AN00009.TXT

Advanced Communication Board Developer's Toolkit 1995
    

    ----------------------------------------------------------------------
    ----------------------------------------------------------------------

Subject:  HDLC and HDLC Subsets (SDLC, LAP, etc.)

The High-level Data Link Control (HDLC) protocol was developed in the 1970s by
the International Organization for Standardization (ISO). This synchronous
protocol involves sending information in a block that contains several defined
fields. Each block of information starts and ends with a distinct
synchronizing character called the sync flag. Each block of information 
contains an address field, control field, a variable length data field, and a
frame check sequence. The combination of all fields, beginning and ending with
a sync flag is collectively referred to as an HDLC frame. HDLC can be
implemented in a multi-drop or point-to-point configuration. The address field
can be used to determine the destination of the HDLC frame in multi-drop
configurations. The control field is normally eight bits (octet) long.
Different bits in the control field are dedicated to separate link control
duties, for example, a group of three bits is dedicated to counting the number
of frames and another bit is dedicated to labeling the type of information
in the data porting of the frame. For this reason, HDLC is considered a bit-
oriented protocol. The HDLC protocol can be implemented in several ways and
has numerous subsets and slight variations. All ACB adapters can communicate
with the HDLC protocol.


HDLC	ISO 3309 and ISO 4335
	High-Level Data Link Control

SDLC	Systems Network Architecture.
	GC30-3072. IBM Corp., 1981
		 or
	IBM Synchronous Data Link Control - General Information
	GA27-3093 File GENL-09
	Synchronous Data Link Control

ADCCP	ANSI X3.66  American National Standards Institute
	FIPS PUB 71-1 U.S. National Bureau of Standards
	FED-STD-1003A Federal Telecommunications Standards Committee
	Advanced Data Communication Control Procedures

LAP	CCITT I.440/I.441
	Link Access Procedure (Pre HDLC specification)

LAPB	CCITT I.440/I.441
	Link Access Procedure Balanced
	This specification is contained in CCITT X.25

LAPD	1988 CCITT Q.920/921, I.440/I.441
	Link Access Procedure, D channel (ISDN)

LLC	IEEE 802.2
	Logical Link Control

LLAP	ISO
	LocalTalk Link Access Protocol (AppleTalk)