> For the complete documentation index, see [llms.txt](https://isdn.learn-telecom.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://isdn.learn-telecom.com/layers/data-link-layer.md).

# 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.