# Physical Layer The ISDN Physical Layer is the first layer in the OSI model for Integrated Services Digital Network (ISDN) networking. It is responsible for the transmission and reception of raw bit streams over a physical medium. Key characteristics include: * **Interface Types**: Supports two main interfaces - Basic Rate Interface (BRI) and Primary Rate Interface (PRI). * **BRI**: Consists of two 64 Kbps B-channels and one 16 Kbps D-channel. * **PRI**: Includes 23 B-channels and one 64 Kbps D-channel in North America or 30 B-channels and one D-channel in Europe. * **Transmission Medium**: Typically uses twisted-pair cables, but may also use fiber optics. * **Functions**: Converts digital signals to electrical signals and vice versa, ensuring proper timing and synchronization. It serves as the foundation upon which higher ISDN layers build, facilitating data exchange over ISDN connections. --- # 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/layers/physical-layer.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.