Incremental Backup

[myth007_03]

In incremental backup we break a file in block and compare the block with previous block and if it is same then we send it, but the problem is that if we add something in between then all blocks are shifted downward and now if we break our file in blocks then no block will match and finally we have to send complete file in backup system...
How to get rid of this problem or we hav to send complete file??