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Thread ID: 51469	2004-11-21 21:32:00	CRC	yingxuan (3330)	Press F1

Post ID	Timestamp	Content	User
294718	2004-11-21 21:32:00	I got a question A set of data may be stored as a block with cyclic redundancy check bits for protection against errors. Explain why you cannot retrieve a single byte of data directly from such a block. I know that the crc protects blocks of data called frames but i do not understand why you can't retrieve a single byte?	yingxuan (3330)
294719	2004-11-21 21:55:00	Sounds like homework.... Insufficient information. There are so many types of CRC systems and their operation can vary. What CRC system is it ? (e.g. Bose-Chaudhary etc) In some abstract systems each byte can have a binary value added or subtracted on the LS bit, and the CRC carries that info. Then retrieving a single Byte could give an incorrect value unless it was tested and converted by reference to the CRC checksum. But that is just speculation, as it varies by CRC type.	godfather (25)
294720	2004-11-22 02:37:00	A CRC is just a piece of data . It is one or more bytes . CRC bytes are no different from any other bytes . CRC is calculated and checked by hardware in many applications, so it is hidden , but it can be done by software . . What an individual byte means depends on the application . Whether you can "retrieve" a single byte depends on the medium . For example you can certainly read a single byte of the BIOS "CMOS" memory . That has two bytes which contain a checksum (error checking . . . similar to CRC . It would be pretty hard to do it with an Ethernet packet, because the CRC is done with hardware, and it normally returns full blocks . It would be quite easy to count bytes in a simple serial stream . But it might be harder in a synchronous serial channel, again because of the hardware . Disk blocks contain CRC, and usually the system disk access routines return only full blocks with just the data . But it wouldn't be very hard to access single bytes on a floppy disk, anyway . Modern hard disks (RLL, GCR . . . . ) would make it hard . :D The question is a bit imprecise . As you present it, I would have to call it a "wrong" question . :D	Graham L (2)
294721	2004-11-22 03:51:00	think of having a lst of 100 numbers, if you add up all the numbers you get a checkc sum. if you then change any of the number in the lst your check sum is worng. thing is you dont know from this as to which number is wrong. if you used HAM encoding then you can tell which number is worng.	robsonde (120)
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