# Integrated Services Digital Network **Integrated Services Digital Network (ISDN)** is a set of communication standards for simultaneous digital transmission of voice, video, data, and other network services over the traditional circuits of the public switched telephone network. It provides a reliable and efficient means of transferring data, allowing users to connect multiple network services through a single line without additional phone lines. ![](/files/8szwL5w5OID95X95NTps) 15 digits are used in ISDN addressing as defined in E.164 ### **Addressing and Numbering** In ISDN, a 15-digit number is used for addressing as per the E.164 recommendation. This numbering plan ensures that each device on the network has a unique identifier, facilitating efficient routing and connectivity. The structure of the E.164 number includes: * **Country Code (CC):** The international code identifying the country. * **National Destination Code (NDC):** Specifies the region or network operator within the country. * **Subscriber Number (SN):** The number assigned to individual subscribers. These components enable seamless international communication through standardized numbering. --- # 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://isdn.learn-telecom.com/integrated-services-digital-network.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.