ISDNQ931

ISDNQ931 refers to the Q.931 signaling protocol used in ISDN networks to control call setup, maintenance, and release over the D-channel. It is an ITU-T standard and forms the layer 3 signaling used between user equipment and the network signaling entity on ISDN interfaces, including both Basic Rate Interface (BRI) and Primary Rate Interface (PRI). Q.931 messages are carried over LAPD, the ISDN Link Access Procedure on the D-channel, and work in conjunction with lower layers to manage the signaling path.

The protocol defines a set of messages that govern the lifecycle of a call. Core messages include Setup for initiating a call, Progress or Alerting to indicate call status, Connect to establish a bearer for the media path, and Release or Release Complete to terminate the call. Additional messages support call modifications and network signaling control. The messages carry information elements such as the called party number, calling party number, bearer capability, channel identification, and facility data, allowing the network to negotiate call parameters and features during setup.

The typical call flow involves a terminal sending a Setup message to the network, which responds with procedures such as Call Proceeding or Alerting. When the called party answers, a Connect message is exchanged to establish the connection. If the call is abandoned or completed, a sequence of Release and Release Complete messages ensures proper tear-down and billing data generation. Q.931 thus provides a standardized, interoperable means of ISDN call control across equipment from different vendors.

ISDNQ931 remains a foundational element of ISDN signaling and is referenced in ITU-T Q-series standards. While newer signaling approaches exist for IP-based networks, Q.931 continues to be encountered in traditional ISDN deployments and interworking scenarios.