Question

Data Commincations: Error Detection and Correction

Help understanding Excerpt:

Text reads:

"The Internet checksum provides greater error-detection capability than a parity bit or two-dimensional parity scheme but is considerably less effective than the cyclic redundancy check (CRC), discussed next. The primary reason for its adoption in Internet protocols is efficiency. Most of these protocols are implemented in software and the Internet checksum, involving simple addition and comparison operations, causes very little overhead. It is assumed that at the lower link level, a strong error-detection code such as CRC is used, and so the Internet checksum is simply an additional end-to-end check for errors."

What does the author mean by "lower link level?" What is meant by that?

Answer 1

Lower link level- means data link layer;
Data link layer implements CRC codes for error detection.
CRC codes are caluculated by dividing message with generator polynomial, it is based on binary division, the remainder is subtracted with message, and the result is transmitted to the receiver.
Receiver , checks the CRC code to determine whether it received data correctly or not. Receiver divides the received message with generator polynomial , if the remainder is zero, then its accepts the data frame , if the remainder is not zero , it rejects the frame becuase it consists of data with errors.
Data link layer is lower level in communication , its functionality to divide packets into frames by adding errror detection codes , flowcontrol and error control.
IP checksum:
Check sum is implemented in transport level, end to end level, while computing checksum, for the entire block check sum is computed, where as CRC is computed for each frame.
To compute checksum of block, data is divided into m blocks and all blocks are added using binary addition , and receiver recomputes check sum to check whether error occured or not.