Public switched telephone network

With a telephone network (or outdated telephone network ) refers to a communication system which is designed for the handling of telephone calls (telephony). Telephone network is an idealistic concept and thus independent of the technology or architecture being used. A telephone network is available if the following properties that go beyond the requirements of a communication system are present:

• A conversation partner can be selected or called by entering a phone number.
• To establish the connection between the interlocutors is mainly used for the exchange of language.
• When the call is established connection is broken down again so that the resources used ( telephone, network resources) are new call connections available.

A telephone network including all operating means used, which are used to establish a telephone call immediately. A telephone network can be public, then one speaks of a public telephone network (for example, the fixed network or mobile network), or it can be private, then one speaks of private telephone networks ( for example, company phone networks or the telephone network of the German Federal Armed Forces).

Development of the telephone network

At the beginning of the development of the telephone network in 1877 were lines were connected directly to each other by the two telephones. There was no way to reach other users.

As of 1881 arose telephone exchanges, in which different participants could be connected to each other by manually repositioning. The agency ran over people (the " Lady of the Office"), which called the caller the desired participant.

1892 an independent exchange was invented, which was used from 1908 on the local networks and from 1923 in the trunk exchanges.

The predominant until about 1990 telephone network consisted of single lines that could transmit analog audio signals. The bandwidth was limited here to the frequency range 300-3400 Hz. In addition to the speech signals signals as the ring signal (ringing ), call progress tones ( dial tone, busy, see also: acoustic telephone signals) are still and tariff units (for the charge indicator ) transmitted to the subscriber apparatus. The local exchanges provide the connected devices with a DC voltage of about 60 volts, but only when the handset is off-hook. This supply is referred to as power.

Since about 1980 the analog telephone network ( AFEN ) for analog narrowband lines ( analog telephone ports ) (see also narrowband communications ) to a digital Integrated Services Universal Network (ISDN ) is removed, processed through not only voice services, but also a variety of other digital services have been integrated. Since then, the term " telephone network " is outdated. Also, mobile and internet use parts of today's telephone network.

With the call-processing -performance features, a telephone network a range of services for the end user are available.

For analog phones only frequencies from about 300 Hz to 3.4 kHz transmitted ( ie bandwidth 3.1 kHz). CCITT the signal - to-noise ratio will be better than 34dB. Falls below 18dB to language comprehension is severely limited. The fundamental frequency of male speaker is just under 100 Hz women talk almost an octave higher with a fundamental frequency of approximately 180 Hz that the caller can still tell whether he is talking with a man or a woman, is due to the overtones of the language. These higher frequencies are transmitted on the phone, and the human brain reconstructs the overtones of the fundamental frequency. This psychoacoustic phenomenon is called residual effect.

With an ISDN subscriber line a much higher bandwidth than 3.1 KHz is used since the introduction of broadband services on the copper pair: In addition to ADSL and SDSL who used a frequency range up to about 1 MHz, with VDSL already the frequency range is up to 17.6 MHz and used in other countries with a gross data rate of 200 Mbit / s up to 30 MHz.

Power levels and the structures of a telephone network

A subdivision of today's telephone networks arises from their different functions and are now used for the operation of a telephone network:

This subnet is used for efficient binding of the participants in the interconnection network.

This subnet is essentially formed by the exchanges. Mission is to connect and manage the communication channels between the participants.

As long as only analogue exchanges were in the telephone network, had all the information that was necessary for call establishment and termination, are transmitted in the voice channels with (elected measure, participants took off / hook, create ringtone, etc.). Since the introduction of digital exchanges voice communication and transmission of the signaling information are separated. About the signaling network call origination and termination is controlled. That is, it is transmitted and found the switch of the desired party the dialed number, all participants actions ( Lift the handset, select, apply, create ringtone, etc.) transmitted on the subject, and the assembly and disassembly of the voice channels is coordinated here on. All facilities of the telephone network are connected via the signaling network. Central point of the network, signal transfer points (STP) that use the signaling system No. 7 as a signaling protocol.

About data networks services such as Internet, email, etc. are handled. Should a participant a data connection (eg DSL ), this is already recognized in the local exchanges and connected from there an appropriate connection to the access node to the Internet.

Each sub-network forms a so-called network level.

Technology and Trends

New Techno Gien in the transmission and the higher bandwidth availability determine already the next development steps of the telephone network. While in the ISDN dialog services are still mainly available that provide the participants the same bit rate available even distribution services (broadcasting) such as radio, television or video -on-demand will be able to be integrated with the asymmetric bit rates of the DSL technology.

Today, networks are located in the building next generation network (NGN ) and IP Multimedia Subsystem ( IMS) networks. These networks are no longer referred to as the actual phone networks, but are generally regarded as communication networks, which include the function of a telephone network. The German Telekom plans to replace by 2015 all existing ISDN and analogue connections by NGN.