High-level Data Link Control (HDLC) is a group of protocols for transmitting information between nodes in a network. In HDLC, data is organized into frames and sent across a network to a destination that verifies its successful arrival. The HDLC protocol used by the network or node may also manage the flow or pacing at which the information is sent. HDLC is one of the most commonly-used protocols in what is layer 2 of the Open Systems Interconnection (OSI) model.
In the OSI model, Layer 1 is the detailed physical level that involves actually generating and receiving the electronic signals. Layer 3 is the higher level that has knowledge about the network, including access to router tables that indicate where to forward or send data. On sending, programming in layer 3 creates a frame that usually contains source and destination network addresses. HDLC (layer 2) encapsulates the layer 3 frame, adding data link control information to a new, larger frame. Other layers also exist in the OSI model.
The Point-to-Point protocol (PPP) provides a standard method for transporting multi-protocol datagrams across point-to-point links, for example, across the Internet. PPP may be comprised of a method for encapsulating multi-protocol datagrams; a link control protocol (LCP) for establishing, configuring, and testing a data-link connection; and a family of Network Control Protocols (NCPs) for establishing and configuring different network layer protocols. The PPP packets may use HDLC-like framing. For instance, PPP may use an HDLC link layer. PPP may also use the HDLC Frame Check Sequence for error detection.
Each piece of data may be encapsulated in an HDLC frame by adding a header and a trailer to the frame. The header may include an HDLC address and an HDLC control field. The trailer may be found at the end of the frame. The trailer may include a Cyclic Redundancy Check (CRC), which detects any errors that occur during the transmission of the frame. Other fields may also be included in the frame.
A device, for example, a receiver, using the HDLC protocol may operate in any number of modes. Normally, the receiver may be in an idle state and wait for the start of a frame. The receiver may enter hunting mode wherein the receiver tries to determine the beginning of a frame. For instance, the beginning of a frame may be indicated by a special flag. The receiver may operate in other modes, as well.