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Structure of the telephone network (landline)
"Self-connection office" board, Nuremberg Transport Museum

A switching center is a node within a communications network that enables communications connections to be established as desired.

Types of exchanges

The switching centers were developed in several steps, with the initially common analog switching centers being replaced by digital switching centers in the early 1980s.

Manual telephone exchange around 1890 on a postage stamp of the Deutsche Bundespost from 1990

Analog exchanges

Analog exchanges switch an analog electrical signal via electromechanical and (rarely) electronic assemblies. The last analog exchange in Germany's public telephone network was replaced by a digital system in December 1997.

Manual mediation

Initially, telephone calls were switched manually ( “Miss from the office” ). The switching took place in telephone exchanges, z. B. using flap cabinets .

Manual switching is no longer available in public networks today. On May 16, 1966, the last manual exchange in the network of the former Deutsche Bundespost was switched off in Uetze near Hanover.

At the Reichsbahn and in the military sector, hand-operated telecommunications equipment operated by field telephones was still in use as OB technology until the end of the 1990s .

Rotary dial in a local exchange

Electromechanical exchanges

The next stage of development was the introduction of automatic switching centers, formerly known as "self-connection exchange". Here the mediation carried out by electromechanical voters (mainly Strowger switch , rotary knobs, motor control knob ; see also autodialer ). These dialers were controlled directly by the pulses generated by the phone when dialing the number.

With the introduction of the self-dialing remote service (SWFD), the local area codes / OnKz (“area codes”) were added to the subscriber numbers . Certain groups of digits of a phone number with area code were processed by special hierarchy levels at exchanges. The exchange types existed

  • Foreign exchange (AVSt),
  • Central exchange (ZVSt) (first digit of the local area code),
  • Main exchange (HVSt) (second digit of the area code),
  • Node switching center (KVSt) (third digit of the local area code),
  • Local exchange (OVSt) (last digit of the local network code), also known as final exchange (EVSt), subscriber exchange (TVSt) or (dialing) exchange (subscriber number). In the OVStn, the charges to be billed were recorded by means of a charge counter .

Such exchanges no longer exist in public telephone networks today. Analog electronic circuits could later also process the signals of the multi-frequency dialing method (DTMF).

Digital switching centers

Digital exchanges ( Nortel )

Digital switching centers convert the analog language into digital data packets and switch a digital signal via electronic assemblies. Another feature of digital networks is that the subscriber's voice signals and the signaling information required to set up and clear a telephone call are transported in different subnetworks.

Digital electronic exchanges

From the beginning of the 1980s, the digital switching centers were introduced. They convert the analog voice signal into a digital signal. At first the analog network structure was retained; only the older, analogue transferring exchanges were replaced by digital electronic ones. In the network of the German Federal Post Office a distinction was made between

  • Foreign exchanges (DIVA),
  • Remote Switching Centers ( DIVF ) and
  • Local exchanges ( DIVO )

Digital switching centers with a meshed network structure

In the mid-1990s, the switch to digital switching centers in the Deutsche Telekom network was completed. A meshed network structure was introduced for the digital network, in which the exchanges for the long-distance network were no longer hierarchical, but rather meshed with one another. The digital switching centers are typified as follows:

  • Local network switching unit (VE: O),
  • Switching unit long-distance network (VE: F),
  • Switching unit with network transition function (VE: N),
  • Foreign exchange unit (VE: A).

Packet switches

While the previous digital switching network is based on wired switching, a replacement by packet-switched technology ( Next Generation Network , IP Multimedia Subsystem , Next Generation Mobile Networks ) has largely been completed. The function of a digital switching center is split up into the function of a media gateway controller , which organizes the path of voice and data traffic through the network, and media gateway , which forwards the voice and data packets according to the specifications of the media gateway controller.

Mediation types

Line switching

In the case of line switching , the terminals are assigned a channel with a fixed bandwidth for the entire duration of the message exchange . For this purpose, feeder lines and subscriber lines are permanently connected to one another via a switching network for the duration of the connection .

Until the 1990s were in the switching networks complex electromechanical components used by the exchanges. These included, in particular, step-by-step selectors such as the lever -operated rotary selector and the precious metal contact motor rotary selector . These took up a lot of space and were very worn out, so that the operation of an exchange was quite labor-intensive. In addition, there were still voters loose exchanges, in which the connection example using the crossbar switch was made. From the 1970s, a then new type of dialless switching technology was used in private branch exchanges that worked with ESK relays . These relays had a high level of operational reliability with little maintenance and a long service life. Nevertheless, due to the increasing competition from semiconductor components, which offered considerably more advantages, this technology was not able to establish itself across the board.

Since the 1970s, one began to gradually replace the electromechanical components in the switching matrices with electronic components, for example with field effect transistors and integrated circuits . With the changeover from direct control to indirect control, it was possible to offer new performance features and, for example, to enable multi-frequency dialing .

Another milestone was the switch from analog to digital switching technology. The voice signal is no longer galvanically switched through, but transmitted via a digital switching network in the form of a 64 kbit / s PCM data stream.

Packet and cell switching

In the packet switching , the incoming message blocks (are controlled by a switching software data packets ) stored and corresponding to the head of the message block ( header forwarded destination information contained) via a further conduit portion. Since there are frequent pauses in a conversation during which a permanently allocated line would be virtually unused, lines can be used to capacity so that data packets from different calls are transported on one line.

Packet-switching networks such as the Datex-P network have switching centers.

The nodes that are used in IP networks to forward the packets (IP routing and IP switching) are usually not referred to as exchanges. This is also not common with the frame switching used by Ethernet switches. In the case of cell switching used by ATM , however, the term exchanges is again used, but only when it comes to large public networks (which are very rare, there are only a significant number of ATM exchanges in the Department of Defense ). In the case of ATMs in company networks, only the expression "switch" is common.

The IP telephony uses IP for voice transmission and transmits the IP packets over the Internet . The devices that control the message paths for the IP packets are not called switching centers because their functions differ greatly from those of traditional switching centers. Instead, new terms such as softswitch and media gateway are used .


Digital switching network

Digital switching center (Nortel)

In contrast to earlier telephone networks , today's telephone networks no longer have a distinct hierarchical structure. In the Deutsche Telekom network, for example, only two of the four previous hierarchy levels remain.

Remote exchanges (VE: F, old name: DIVF ) form the top level. These switching centers are strongly meshed with one another. These switching centers often have network transition functions (VE: N) in order to be able to forward calls from their own network to the networks of other national telephone companies .

A through -going exchange is an exchange that only handles traffic between exchanges, but to which as a rule no subscribers are connected. Long distance exchanges are transit exchanges.

Foreign exchanges (VE: A) mediate traffic between different countries. Foreign exchanges are connected to long-distance exchanges. You have no switching function within your own network.

Local exchanges (VE: O, old name: DIVO ) form the lowest level. You manage the customer connections . Several local exchanges are star-connected to a long-distance exchange.

A local exchange can manage 10,000 to over 100,000 subscribers , depending on the configuration . Several local exchanges can therefore exist in large cities. The identification of local exchanges by the first 1–3 digits of a telephone number has often been adopted by the former analog network, it means that the telephone numbers of a district always begin with the identical 1.– 3. Digits. It is no longer possible to assign certain number ranges to an exchange in all cases. On the one hand, newly assigned numbers are no longer distributed geographically; instead, in Germany the numbers are now assigned to the telephone providers in blocks. On the other hand, the phone number can be geographically ported by moving, i.e. taken to a different number range. When the associated connection is no longer available, however, the geographically ported call number returns to the original exchange area.

Many smaller communities share a local exchange (parent VSt), which together with the cable network forms the local network of a place. Sub-exchanges in other locations can be connected to this parent VSt. The sub-switching centers usually only take on a concentration of the offer . They are also known as the Remote Peripheral Unit (APE). The need to use remote switching equipment arises from the fact that the connection line to the subscriber cannot be extended at will. The loop resistance and the attenuation set physical limits that could otherwise only be pushed out with an additional device in the connection area.

Former analog exchange network

Rear cabling of a rotary selector bank in the analog exchange network

Since each dialed digit of a phone number was evaluated individually in the analog network, it was necessary to structure the network hierarchically.

Local network

The lowest level essentially consisted of one or more local or terminal exchanges, with one or more digits assigned to each exchange.

The numbers 1 to 8 were assigned to the individual areas of an OVSt or the EVSt'n; however, if the first digit was a 1, it could not be followed by a further 1, unless it was an emergency number or a special number . For example, a VSt with the numbers 2 and 3 could be provided for the core city, the 5 and 6 with another VSt for districts in the west and north, the 7 for a district in the east of the city and the 8 for an adjacent town - likewise with own tax. This division kept the line length to the participants within reasonable limits.

The 9 was originally a traffic elimination number for the simplified self-dialing remote service (vSWFD). It consisted of cross-connection routes to neighboring cities, to which there was a lot of telephone traffic. Later, after the vSWFD was abolished, the number 9 was used in order to be able to offer ISDN in local networks that were still connected to an analog exchange . These connections were then supplied from other exchanges. Not all local networks used Gasse 9 for this; with some local networks this was already occupied, so that one dodged to other streets .

From 1952 (previously manually mediated) the number 0 in the transition technology-I (2-wire technology) and -II (from 1956, 4-wire technology) became the traffic elimination number in the SWFD. At the beginning of the 1960s, the T62 remote dialing system with a uniform numbering plan was introduced.

Long distance network

By choosing 0 as the first digit, a connection was established to the telephone exchange network ( FVSt ), which consisted of three hierarchical levels :

  1. Central switching offices (ZVSt, also ZA for central office)
  2. Main switching offices (HVSt, also HA for main office)
  3. Node switching centers (KVSt, also KA for node exchange)

The local exchanges (OVSt), also called local office (OA) or end office (EA), no longer belonged to the long-distance network.

The second digit of an area code established a connection to the top hierarchical level (ZVSt), unless it was another 0 , because then a connection to the international exchange was established.

The following digits led through the hierarchy levels via HVSt (3rd digit) and KVSt (4th digit) to OVSt (5th digit).

Central switching areas

The number of central exchanges was specified on the basis of the existing digits (0 to 9). However, in the event of an early reunification, at least ten ZVStn would have been required, which is why two-digit ZA codes had to be provided (2 to 9 and 12 to 19).

Map of the federal German telephone network with hierarchically represented switching centers 1970, in the Official Telephone Book of Berlin (West) 1970/71, digitized by the Central and State Library Berin,

Central office codes
  • 1 → no central switching area, was used for special numbers and as a lane for the originally planned two-digit ZVStn. reserved
  • 11 → Special services
  • 12 → Rostock (planned)
  • 13 to 18 → reserve
  • 19 → Nuremberg (planned, "provisional" instead 9)
  • 10 → remote office (manual switching)
  • 2 → Düsseldorf
  • 3 → Berlin
  • 4 → Hamburg
  • 5 → Hanover
  • 6 → Frankfurt am Main
  • 7 → Stuttgart
  • 8 → Munich
  • 9 → Nuremberg (initially provisional, would have been given to Leipzig in the event of early reunification )
  • 0 → traffic elimination number (international exchange)

Each ZA could supply up to ten HÄ (main offices) and these in turn theoretically up to nine (special case, since the 1 for the local office at the location of the HA is lost) KÄ (node ​​offices), which is also explained by the existing number of digits from 0 to 9 .

The HA at the location of the ZA was given the number 1 (if more than nine KÄs were to be connected to this HA, the 0 was also used for the other KÄs; this reduces the number of HÄs that can be connected to this ZA to nine; the number of this HA connectable KÄ increases from nine to nineteen).

The local network at the location of an HA received a shortened area code with the last digit 1 in the place of the KA.

The local network at the location of a KA receives a code ending with 1 .


This hierarchical network structure also existed in the telecommunications network of the GDR , with the difference that the ZVSt network level did not exist. There were numerous bundles of branches and meshes between neighboring nodal networks. Since, with one exception, only directly controlled switching systems were used, there were usually two different area codes for a local network, depending on the local network of the caller . One preselection for the control path and one for the mesh path. The connection fees with the Maschenweg were cheaper than with the regular route. Area codes for standard route began with either 00 or 09, international codes began with 06; an automatic long-distance call to Czechoslovakia began, for example. B. with 0642. West Berlin could not be reached abroad due to the four-power status from the perspective of the GDR with regard to East Berlin and accordingly could be reached from there via the area code 849.


In contrast to the original West German plan, which provided for alleys 03, 09 and 012, the new federal states have been supplied mainly through alley 03 since reunification . Here the area code can be five digits (for example 034567 / xxxx). However, this would not have been possible with the original analog technology; a single ZVSt would not have been enough for such a large area.

The length of a phone number is coordinated internationally. Up to December 31, 1996, a maximum of 12 digits were allowed, including the international area code and the digits of the direct dialing , but without traffic exclusion digits . Since then, according to ITU-T recommendation E.164, up to 15 digits have been permitted, without traffic elimination digits.

The changeover took place step by step with the commissioning of the digital telephone exchanges ( DIVF ) responsible for the surrounding OVSt'n . The AVON , in which the current area codes were announced, appeared as required.


A subscriber in the Bad Zwischenahn local network had the number 04403 / xxxx: The first 4 set up a connection via Hamburg (ZVSt). The second 4 led this connection to the HVSt in Oldenburg and the following 0 initiated a connection to the KVSt (in this example also at the Oldenburg location). By dialing number 3 , the connection to the OVSt Bad Zwischenahn was fully established and the evaluation of the local number began.

However, if a subscriber from the Rastede local network with the area code 04402 wanted to reach this number in Bad Zwischenahn, it was uneconomical to use the hierarchical route via Hamburg. Cross-connection routes have been established for such goals . In this example, both local networks were connected to the same KVSt (440 - Oldenburg) and the connection route was only established via Oldenburg.

Cross-connection routes were set up according to economic aspects and have further refined the mesh network existing at the top level between the ZVStn at the HVStn and KVStn levels. With frequent telephone traffic between two local networks, there were also cross-connection paths between the OVStn, so that hierarchical connection establishment was no longer necessary.


In Austria, the analog network structure was similar to that in Germany, but the individual network levels had different names:

  • Main area office (corresponds to the German central exchange; in Vienna , Linz , Salzburg , Innsbruck , Klagenfurt and Graz )
  • Node office
  • Verbundamt
  • End office
  • Partial office (in places with several exchanges, the first digits of the telephone number could be differentiated)

The transit office was located in Vienna as a foreign exchange. Since December 14, 1972, the automation of automatic dialing traffic in Austria as well as to Germany, Switzerland and Italy was completed.


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See also

Web links

Commons : Central Office  - collection of images, videos and audio files
Commons : Switching Equipment  - Collection of pictures, videos and audio files

Individual evidence

  1. a b Key figures for the German long-distance election from 1958
  2. ^ Telephone book for the capital of the German Democratic Republic Berlin , 1989 edition - Deutsche Post, Berlin