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Q.931 is an ITU-T recommendation that defines the signaling protocol used in the ISDN (Integrated Services Digital Network) for call setup and teardown between network elements. Here are some key points about Q.931:

  1. Signaling Protocol: Q.931 specifies the signaling protocol used for establishing, maintaining, and releasing connections between ISDN devices. It is a digital signaling protocol that operates at the network layer (Layer 3) of the ISDN protocol stack.
  2. Call Setup: Q.931 defines the messages and procedures for call setup. When a call is initiated, Q.931 signaling is used to exchange information between the calling and called parties, including the addresses, call type, capabilities, and other call-related parameters.
  3. Call Teardown: Q.931 also specifies the procedures for call teardown. When a call is terminated, Q.931 signaling is used to exchange the necessary information to release the call resources and notify the parties involved.
  4. Message Structure: Q.931 messages consist of a fixed-length header and variable-length information elements. The header contains information such as the message type, call reference, and message length. The information elements carry the call-related parameters and other signaling information.
  5. Message Transfer: Q.931 messages are transferred between ISDN devices using the Data Link Layer protocol, such as LAPD (Link Access Procedure on the D channel), which provides reliable and error-free data transfer over the ISDN D channel.
  6. Call Control Procedures: Q.931 defines various call control procedures, including call setup procedures, call progress procedures, call supervision procedures, and call release procedures. These procedures ensure the proper establishment, progress, and termination of ISDN calls.
  7. Supplementary Services: Q.931 also supports supplementary services, which are additional features and functionalities offered by the ISDN. Examples of supplementary services include call forwarding, call transfer, call waiting, and conference calling. Q.931 defines the signaling procedures for activating and deactivating these supplementary services.
  8. ISDN Variants: Q.931 is used in various ISDN variants, including the ISDN Basic Rate Interface (BRI) and the ISDN Primary Rate Interface (PRI). The ISDN BRI provides two B channels (for data and voice) and one D channel (for signaling), while the ISDN PRI provides multiple B channels and one or more D channels.

Q.931 plays a crucial role in establishing and managing calls within the ISDN network. It ensures the proper exchange of signaling information between ISDN devices, allowing for effective call setup, teardown, and supplementary service activation.

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