| 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.) |
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Variations of HDLC are also used for the public networks that use the
X.25 communications protocol and for
frame relay, a protocol used in both
and
wide area network, public and private. |
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| 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). |
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| The
following table summarizes the HDLC variations and who uses them. |
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| HDLC SUBSET |
USES |
| 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) |
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