Question

a crc 32 checksum in the final field of an ethernet frame serves what purpose?

Answer 1

1. CRC32: Checks sum field in final ethernet field consist of 32 bit. It represents the hash code of the data generated over destination address,source address, length and data field(which are again fields of ethernet frame).

2. What is hash code here? -- This comes under Data Algorithm part, where to access the data related to particular field correctly, we use hashcodes. For ex: if we wish to access check sum related to source address, it can be quickly searched and extracted. Why we use hash code here is----The efficiency for searching any item using hashcode take lesser time than any other method available.

3. Purpose:

All frames and the bits, bytes, and fields contained within frames, are susceptible to errors from a variety of sources(Ex.Noise from external sources, Tranmission media etc).  The CRC32 field contains a number that is calculated by the source node(Computer) based on the data in the frame.

When the destination node receives the frame the CRC number is recalculated and compared with the CRC number included in the frame. If the two numbers are different, an error is assumed and the frame is discarded.

The CRC32 in ethernet frame provides error detection only. Error recovery must be performed through separate means. Ethernet, for example, specifies that a damaged frame should be discarded and does not specify any action to cause the frame to be retransmitted. Other protocols, such as Transmission Control Protocol (TCP), can notice the data loss and initiate retransmission and error recovery.