A telephone network (or outdated telephone network ), also PSTN ( English Public Switched Telephone Network ), is understood to be a communication system that is designed for the handling of telephone calls ( telephony ). Telephone network is an ideal term and is therefore independent of the technology or architecture used. A telephone network exists when the following properties that go beyond the requirements of a communication system are present:
- A conversation partner can be dialed or called by entering a phone number.
- The establishment of the connection between the interlocutors mainly serves the exchange of language.
- After the call has ended, the established connection is cleared down again so that the equipment used (telephone, network resources) is available for new call connections.
A telephone network includes all resources that are used directly to set up a telephone call. A telephone network can be public, one then speaks of a public telephone network (for example the fixed network or cellular network ), or it can be private, one then speaks of private telephone networks (for example company telephone networks or the telephone network of the German Armed Forces).
Development of the telephone network
At the beginning of the development of the telephone network in 1877 there were lines through which two telephones were directly connected to one another. There was no way to reach other participants.
From 1881 telephone exchanges were built in which different participants could be connected to one another by manual plugging. The transfer was carried out via people (the " Miss from Office ") to whom the caller named the desired subscriber.
The telephone network that was predominant until around 1990 consisted of individual lines that could transmit analog audio signals. The bandwidth was limited to the frequency range from 300 to 3400 Hertz . In addition to the voice signals , signals such as the ringing signal (ringing), audio tones (e.g. ringing tone, busy tone) and tariff units (for the charge display ) are transmitted to the subscriber set. The local exchanges supply the connected devices with a direct voltage of around 60 volts, but only when the receiver is off the hook. This supply is called the feed .
Since around 1980, the analogue telephone network ( AFeN ) for analogue narrowband connections (analogue telephone connections) (see also narrowband communication ) has been expanded into a digital service-integrated universal network ( ISDN ) that not only handles voice services, but also integrates a large number of other digital services. Since then, the term "telephone network" has become obsolete. Also, mobile and Internet use parts of today's telephone network.
With the switching-related performance features , a telephone network provides numerous services for the end user.
With analog telephones only frequencies of approx. 300 Hz to 3.4 kHz are transmitted (bandwidth 3.1 kHz). According to CCITT , the signal-to-noise ratio should be better than 34 dB. If it falls below 18 dB, speech understanding is significantly restricted. The basic frequency of male speakers is almost 100 Hz. Women speak almost an octave higher with a basic frequency of approx. 180 Hz. The fact that the other person on the phone can still tell whether he is speaking to a man or a woman is due to the overtones of the language. These higher frequencies are transmitted on the telephone, and the human brain reconstructs the basic frequency from the overtones. This psychoacoustic phenomenon is called the residual effect .
In an ISDN - subscriber line is since the introduction of DSL services on the copper twisted pair uses a much higher bandwidth than 3.1 kHz: In addition to ADSL and SDSL , who used a frequency range up to about 1 MHz, at will VDSL already the frequency range up to 17.6 MHz and used in other countries at a gross data rate of 200 Mbit / s up to 30 MHz.
Network levels and the structures of a telephone network
A subdivision of today's telephone networks results from their different functions. Various sub-networks are used today to operate a telephone network:
This sub-network is used for the efficient connection of the individual participants to the connection network, which is wired in several hierarchical levels.
This sub-network is essentially formed by the switching centers as nodes and the connecting lines between these nodes. The task is to switch and manage the communication channels between the participants, as well as to enable the connection via higher-level or to remote exchanges and to the individual network participants.
As long as there were only analog exchanges in the telephone network, all information that was necessary to set up and clear calls had to be transmitted in the voice channels (code number selected, subscriber picked up / hung up, ringing tone, etc.).
Since the introduction of digital exchanges, voice transmission and the transmission of signaling information have been separated. The call setup and cleardown is controlled via the signaling network. This means that the dialed number is transmitted and the exchange of the desired participant is found, all participant actions are transmitted via this (pick up the receiver, dial, hang up, put on the ring tone, etc.), and the establishment and dismantling of the voice channels is also coordinated. All systems in the telephone network are connected to one another via the signaling network. The core of this network are Signaling Transfer Points (STP), which use signaling system no. 7 as the signaling protocol.
- Data network
Services such as the Internet, e-mail, etc. are handled via data networks. If a subscriber wishes a data connection (e.g. via DSL), this is already recognized in the local exchanges and a corresponding connection to the dial-in node is switched from there to the Internet.
Each individual sub-network belongs to a so-called network level.
New technologies in transmission and their higher bandwidth availability are already determining the next steps in the development of the telephone network. While dialog services are predominantly available in ISDN that provide the subscribers with the same bit rate, the asymmetrical bit rates of DSL technology can in future be used to integrate not only IP telephony but also distribution services ( broadcasting ) such as radio , television or video-on-demand .
Networks under construction today are Next Generation Network (NGN) and IP Multimedia Subsystem (IMS) networks. These networks are no longer referred to as actual telephone networks, but are generally viewed as communication networks which include the function of a telephone network. The German Telekom plans to replace by 2018 all existing ISDN connections by NGN. Analog connections will continue to be offered via MSAN technology.
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- MSAN POTS | Telekom business customers. Retrieved April 9, 2019 .