Re: CRC versus CheckSum
- From: Jim Granville <no.spam@xxxxxxxxxxxxxxxxxxxxxx>
- Date: Mon, 12 Nov 2007 13:46:22 +1300
Robert Adsett wrote:
In article <2ridnYl6zIxM-6ranZ2dnUVZ_hudnZ2d@xxxxxxxxxxxx>, msg says...
Hi,
I have a situation where I have a byte in which to place either a checksum
or a CRC.
I am on an embedded system.
I am using a checksum that is
BYTE create_csum(BYTE data_in,BYTE csum)
{
csum += data_in + 3;
while ((csum == 0) || (csum == 0xff))
csum += 3;
return(csum);
}
This function elimitates 0x00 and 0xff being calculated.
Our packets cannot have bytes in a packet swapped in order (the transport
layer sees to that) and we are not using the CRC/Checksum to fix bad
packets just to reject them. I contend that in this case the checksum is
jsut as good as the CRC.
Any opinions?
In some sense you are right. It depends on the pattern of errors you expect to correct. CRCs tend to be optimized for detecting short burst errors such as might occur during communication. Simple checksums tend to have a more even distribution of the errors you can detect. Neither detect all errors (there are after all the same number of states for the sum in either case).
For communications I suspect CRC would be better under the assumption that any interference is likely to occur in short bursts.
Write some test code, and try it on what you consider likely error patterns, on both schemes, and see :
Most error sense systems try to cover like this :
100% Single bit errors
xx% Two bit errors
YY% Three bit errors
etc
In a comms link, things like stretched bits, shifted bits
and dropped bits are likely occurances.
The Sum should be designed so a stuck-line gives an invalid answer,
so commonly the sum complements the bytes
eg - everything _including_ the sum = 0FFH on RX, and as Bytes INC, the CkSum decrements.
-jg
.
- Follow-Ups:
- Re: CRC versus CheckSum
- From: Robert Adsett
- Re: CRC versus CheckSum
- References:
- CRC versus CheckSum
- From: msg
- Re: CRC versus CheckSum
- From: Robert Adsett
- CRC versus CheckSum
- Prev by Date: advice on occasional memory error
- Next by Date: Re: advice on occasional memory error
- Previous by thread: Re: CRC versus CheckSum
- Next by thread: Re: CRC versus CheckSum
- Index(es):
Relevant Pages
|
