The Q.931 Recommendation ( Recommandation ) of the ITU -T ( formerly CCITT) describes layer 3 of the Protocol (ISDN user - network interface layer 3 specification for basic call control ) and is used for signaling in ISDN and H.323.

A Q.931 frame is composed of:

  • Protocol discriminator (PD ), protocol discriminator
  • Call reference value ( CR) connection detection
  • Message type (MT) message type
  • Information element (IE), information elements
  • ISDN
  • Network protocol
  • Switching technology
  • ITU Recommendation