4.2: How do Errors Happen?

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Page ID: 50171

Paul Penfield, Jr.
Massachusetts Institute of Technology via MIT OpenCourseWare

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The model pictured above is quite general, in that the purpose might be to transmit information from one place to another (communication), store it for use later (data storage), or even process it so that the output is not intended to be a faithful replica of the input (computation). Different systems involve different physical devices as the channel (for example a communication link, a floppy disk, or a computer). Many physical effects can cause errors. A CD or DVD can get scratched. A memory cell can fail. A telephone line can be noisy. A computer gate can respond to an unwanted surge in power supply voltage.

For our purposes we will model all such errors as a change in one or more bits from 1 to 0 or vice versa. In the usual case where a message consists of several bits, we will usually assume that different bits get corrupted independently, but in some cases errors in adjacent bits are not independent of each other, but instead have a common underlying cause (i.e., the errors may happen in bursts).