Signaling protocol

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Signaling protocols are communication protocols that the signaling used.

Telecommunication networks

In telecommunications, a distinction signaling protocols (English: signaling protocols ) used to be, Zeichengabe- or telecommunications method called signaling systems (English: signaling system ). Protocols are procedural rules with which control messages are encoded, decoded and interpreted. Several protocols, arranged in so-called layers (see also ISO / OSI reference model ), serve to cover a network requirement and form a signaling system. Protocols are usually subject to standardization in order to be able to combine products from different telecommunications manufacturers . The protocols standardized in international and national committees contrast with proprietary protocols that are not subject to any standardization and are mostly issued by individual manufacturers.

A well-known example of a signaling system is the Signaling System No. 7 . A signaling protocol therein would be, for example, SCCP or ISUP .

Types of signaling protocols

Transmission path of the signaling information

In principle, a distinction must be made between the following variants of the transmission of the signaling information:

  • User channel signaling (in-band signaling)
    Here the signaling information is also transmitted in the user channel. Used for example in analog - phones and (formerly) in analog telephone networks . Traffic channel signaling method is also known as channel-associated signaling (engl. Channel-associated signaling )
  • Ancillary channel signaling (out-of-band signaling)
    Here, the signaling information is transmitted in a separate signaling channel that is directly assigned to the user channel. Used, for example, in ISDN telephones and in digital telephone networks .
  • Central channel
    signaling Here, the signaling information for a larger number of useful channels, which can be distributed over several trunk groups, is transmitted in a common signaling channel. With the Signaling System No. 7 the case.

Function in the telephone network

The types of signaling are also differentiated according to their function in the telephone network, whether they define interfaces between network nodes ( NNI ) or between network nodes and connections ( UNI ).

  • Examples of signaling protocols on user interfaces (UNI):
  • Examples of signaling protocols between network nodes within a telephone network (NNI):
    • Signaling System No. 7 (digital, used today with almost no exception)
    • Signaling System No. 5 (last analogous procedure, only international)
    • Pulse dialing (outdated method, no longer used for telephone networks in Europe).

Transmission type of the signaling information

Another distinction is made between analog and digital transmission of the signaling protocols. Although analog switching technology and its analog signaling between switching nodes are no longer used in Europe today, they used to play an important role in the first construction of a global telephone network. In principle, characters or signals are always digitally coded; they convey discrete information on a certain processing level . However, the difference is in the physical layer of transmission that was used in the older analog technologies to distinguish characters. They are not binary coded here, as in the so-called digital signaling , but their differentiation is based, for example, on the length of a signal, see pulse identification method 50 ( IKZ 50 ) or the pitch of a transmitted tone, such as in multi-frequency dialing methods.

  • Examples of analog signaling protocols:
    • IKZ 50
    • Pulse dialing
    • Multi-frequency dialing
  • Examples of digital signaling protocols:
    • Signaling System No. 7,
    • DSS1 (ITU-T Q.931)

Packet-switching procedures

In modern packet-switched networks, signaling information is also carried over IP connections . An example of this is

In the field of VoIP , new, protocol-transparent signaling is becoming more and more important. Fall under this


In the Internet , signaling is used to configure middleboxes or end nodes to create permanent ( hard states ) or volatile ( soft states ) states. Middleboxes are nodes that are positioned between a sender and receiver and, in addition to forwarding data packets, change or evaluate them.

Examples of signaling protocols are protocols for

A generic protocol framework for signaling was developed in the IETF Working Group Next Steps in Signaling . For this purpose, RFCs were created for


  • Volker Jung, Hans-Jürgen Warnecke (Hrsg.): Handbook for telecommunications. Springer Verlag, Berlin / Heidelberg 1998, ISBN 978-3-642-97703-9 .
  • Wolf-Dieter Haaß: Handbook of the communication networks. Introduction to the basics and methods of communication networks, Springer Verlag Berlin Heidelberg, Berlin Heidelberg 1997, ISBN 978-3-642-63825-1 .

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