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What is data transparency in HDLC?

Published in HDLC Transparency 2 mins read

Data transparency in High-Level Data Link Control (HDLC) refers to the ability of the protocol to transmit any arbitrary bit pattern as data without misinterpreting it as control information. Essentially, it ensures the separation of data from control signals (30-Jul-2020).

To achieve this, HDLC utilizes a technique called bit stuffing.

Bit Stuffing Explained

Bit stuffing is a method used to prevent the unintentional appearance of the flag sequence (01111110) within the data being transmitted. The flag sequence is used to mark the beginning and end of HDLC frames. If the same bit pattern appears in the data, the receiver might incorrectly interpret it as the end of the frame, leading to errors.

Here's how bit stuffing works:

  1. Transmitter: The transmitter monitors the data stream for five consecutive 1s.
  2. Stuffing: After encountering five consecutive 1s, the transmitter automatically inserts a '0' bit into the data stream, regardless of the next bit's value.
  3. Receiver: The receiver monitors the incoming data stream for five consecutive 1s.
  4. Destuffing: Upon detecting five consecutive 1s followed by a '0', the receiver removes the '0' bit.

Example

Let's say the data to be transmitted is: 110111111100

  1. Original Data: 110111111100
  2. After Bit Stuffing: 1101111101000 (A '0' is inserted after the fifth consecutive '1')

The receiver, upon receiving 1101111101000, will remove the stuffed '0' to recover the original data 110111111100.

Why is Data Transparency Important?

Data transparency is crucial for the following reasons:

  • Error-free Transmission: It ensures the correct interpretation of data by preventing confusion with control sequences.
  • Versatility: Allows transmission of any type of data, regardless of its bit pattern.
  • Reliability: Improves the reliability of data communication by eliminating false frame delimiters.

In essence, data transparency, achieved through bit stuffing, is a core element of HDLC, enabling robust and reliable data transmission over point-to-point and multipoint links in the Data Link Layer (DLL).

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