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NE40E V800R010C00 Feature Description - WAN Access 01

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HDLC Principles

HDLC Principles


Synchronous data link protocols include character-oriented, bit-oriented, and byte-oriented protocols.

IBM put forward the first character-oriented synchronous protocol, called Binary Synchronous Communication (BISYNC or BSC).

Later, ISO put forward related standards. The ISO standard is ISO 1745:1975 Information processing - Basic mode control procedures for data communication systems.

In the early 1970s, IBM introduced the bit-oriented Synchronous Data Link Control (SDLC) protocol.

Later, ANSI and ISO adopted and developed SDLC, and then later put forward their own standards. ANSI introduced the Advanced Data Communications Control Protocol (ADCCP), and ISO introduced HDLC.

HDLC Features

HDLC is a bit-oriented code-transparent synchronous data link layer protocol. It provides the following features:

  • HDLC works in full-duplex mode and can transmit data continuously without waiting for acknowledgement. Therefore, HDLC features high data link transmission efficiency.

  • HDLC uses cyclic redundancy check (CRC) for all frames and numbers them. This helps you know which frames are dropped and which frames are repeatedly transmitted. HDLC ensures high transmission reliability.

  • HDLC separates the transmission control function from the processing function and features high flexibility and perfect control capabilities.

  • HDLC is independent of any character encoding set and transparently transmits data.

  • Zero-bit insertion, which is used for transparent data transmission, is easy to implement on hardware.

HDLC is especially used to logically transmit data that is segmented into physical blocks or packages. These blocks or packages are called frames, each of which is identified by a start flag and an end flag. In HDLC, all bit-oriented data link control protocols use a unified frame format, and both data and control information are transmitted in frames. Each frame begins at and ends with a frame delimiter, which is a unique sequence of bits of 01111110. The frame delimiter marks the start or end of a frame or marks for synchronization. The frame delimiter is invisible inside a frame to avoid confusion.

Zero-bit insertion is used to ensure that the sequence of bits used for the flag does not appear in normal data. On the transmit end, zero-bit insertion monitors all fields except the flag and places a 0 after five consecutive 1s. On the receive end, zero-bit insertion also monitors all fields except the flag. After five consecutive 1s are found, if the following bit is a 0, the 0 is automatically deleted to restore the former bit flow. If the following bit is a 1, it means that an error has occurred or an end delimit is received. In this case, the frame receive procedure is generally either restarted or aborted.

Updated: 2018-07-04

Document ID: EDOC1100027168

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