# Glossary: ISDN ### TelcoBridges and ISDN Tmedia and Tdev systems can be configured to provide an ISDN signalling stack on the trunks of one or more units to meet system configuration requirements. ISDN signaling stacks are signaling resources that are assigned to a specific line of the Tmedia or Tdev unit. Based on the [Q.931](http://en.wikipedia.org/wiki/Q.931) switch variant that will be used by the system, a selection of ISDN variants are available. For each line service requiring ISDN signaling, an ISDN signaling stack is created with its own distinct signaling variant. \ ![](https://docs.telcobridges.com/w/images/4/43/ISDN_signaling_stacks.jpg) \ Note: The trunks of each Tmedia or Tdev unit can be configured to have their own ISDN signaling stack. ### References * [Wikipedia article](http://en.wikipedia.org/wiki/Integrated_Services_Digital_Network) * [Supported ISDN standards and variants](/hidden-pages/glossary-isdn/isdn-specification.md) --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://prosbcdocs.telcobridges.com/hidden-pages/glossary-isdn.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.