HDLC abbriviation of High-level Data Link Control is a transmission protocol used at the data link layer (layer 2) of the OSI seven layer model for data communications. The HDLC protocol embeds information in a data frame that allows devices to control data flow and correct errors. HDLC is an ISO standard developed from the Synchronous Data Link Control (SDLC) standard proposed by IBM in the 1970's. HDLC NRM (also known as SDLC) .
HDLC History
HDLC is based on IBM's SDLC protocol, which is the layer 2 protocol for IBM's Systems Network Architecture (SNA). It was extended and standardized by the ITU as LAP (Link Access Procedure), while ANSI named their essentially identical version ADCCP.
Derivatives have since appeared in innumerable standards. It was adopted into the X.25 protocol stack as LAPB, into the V.42 protocol as LAPM, into the Frame Relay protocol stack as LAPF and into the ISDN protocol stack as LAPD.
HDLC was the inspiration for the IEEE 802.2 LLC protocol, and it is the basis for the framing mechanism used with the PPP on synchronous lines, as used by many servers to connect to a WAN, most commonly the Internet.
A mildly different version is also used as the control channel for E-carrier (E1) and SONET multichannel telephone lines. Cisco HDLC uses low-level HDLC framing techniques but adds a protocol field to the standard HDLC header.
HDLC is a bit oriented protocol that supports both half-duplex and full-duplex communication over point to point & multipoint link.
For any HDLC communications session, one station is designated primary and the other secondary.
Modes of HDLC
A session can use one of the following connection modes, which determine how the primary and secondary stations interact.
• Normal unbalanced: The secondary station responds only to the primary station.
• Asynchronous: The secondary station can initiate a message.
• Asynchronous balanced: Both stations send and receive over its part of a duplex line.
This mode is used for X.25 packet-switching networks.
Types of Frames in HDLC
HDLC defines three types of frames:
1. Information frames (I-frame)
2. Supervisory frame (S-frame)
3. Unnumbered frame (U-frame)
To get detailed information about HDLC Protocol, must visit the Tech-Blogs of MSA Technosoft.
No comments:
Post a Comment