CRC - Cyclic Redundancy Check
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Cyclic Redundancy Check (CRC) is a common method for protecting binary data that way. Different CRCs exist, which in the past has resulted in a naming scheme. The standard ITU-TSS CRC, often written as a formula: G(x)=x16+x12+x5+1. Different methods exist to calculate a check number for binary data, to be able to see if the data is not altered, for example, after being sent through some communication channel. Characteristic of this CRC is its 16 bits size and its initial value $FFFF. Although, you can encounter an initial value $0000 too. CRC is an industry standard method of ensuring data integrity. To ensure that a message made it to the destination in tact, the most straightforward method would be to check it "word for word" or "bit for bit" to make sure that it is what it should be. But that would mean sending the message at least twice so that two messages could be compared with each other to make sure that they agree. So as not to waste that bandwidth, a shorter representation of the message is tacked on the end. This representation is called the CRC. Obviously since the CRC is shorter than the actual message, it isn't a perfect "bit for bit" check of integrity. The longer the CRC, the better its ability to flag an error in the message. Common CRC lengths range from 8 bits to 32 bits. The 32-bit CRC that Locus uses, in conjunction with the rest of the correlation mechanisms in the radio, ensure that packets passed to the user contain accurate data only. Checksum A simple error-detection scheme in which each transmitted message is accompanied by a numerical value based on the number of set bits in the message. The receiving station then applies the same formula to the message and checks to make sure the accompanying numerical value is the same. If not, the receiver can assume that the message has been garbled. |
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