# Entropy and Information Theory

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Entropy and Information Theory
Aida Austin 4/24/2009

Overview What is information theory? Random variables and entropy
Entropy in information theory Applications Compression Data Transmission

Information Theory Developed in 1948 by Claude
E. Shannon at Bell Laboratories Introduced in “A Mathematical Theory of Communication'' Goal: Efficient transmission of information over a noisy network Defines fundamental limits on compression needed for reliable data communication Claude E. Shannon

Random Variables A random variable is a function.
Assigns numerical values to all possible outcomes (events) Example: A fair coin is tossed. Let X represent a random variable. Possible outcomes:

Entropy in Information Theory
Entropy is the measure of the average information content missing from a set of data when the value of the random variable is not known. Helps determine the average number of bits needed for storage or communication of a signal. As the number of possible outcomes for a random variable increases, entropy increases. As entropy increases, information decreases Example: MP3 sampled at 128 kbps has higher entropy rate than 320 kbps MP3

Applications Data Compression MP3 (lossy) JPEG (lossy) ZIP (lossless)
Cryptography Encryption Decryption Signal Transmission Across a Network Text Message Cell phone

Data Compression “Shrinks” the size of a signal/file/etc. to
reduce cost of storage and transmission Smaller data size reduces the possible outcomes of the associated random variables, thus decreasing the entropy of the data. Entropy - minimum number of bits needed to encode with a lossless compression. Lossless (no data lost) if the rate of compression = entropy rate

Signal/Data Transmission
Channel coding reduces bit error and bit loss due to noise in a network. As entropy increases, the likelihood of valuable information transmitted decreases. Example: Consider a signal composed of random variables. We may know the probability of certain values being transmitted, but we do not know the exact values will be received unless the transmission rate = entropy rate.

Questions?

Resources http://www.gap-system.org/~history/PictDisplay/Shannon.html