> For the complete documentation index, see [llms.txt](https://tmediadocs.telcobridges.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://tmediadocs.telcobridges.com/telecom-references/transmission-and-lines/hdlc.md). # HDLC High-Level Data Link Control (HDLC): the framing protocol that encapsulates SS7 and other traffic, and why a full-duplex SS7 link needs two controllers. High-Level Data Link Control (HDLC) is a framing protocol used to encapsulate other network protocols such [SS7](/telecom-references/ss7-and-signalling/ss7.md), [ISDN](/telecom-references/ss7-and-signalling/isdn.md), X.25, PPP and others. An HDLC controller, essentially special-purpose circuitry on a motherboard, provides the ability to receive HDLC packets. For a full-duplex (FDX) SS7 link, 2 HDLC controllers are required. ### TelcoBridges and HDLC TelcoBridges supports HDLC via its [Tmonitor](https://tmediadocs.telcobridges.com/products/tmonitor/tmonitor) family of products that support activities such as [network monitoring](https://tmediadocs.telcobridges.com/products/tmedia/features-and-solutions/features/network-monitoring), [lawful intercept](https://tmediadocs.telcobridges.com/configuration/web-portal-3-3/lawful-intercept), and [location-based services](https://tmediadocs.telcobridges.com/products/tmedia/features-and-solutions/solutions/location-based-services). The entry-level [TM1000](https://tmediadocs.telcobridges.com/products/tmonitor/tm1000) device features up to 512 HDLC controllers, while the [TM3000](https://tmediadocs.telcobridges.com/products/tmonitor/tm3000) devices support up to 2048 HDLC controllers. ### References * [Wikipedia article](http://en.wikipedia.org/wiki/High-Level_Data_Link_Control) --- # Agent Instructions This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com. ## Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation by asking a question. Perform an HTTP GET request on the following URL with the `ask` and `goal` query parameters: ``` GET https://tmediadocs.telcobridges.com/telecom-references/transmission-and-lines/hdlc.md?ask=&goal= ``` `ask` is the immediate question: it should be specific, self-contained, and written in natural language. `goal` is what the user is ultimately trying to achieve, the reason they need the answer. Sharing it helps GitBook give you a better, more relevant answer. A goal is most helpful when it describes the outcome the user wants rather than restating the question. For example, with `ask=how do I create an API token`, a goal like `automate deployments from our CI pipeline` lets GitBook tailor the answer to that use case. 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.