Question

You receive the following data fragment: A ESC FLAG A B A FLAG FLAG C B ESC FLAG ESC ESC ESC FLAG FLAG. You know that the protocol uses byte stuffing. Show the contents of each frame after destuffing.

Short answer

After destuffing: Frame 1 = A ESC, Frame 2 = A B A, Frame 4 = C B ESC. Frames 3 and 6 are empty.

Step-by-step solution

Understand Byte Stuffing

In byte stuffing, special characters like ESC (escape) and FLAG are used to differentiate frame boundaries and replace occurrences of these characters within the data. If ESC or FLAG appears as part of the data, an additional ESC character is inserted before them whenever they occur in the data.

Identify Data Fragments

The data fragment is broken into frames using the FLAG character as the delimiter. Check for the sequence FLAG and assign each contiguous set of data between subsequent FLAGs as potential frames. The initial data appears as: FRAME 1 = A ESC FRAME 2 = FLAG A B A FRAME 3 = FLAG FRAME 4 = C B ESC FRAME 5 = FRAME 6 = ESC ESC ESC.

Destuff Frame 2

In FRAME 2 (A B A), check for an ESC character. Here, there is no ESC character, so this frame is unchanged.

Destuff Frame 4

In FRAME 4 (C B ESC), check for 'ESC'. Observe the sequence and determine if ESC is prefixed to FLAG or ESC; since the sequence shows 'ESC' without another ESC immediately after, it is part of valid data.

Analyze Empty Frames

Analyze FRAME 3 and FRAME 6 where no data exists. These frames are identified by consecutive FLAGs but have no content.

Summarize Contents After Destuffing

FRAME 1 contains A ESC, FRAME 2 contains A B A, FRAME 4 contains C B ESC. FRAME 3 and FRAME 6 are simply flags with no data.

Key concepts

Byte Stuffing

Byte stuffing is a technique used in computer networks to handle frames of data in a consistent and error-free manner. It helps to differentiate between data that is part of the frame and special control information that indicates the start or end of a frame.

This method involves inserting a special character, often referred to as the **escape character (ESC)**, into the data stream whenever a special delimiter character like **FLAG** or **ESC** itself appears as part of the data.

• The escape character precedes a FLAG or ESC character that is part of the actual data, rather than being interpreted as a control code.
• This ensures that the data is transmitted correctly without being mistaken for frame boundaries.

Byte stuffing thus maintains the integrity of data and control information, ensuring reliable communication between network entities.

Data Frames

Data frames in networking are self-contained units of data used for communication over a network. They include not just the actual data being sent but also both header and footer information to help manage the transmission process.

The FLAG character is commonly used to indicate the start and end of these data frames. This helps to:

• Parse and separate individual frames from a continuous data stream.
• Avoid data overlap or loss during the transmission process.
• Identify empty frames, which may occur when two FLAGs follow each other consecutively without any data in between.

Using data frames helps maintain organized and correct data transfer, especially when multiple pieces of information are being sent simultaneously.

Destuffing

Destuffing is the reverse process of byte stuffing. It involves removing the escape character (ESC) from the data stream before delivering the information to higher-level protocols or applications.

During destuffing, each frame is examined:

• Any escaped FLAG or ESC character originally inserted to mark boundaries or special conditions must be removed.
• Only actual data remains. This makes sure the received data is exactly what was intended to be sent, without any extra control information interfering.
• Frames without ESC tags simply pass through, as there are no added characters to remove.

Thus, destuffing reverts the data back to its intended format, protected from any protocol interference while ensuring data integrity.

Protocol Design

Protocol design is crucial in network communication. It defines the rules for transmitting data over a network effectively and efficiently. Protocols incorporate byte stuffing as a strategy to manage complex data sequences and ensure no control characters are misinterpreted.

Key aspects of protocol design include:

• **Defining control characters** such as FLAG to easily identify the start and end of data frames.
• Establishing rules for **byte stuffing and destuffing** to handle special characters within the data, maintaining a clear separation between data and control information.
• **Checking for errors and ensuring data integrity** during transmission by employing checksum or error detection mechanisms.

Good protocol design contributes to robust and reliable network communication, allowing devices to understand and exchange data predictably and accurately.