Data link protocols almost always put the CRC in a trailer rather than in a header Why?

Answer 1

As per my knowledge, Data link protocols often place the Cyclic Redundancy Check (CRC) in a trailer rather than a header for several reasons:

Efficient Error Detection:

Placing the CRC at the end of the frame allows the receiving device to perform error detection on the entire frame, including both the header and the data. This ensures a more comprehensive check of the transmitted information.

Simplified Processing:

By placing the CRC in the trailer, the processing logic in network devices can be simplified. Devices can start processing the frame immediately upon receipt and continue until they reach the trailer, where they can perform a final error check. This streamlined process can contribute to faster and more efficient data handling.

Sequential Processing:

The sequential arrangement of header, data, and trailer allows for a logical flow in processing. The header is processed first to extract essential information, followed by the data, and finally, the trailer for error checking. This sequential processing aligns well with the typical flow of data handling in network devices.

Consistency with Standards:

Many data link layer standards, such as Ethernet, have established practices of placing the CRC in the trailer. Adhering to these standards ensures interoperability between different devices and systems, promoting a consistent and reliable communication environment.

Frame Integrity:

Placing the CRC at the end helps ensure the integrity of the entire frame. If the CRC check fails, the entire frame can be discarded, preventing the delivery of corrupted data to higher layers in the networking stack.

Answer 2

The CRC is computed while the packet is being transmitted and then incorporated in a trailer. Similarly, the receiver computes the CRC and compares it with the transmitted one. From both points of view, it is better to have the CRC in a trailer. Because then you can compute the CRC as you transmit the packet, and then splat out the final CRC at the end. If you put it in the header, you would have to make *one* pass over the packet computing the CRC, then send the CRC, and then a *second* pass actually stuffing each byte out the interface
The CRC is computed during transmission and appended

to the output stream as soon as the last bit goes out onto the wire. If the CRC were in the

header, it would be necessary to make a pass over the frame to compute the CRC before transmitting.

This would require each byte being handled twice-once for checksumming and once for transmitting.

Using the trailer cuts the work in half.