1. CS 111 – Sept. 13 Error detection Error correction Review/practice chapter 1 questions Commitment: –Please read sections 2.1 and 2.2

2. Transmission errors When you send data over a network, there could be rare random flipping of bits. Error Detection Error Correction One method of detection is using a parity bit –Add 9 th bit to each byte during transmission –Goal is that each byte has even # of 1’s –Receiver checks each byte. –… Catches many but not all errors.

3. 2-d parity 101101110 000111010 110101011 000111001 101010010 110010111 010100110 100101100 110001000 The 9 th byte is called a check byte.

4. Error correction Useful if you think there may be a lot of potential errors, such as a noisy transmission medium. Devise a “code” so that each symbol’s bit pattern is quite distinct from all the others. –In practice, this means longer codes. In other words, the 8-bit ASCII code would not be enough. One technique: Hamming code –Example code p. 71 –Idea for assigning code is Hamming distance: comparing codes, count how many bits differ. –When you receive an erroneous code, see which letter it’s closest to. Then you can make a correction.