CS 111 – Sept. 13

• Error detection• Error correction• Review/practice chapter 1 questions

• Commitment:– Please read sections 2.1 and 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 9th 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.

2-d parity

1 0 1 1 0 1 1 1 0

0 0 0 1 1 1 0 1 0

1 1 0 1 0 1 0 1 1

0 0 0 1 1 1 0 0 1

1 0 1 0 1 0 0 1 0

1 1 0 0 1 0 1 1 1

0 1 0 1 0 0 1 1 0

1 0 0 1 0 1 1 0 0

1 1 0 0 0 1 0 0 0

The 9th byte is called a check byte.

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.