HDLC (High-level Data Link Control) is a group of protocols or rules for transmitting data between network points (sometimes called nodes). In HDLC, data is organized into a unit (called a frame) and sent across a network to a destination that verifies its successful arrival. The HDLC protocol also manages the flow or pacing at which data is sent. HDLC is one of the most commonly-used protocols in what is layer 2 of the industry communication reference model called Open Systems Interconnection (OSI). (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.
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Now an ISO standard, HDLC is based on IBM's SDLC protocol, which is widely used by IBM's large customer base in mainframe computer environments. In HDLC, the protocol that is essentially SDLC is known as Normal Response Mode (NRM). In Normal Response Mode, a primary station (usually at the mainframe computer) sends data to secondary stations that may be local or may be at remote locations on dedicated leased lines in what is called a multidrop or multipoint network. (This is not the network we usually think of; it's a nonpublic closed network. In this arrangement, although communication is usually half-duplex.)
In the X.25 version of HDLC, the data frame contains a packet. (An X.25 network is one in which packets of data are moved to their destination along routes determined by network conditions as perceived by routers and reassembled in the right order at the ultimate destination.) The X.25 version of HDLC uses peer-to-peer communication with both ends able to initiate communication on duplex links. This mode of HDLC is known as Link Access Procedure Balanced (LAPB).
The following table summarizes the HDLC variations and who uses them.
|NRM (Normal Response Mode)||Multipoint networks that typically use SDLC|
|LAP (Link Access Procedure)||Early X.25 implementations|
|LAPB (Link Access Procedure, Balanced)||Current X.25 implementations|
|LAPD (Link Access Procedure for the Integrated Services Digital Network D channel)||ISDN D channel and frame relay|
|LAPM (Link Access Procedure for Modems)||Error-correcting modems (specified as part of V.42)|