PRI ports use a separate channel called the D-Channel for call-control and signaling messages. This is called out-of-band signaling. The D-Channel associated with the set of B-Channels is an ISDN-PRI signaling link’s port (ISDN-PLK). The signaling protocol used on the ISDN-PRI signaling-link port’s D channel is defined by one of the four selectable ISDN-PRI Specifications: Avaya, CCITT, ECMA, and ANSI.

The ISDN-PRI Specification defines the possible service states for a B-Channel. The service state is negotiated with the far-end terminal adapter, changes over time, and can have far- and near-end components. The service state is initialized to Out-Of-Service/Far-End, and an attempt is made to negotiate it to In-Service. Use status pri-endpoint extension to display the service state of a particular PRI endpoint port’s B-Channel.

If a call is present, the Specification defines the permissible call states as well. There are tests in the short and long test sequences for the PRI endpoint port designed to audit these states and ensure agreement between both ends of the PRI wideband connection.