# Data Link Layer The ISDN (Integrated Services Digital Network) data link layer ensures reliable data transfer across the ISDN network. It operates at Layer 2 of the OSI model and is responsible for framing, flow control, and error management. Key protocols used at this layer include: * **LAPD (Link Access Protocol for D-channel):** This protocol facilitates signaling across the ISDN's D-channel, managing multiple logical connections and ensuring data integrity and orderly delivery. * **HDLC (High-Level Data Link Control):** ISDN utilizes HDLC-like framing for B-channels to handle data encapsulation and error detection through checksums. These protocols collectively ensure efficient and error-free communication between network devices over ISDN links. --- # 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/data-link-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.