax from an analog connection ( ie without service identifier ), an MSN is calling, which differs only by service identifier between fax and telephone. In practice, therefore usually did not rely on this feature, but awarded for each device one of the ten MSNs. In addition, numerous ISDN call processing features ready, the control information - as well as the signaling to establish and release of connections - a separate data channel ( D channel) to be transferred.

The digital transmission over analog technology enables numerous quality improvements: The signals can be transmitted without loss in continuous digital transmission. With analog transmission, the signal is only amplified, not regenerated. Not only the useful signal is amplified, but also noise and extraneous voltages. Is the longer the link, the lower case of analog transmission, the signal -to-noise ratio, therefore, the quality of the transmission deteriorates. The voice quality of digital transmissions is therefore much better. Furthermore, data transfers are faster because no modem must be inserted, but the data is transmitted directly through the network. In principle, the transmission via a connecting line with the use of effective coding and modulation method is much faster than the ISDN - rate of 2 x 64 kbit / s to be (for example, at high speed ), the limitation to the typical for voice frequency range of 300 Hz to 3400 Hz However, the transmission and switching systems limits the speed.

To connect analog devices such as phone, fax, answering machine, or modem to an ISDN line, you need an a / b converter, also called a terminal adapter ( abbreviated TA) is called, or an ISDN PBX with analog extension ports.

Disadvantage of ISDN technology over analog connections is that operation of a simple corded telephone is not provided without an independent power supply in normal operation - at least either the NT or the ISDN telephone must be powered externally according ISDN specification in control mode. Exception is the emergency mode, in which, if the NT is not supplied with mains voltage, the supply voltage on the S0 bus is reversed and thus the ( then only allowable ) the terminal is signaled that it must reduce its consumption.

Publicly available connection types

An ISDN line is available in two versions: a basic access ( at a Uk0 interface ) or primary rate interface (on a Uk2 or UG2 interface).

Basic access

A base terminal has two bearer channels (B channels ), a channel for control information ( the D channel) and a channel for synchronization and control purposes. Each of the two bearer channels provides 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 The frame identifier (synchronization ), and for maintenance purposes more 16 kbit / s is such that the gross bit rate at the base terminal is 160 kbit / s.

Base terminals are available as

  • Multipoint connection ( point-to- multipoint) to connect up to eight ISDN terminals
  • System connection (Point- to-Point ) to connect a single telecommunications device, such as a telephone system

Primary Rate Interface

A PRI has 30 user channels with 64 kbit / s (USA and some other countries 56 kbit / s) and a control channel with 64 kbit / s, as well as an additional channel for synchronization and maintenance with further 64 kbit / s It is only available as a system port and is used for connecting telephone systems or for 2-Mbit/s-Festverbindungen.

Providers in Germany

In Germany may 1998 offer in addition to Deutsche Telekom and other network operators phone terminals, the alternative network operators rent usually the so-called last mile, so the connection line from the local exchange to the home of the participant, by Deutsche Telekom since the third stage entry into force of postal reform and use to connect the subscriber to the own switching technology. Furthermore, network operator and Internet service provider ISDN connection services can offer on the basis of Call-by-Call/Internet-by-Call and preselection of existing T- ISDN connections Telekom.

In modern times there is increasingly the phenomenon of so-called "non-genuine " ISDN connection. Here is a supplier to the customer via an IAD an S0 interface, but without support all ISDN features. It is then usually to connectors on the basis of NGN. A classic transmission by Uk0 in the baseband then is no longer present.

Physical Specifications

Cabling in multi- device connection ( point-to- multipoint)

In a multipoint connection, the connection to the local exchange as well as with an analog connection over a copper pair. The old telephone socket is actually superfluous, however, remains mostly for cost reasons ( for connecting an NT by the customer; NT with self-assembly ) are made. In general, the NT is connected via a supplied special cable to the telephone socket. The NT converts the digital signal from the incoming two-wire UK0 on the four-wire S0 interface.

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

Do existing on the NT mating possibilities not or should the terminals are placed apart, when needed up to 150 m long passive S0 bus can be connected. For cables should be used with at least 0.6 mm core diameter, special shielding is not usually required; Lines of category 3 are sufficient. At a maximum of twelve IAE or UAE sockets at the same time a total of up to eight devices can be connected to up to four devices can be converted power from the NT with current ( 12:8:4 rule). The end of the S0 bus should be terminated with two 100 - Ω termination resistors. These terminators prevent reflection of the signal at the open end of the bus system. Only at a theoretically infinitely long loop scheduling could be neglected. An installation with the NT in the Busmitte required at both ends of the bus terminating resistors, the resistors in the NT must be switched off in this case.

The NT is not a device but a network component: the transition from the public telephone network to the subscriber 's home network ( with all the rights and obligations ) is not like the analog connection is called the 1st TAE, but the NT. Are in the house ( circuit technology ) before the NT still analog peripherals ( for example, additional alarm or change-over switch ), they must be removed before using the ISDN connection.

Eumex 220PC PBX

NT Deutsche Telekom

NT Deutsche Telekom

Wiring the system connection (Point-to - Point)

In a system access only one ISDN device is connected to the NT or NTPM. This is a telephone system in general.

  • With a base terminal of the wiring is principally described as under -multipoint connection, with the difference that a maximum of one can be used. The connection of the NT to the house power supply is not required ( see Power Supply for S0 ).
  • For a Primary Rate Interface cabling is usually six-wire; two pairs for the S2M interface and a twin wire for the power supply of NTPM, as this is usually supplied by the telephone system with power.
  • The cable length between the NT and the telephone system, as the only connected device can be a maximum of 500 to 1000 m depending on the cable type used.
  • In addition to special ISDN cables ( Western plug, two pairs of cables used ) can also network cables are used to connect the devices.

Power supply

Control power supply

To supply power to the connected equipment, the equipment connected to the house power supply NT generates a supply voltage of 40 V. This is conducted via the S0 bus to the terminals and may be loaded with a maximum of 4.5 W. The supply takes place through the coupling into the signal wires. In order not to hinder the transmission and reception electronics, the voltage between the pairs for the transmit and receive directions is established. Within a pair of wires so no voltage is found. This concept is also referred to as a remote power supply. With control power to the positive terminal of the power supply to pins 3 and 6 of the NT, the negative pole to the pins 4 and 5 is

The connection of the NT to the 230 V supply is only necessary if devices directly (for example, an ISDN telephone ) should be connected to the NT or a clamped- S0 bus without their own power supply. Do all connected devices their own power supply (eg a cordless telephone or a PBX ), the NT does not need to be connected to the 230 V supply, the power for his own business always receives the NT from the central office via the local loop. The latter type of installation saves energy and can have a positive impact on the life of the NT, as the integrated power supply is not then in operation and less heat is generated.

Emergency power supply

Thus, even during a power failure in the house until an emergency call to police or fire department may be discontinued, appropriate ISDN telephones can also be supplied independent of the local power supply from the local exchange with power (emergency operation). The power supplied by the NT case of power failure is limited to 400 mW. In emergency mode, only a single ( notspeisefähiges and legitimate ) ISDN telephone can be powered, although several notspeisefähige devices can be connected to the S0 bus, but are not suited for restricted power have ( must ), because that would overload the emergency power supply or a safety shutdown occurs. This option to activate the emergency operation on a phone is usually carried out as a mechanical switch. This can also be changed in the case of an already existing emergency operation this setting, since often only the basic functions of the ISDN telephone available: can be telephoned quite normal, but sets sided comfort features with high power consumption, such as free speech and the display function, in non- emergency operating normally.

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

Not all phone types are notspeisefähig. For example, take the base stations of cordless phones definitely a local power supply, which can for example be made from a UPS in case of power failure.

Logical specifications


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

In the U.S. there are ISDN under the name of NI -1 ( National ISDN U.S. Phase 1) and NI -2. The data transmission rate of the user channels (B- channels) is because of the PCM coding and speech compression ( μ -law) used in North America while only 56 kbit / s

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

Voice transmission

The speech signals for transmission in the Euro -ISDN at 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 process), coded into 8 bit per sample. This results in the typical ISDN transmission rate of 64 kbit / s (8000 times per second 8 bits). Is transferred to the frequency range 300-3400 Hz

However, it is also possible to transfer the codec G.722 ISDN (see HD voice ), as this also requires 64 kbit / s bandwidth. This transmits the frequencies from 50 Hz to 7000 Hz, both peers have to be HD -capable.

Data transmission

The B channels are bit-transparent and synchronously, so that any line codes can be used. In order to achieve a doubling of the data transmission rate, the two B - channels of a basic rate access terminals can also be bundled. To use this facility, devices are required which are able to synchronize the two B channels (for example, ISDN PC card, or video conferencing ).

With the help of suitable router several or all bearer channels of a primary rate access can be bundled. This can be achieved data transfer rates up to 1920 kbit / s ( net). This possibility is particularly used for networking remote locations within a corporate network or leased line to the Internet.


V.110 is a protocol of ITU- T for the use of terminal equipment with V-series interfaces (for example, V.24 interface) to integrated services networks. V.110 realized a bit rate adaptation for adapting the data transfer rate slow devices, such as modems to ISDN. The data transfer rates are standardized to 19.2 kbit / s; in most V.110 -compatible terminal adapters but data transmission rates up to 38.4 kbit / s available. Each bit of the V- port is mapped to one bit of the 64 kbit / s stream of the B channel, the remaining capacity being filled with padding bits. In some implementations, lower speeds can be used in multiplex mode. The bit rate adaptation described in V.110 is often used outside of ISDN.


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


The ISDN signaling is at out-of- band - it is transmitted on a separate channel, and not, as in the analogue network, for example by means of multi-frequency dialing mode in the voice channel. Thus the connection is established and the control of call processing features work safer and faster. Technical is used for the signaling of the D-channel, in the base terminals of a data rate of 16 kbit / s and primary rate connections of 64 kbit / s.

In the core network the protocol Signalling System No. 7 is used for signaling between the switching centers on the so-called central signaling channels.

Reference points and interfaces

An ISDN connection consists of two parts: the local loop ( the base connection the UK0 interface; during primary rate access the UK2 interface) and the in-house cabling (at the base terminal of the S0 bus, the PRI, the S2M interface). The local loop is terminated by a network termination ( NT at the base terminal, the PRI NTPM ).

Functional units:

  • ET: Exchange Termination ( and transfer completion ) ( local exchange ) Exchange ( layers 1 to 3)
  • Line transmission equipment
  • Reaction between relatively niedrigratigem subscriber and hochratigem Multiplexanschuss on the exchange side
  • Layer 1
  • Layer 1-3
  • Optional, met ( if any) mediating or concentrating tasks (eg a telephone system )
  • TE2 adapts to the requirements of NT1 and NT2
  • Device that meets all the ISDN interface recommendations
  • Device that does not meet the ISDN interface recommendations

The interface to computer software is usually produced by the CAPI. Under Linux the Hisax drivers have been previously used.

Addressing for ISDN

ISDN addresses are defined by the ITU- T recommendation E.164. The ISDN address consists of the ISDN number and subaddress. The ISDN number is addressed, for example, a participant in a base terminal. The sub-address is a maximum of 32 characters long and is used for example for addressing a host in a LAN ( this must be connected to a suitable gateway on ISDN). The subaddress is transparent for ISDN and known only to the participants -using.

ISDN Emulation over NGN

Since the increasing migration of circuit-switched fixed networks towards NGN network topology provide some providers using IP telephony "ISDN" over existing broadband connections as so-called ISDN - NGN terminals. This is done where the provider does not maintain local exchanges or they are no longer continues to expand and its telecommunications service instead exclusively by means of data connection intermediate ( bitstream access, T- DSL resale ) or offering their own DSLAMs.

Instead of the NT network termination to the ISDN S0 bus provided for ISDN terminals is carried out by an Integrated Access Device and the communication is IP - based on a SIP gateway. Here ISDN typical characteristics are simulated or emulated, but there is no full-fledged DSS1 ISDN connection; due to a lack of data channel only voice services are mostly supported - several ISDN features are therefore not available ( eg, Group 4 fax, B- channel bundling, Datex -P, Park / Unpark ).

Most lack the emergency power supply. In case of failure of the normal power supply to the participants of such emulated ISDN connection is not reachable in the absence of the UPS and can not make calls, unlike a connection with emergency power supply. With ISDN over IP is a proprietary protocol that ISDN with all the features exist also allows for using Voice -over- IP connections, but is rarely used because of the lower cost feasible SIP.