Question

Suppose you want to send some data using the BISYNC framing protocol, and the last 2 bytes of your data are DLE and ETX. What sequence of bytes would be transmitted immediately prior to the CRC?

Short answer

DLE, DLE, ETX

Step-by-step solution

- Understand BISYNC Protocol

The BISYNC (Binary Synchronous) protocol uses special characters for communication control. The DLE (Data Link Escape) character is used to indicate that the following character is a control character rather than data.

- Identify Control Characters Sequence

When DLE is used in a data frame, it indicates that the next character should be interpreted differently. In the BISYNC protocol, if DLE is followed by ETX, it signifies the end of the text frame.

- Apply BISYNC Framing Rules

According to BISYNC rules, if DLE itself appears in the data, it must be doubled to distinguish it from a DLE used to indicate control sequences. Therefore, if the last two bytes of the data are DLE and ETX, we need to encode DLE by doubling it.

- Determine the Transmitted Sequence

Given that our last bytes are DLE and ETX, the transmitted sequence immediately prior to the CRC will have DLE doubled to indicate a data DLE. Thus, the sequence will be DLE, DLE, ETX.

Key concepts

Data Link Escape (DLE) character

The Data Link Escape (DLE) character is vital in communication protocols like BISYNC. It signals that the next character holds special importance and should be treated differently from regular data characters.
DLE is primarily used to signify that the subsequent character is a control character. This way, the receiving end can appropriately interpret the sequence.
For example, if you have a data sequence where DLE appears followed by another character, you should anticipate a specific control sequence rather than typical data. This ensures that the transmitted data maintains integrity and follows the protocol's rules.

Here are some key points to remember about DLE:

• DLE indicates that the following character is a control character.
• It helps differentiate between control sequences and regular data.
• In BISYNC, DLE, when followed by specific characters, holds special meanings.

End of Text (ETX) character

The End of Text (ETX) character is another significant control character in framing protocols. It denotes the end of a text or data frame.
In the context of BISYNC, when DLE is followed by ETX, it indicates the conclusion of the data sequence or frame. This helps the receiver understand where the data ends, making the communication more structured and error-free.
Understanding ETX is crucial in ensuring proper data interpretation and transmission. When sending data, especially using BISYNC, recognizing ETX helps in identifying the end of meaningful data.

Important aspects of ETX include:

• Often used to signify the end of a text frame.
• Interpreted specially when preceded by DLE.
• Ensures proper framing and termination of data sequences.

CRC (Cyclic Redundancy Check)

The Cyclic Redundancy Check (CRC) is a powerful error-detecting mechanism used to ensure data integrity. In BISYNC and other communication protocols, CRC is appended at the end of data frames.
CRC works by performing polynomial division on the data to create a remainder. This remainder, or CRC code, is then transmitted with the data. The receiver performs the same CRC calculation and checks if the remainder matches the sent CRC. If they match, it means the data likely arrived without errors.
CRC is crucial in identifying and correcting errors that might occur during data transmission due to noise, interference, or other issues.

Key points about CRC include:

• Used for error detection.
• Calculates a remainder using polynomial division of data.
• Ensures data integrity by verifying if transmitted and received data match.