Information theory
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Information theory is a branch of applied mathematics and engineering involving the quantification of information. Historically, information theory developed to find fundamental limits on compressing and reliably communicating data. Since its inception it has broadened to find applications in statistical inference, networks other than communication networks, biology, quantum information theory, data analysis^{[1]}, and other areas, although it is still widely used in the study of communication.
A key measure of information that comes up in the theory is known as information entropy, which is usually expressed by the average number of bits needed for storage or communication. Intuitively, entropy quantifies the uncertainty involved in a random variable. For example, a fair coin flip will have less entropy than a roll of a die.
Applications of fundamental topics of information theory include lossless data compression (e.g. ZIP files), lossy data compression (e.g. MP3s), and channel coding (e.g. for DSL lines). The field is at the crossroads of mathematics, statistics, computer science, physics, neurobiology, and electrical engineering. Its impact has been crucial to success of the Voyager missions to deep space, the invention of the CD, the feasibility of mobile phones, the development of the Internet, the study of linguistics and of human perception, the understanding of black holes, and numerous other fields. Important subfields of information theory are source coding, channel coding, algorithmic complexity theory, algorithmic information theory, and measures of information.
Contents[hide] 
[edit] Overview
The main concepts of information theory can be grasped by considering the most widespread means of human communication: language. Two important aspects of a good language are as follows:
First, the most common words (e.g., "a", "the", "I") should be shorter than less common words (e.g., "benefit", "generation", "mediocre"), so that sentences will not be too long. Such a tradeoff in word length is analogous to data compression and is the essential aspect of source coding.
Second, if part of a sentence is unheard or misheard due to noise—e.g., a passing car—the listener should still be able to glean the meaning of the underlying message. Such robustness is as essential for an electronic communication system as it is for a language; properly building such robustness into communications is done by channel coding. Source coding and channel coding are the fundamental concerns of information theory.
Note that these concerns have nothing to do with the importance of messages. For example, a platitude such as "Thank you; come again" takes about as long to say or write as the urgent plea, "Call an ambulance!" while clearly the latter is more important and more meaningful. Information theory, however, does not involve message importance or meaning, as these are matters of the quality of data rather than the quantity of data, the latter of which is determined solely by probabilities.
Information theory is generally considered to have been founded in 1948 by Claude Shannon in his seminal work, "A
Mathematical Theory of Communication." The central paradigm of classical information theory is the engineering problem of the transmission of information over a noisy channel. The most fundamental results of this theory are Shannon's source coding theorem, which establishes that, on average, the number of bits needed to represent the result of an uncertain event is given by its entropy; and Shannon's noisychannel coding theorem, which states that reliable communication is possible over noisy channels provided that the rate of communication is below a certain threshold called the channel capacity. The channel capacity can be approached by using appropriate encoding and decoding systems.
Information theory is closely associated with a collection of pure and applied disciplines that have been investigated and reduced to engineering practice under a variety of rubrics throughout the world over the past half century or more: adaptive systems, anticipatory systems, artificial intelligence, complex systems, complexity science, cybernetics, informatics, machine learning, along with systems sciences of many descriptions. Information theory is a broad and deep mathematical theory, with equally broad and deep applications, amongst which is the vital field of coding theory.
Coding theory is concerned with finding explicit methods, called codes, of increasing the efficiency and reducing the net error rate of data communication over a noisy channel to near the limit that Shannon proved is the maximum possible for that channel. These codes can be roughly subdivided into data compression (source coding) and errorcorrection (channel coding) techniques. In the latter case, it took many years to find the methods Shannon's work proved were possible. A third class of information theory codes are cryptographic algorithms (both codes and ciphers). Concepts, methods and results from coding theory and information theory are widely used in cryptography and cryptanalysis. See the article ban (information) for a historical application.
Information theory is also used in information retrieval, intelligence gathering, gambling, statistics, and even in musical composition.
[edit] Historical background
Prior to this paper, limited information theoretic ideas had been developed at Bell Labs, all implicitly assuming events of equal probability. Harry Nyquist's 1924 paper, Certain Factors Affecting Telegraph Speed, contains a theoretical section quantifying "intelligence" and the "line speed" at which it can be transmitted by a communication system, giving the relation W = Klogm, where W is the speed of transmission of intelligence, m is the number of different voltage levels to choose from at each time step, and K is a constant. Ralph Hartley's 1928 paper, Transmission of Information, uses the word information as a measurable quantity, reflecting the receiver's ability to distinguish that one sequence of symbols from any other, thus quantifying information as H = logS^{n} = nlogS, where S was the number of possible symbols, and n the number of symbols in a transmission. The natural unit of information was therefore the decimal digit, much later renamed the hartley in his honour as a unit or scale or measure of information. Alan Turing in 1940 used similar ideas as part of the statistical analysis of the breaking of the German second world war Enigma ciphers.
Much of the mathematics behind information theory with events of different probabilities was developed for the field of thermodynamics by Ludwig Boltzmann and J. Willard Gibbs. Connections between informationtheoretic entropy and thermodynamic entropy, including the important contributions by Rolf Landauer in the 1960s, are explored in Entropy in thermodynamics and information theory.
In Shannon's revolutionary and groundbreaking paper, the work for which had been substantially completed at Bell Labs by the end of 1944, Shannon for the first time introduced the qualitative and quantitative model of communication as a statistical process underlying information theory, opening with the assertion that
 "The fundamental problem of communication is that of reproducing at one point, either exactly or approximately, a message selected at another point."
 the information entropy and redundancy of a source, and its relevance through the source coding theorem;
 the mutual information, and the channel capacity of a noisy channel, including the promise of perfect lossfree communication given by the noisychannel coding theorem;
 the practical result of the Shannon–Hartley law for the channel capacity of a Gaussian channel; and of course
 the bit—a new way of seeing the most fundamental unit of information
[edit] Ways of measuring information
The choice of logarithmic base in the following formulae determines the unit of information entropy that is used. The most common unit of information is the bit, based on the binary logarithm. Other units include the nat, which is based on the natural logarithm, and the hartley, which is based on the common logarithm.
In what follows, an expression of the form is considered by convention to be equal to zero whenever p is. This is justified because for any logarithmic base.
[edit] Entropy
The entropy, H, of a discrete random variable X is a measure of the amount of uncertainty associated with the value of X.Suppose one transmits 1000 bits (0s and 1s). If these bits are known ahead of transmission (to be a certain value with absolute probability), logic dictates that no information has been transmitted. If, however, each is equally and independently likely to be 0 or 1, 1000 bits (in the information theoretic sense) have been transmitted. Between these two extremes, information can be quantified as follows. If is the set of all messages x that X could be, and p(x) = Pr(X = x), then the entropy of X is defined:^{[2]} (Here, I(x) is the selfinformation, which is the entropy contribution of an individual message.) An important property of entropy is that it is maximized when all the messages in the message space are equiprobable—i.e., most unpredictable—in which case
The special case of information entropy for a random variable with two outcomes is the binary entropy function:
[edit] Joint entropy
The joint entropy of two discrete random variables X and Y is merely the entropy of their pairing: (X,Y). This implies that if X and Y are independent, then their joint entropy is the sum of their individual entropies.
For example, if (X,Y) represents the position of a chess piece — X the row and Y the column, then the joint entropy of the row of the piece and the column of the piece will be the entropy of the position of the piece. Despite similar notation, joint entropy should not be confused with cross entropy.
[edit] Conditional entropy (equivocation)
The conditional entropy or conditional uncertainty of X given random variable Y (also called the equivocation of X about Y) is the average conditional entropy over Y:^{[3]} Because entropy can be conditioned on a random variable or on that random variable being a certain value, care should be taken not to confuse these two definitions of conditional entropy, the former of which is in more common use. A basic property of this form of conditional entropy is that:
[edit] Mutual information (transinformation)
Mutual information measures the amount of information that can be obtained about one random variable by observing another. It is important in communication where it can be used to maximize the amount of information shared between sent and received signals. The mutual information of X relative to Y is given by: where SI is the pointwise mutual information.
A basic property of the mutual information is that That is, knowing Y, we can save an average of I(X;Y) bits in encoding X compared to not knowing Y.
Mutual information is symmetric: Mutual information can be expressed as the average Kullback–Leibler divergence (information gain) of the posterior probability distribution of X given the value of Y to the prior distribution on X: In other words, this is a measure of how much, on the average, the probability distribution on X will change if we are given the value of Y. This is often recalculated as the divergence from the product of the marginal distributions to the actual joint distribution: Mutual information is closely related to the loglikelihood ratio test in the context of contingency tables and the multinomial distribution and to Pearson's χ^{2} test: mutual information can be considered a statistic for assessing independence between a pair of variables, and has a wellspecified asymptotic distribution.
[edit] Kullback–Leibler divergence (information gain)
The Kullback–Leibler divergence (or information divergence, information gain, or relative entropy) is a way of comparing two distributions: a "true" probability distribution p(X), and an arbitrary probability distribution q(X). If we compress data in a manner that assumes q(X) is the distribution underlying some data, when, in reality, p(X) is the correct distribution, the Kullback–Leibler divergence is the number of average additional bits per datum necessary for compression. It is thus defined Although it is sometimes used as a 'distance metric', it is not a true metric since it is not symmetric and does not satisfy the triangle inequality (making it a semiquasimetric).
[edit] Other quantities
Other important information theoretic quantities include Rényi entropy (a generalization of entropy) and differential entropy (a generalization of quantities of information to continuous distributions.)
[edit] Coding theory
 Data compression (source coding): There are two formulations for the compression problem:
 lossless data compression: the data must be reconstructed exactly;
 lossy data compression: allocates bits needed to reconstruct the data, within a specified fidelity level measured by a distortion function. This subset of Information theory is called rate–distortion theory.
 Errorcorrecting codes (channel coding): While data compression removes as much redundancy as possible, an error correcting code adds just the right kind of redundancy (i.e. error correction) needed to transmit the data efficiently and faithfully across a noisy channel.
[edit] Source theory
Any process that generates successive messages can be considered a source of information. A memoryless source is one in which each message is an independent identicallydistributed random variable, whereas the properties of ergodicity and stationarity impose more general constraints. All such sources are stochastic. These terms are well studied in their own right outside information theory.
[edit] Rate
Information rate is the average entropy per symbol. For memoryless sources, this is merely the entropy of each symbol, while, in the case of a stationary stochastic process, it is that is, the conditional entropy of a symbol given all the previous symbols generated. For the more general case of a process that is not necessarily stationary, the average rate is that is, the limit of the joint entropy per symbol. For stationary sources, these two expressions give the same result.^{[4]}
It is common in information theory to speak of the "rate" or "entropy" of a language. This is appropriate, for example, when the source of information is English prose. The rate of a source of information is related to its redundancy and how well it can be compressed, the subject of source coding.
[edit] Channel capacity
Consider the communications process over a discrete channel. A simple model of the process is shown below:
Here X represents the space of messages transmitted, and Y the space of messages received during a unit time over our channel. Let p(y  x) be the conditional probability distribution function of Y given X. We will consider p(y  x) to be an inherent fixed property of our communications channel (representing the nature of the noise of our channel). Then the joint distribution of X and Y is completely determined by our channel and by our choice of f(x), the marginal distribution of messages we choose to send over the channel. Under these constraints, we would like to maximize the rate of information, or the signal, we can communicate over the channel. The appropriate measure for this is the mutual information, and this maximum mutual information is called the channel capacity and is given by: This capacity has the following property related to communicating at information rate R (where R is usually bits per symbol). For any information rate R < C and coding error ε > 0, for large enough N, there exists a code of length N and rate ≥ R and a
decoding algorithm, such that the maximal probability of block error is ≤ ε; that is, it is always possible to transmit with arbitrarily small block error. In addition, for any rate R > C, it is impossible to transmit with arbitrarily small block error.
Channel coding is concerned with finding such nearly optimal codes that can be used to transmit data over a noisy channel with a small coding error at a rate near the channel capacity.
[edit] Channel capacity of particular model channels
 A continuoustime analog communications channel subject to Gaussian noise — see Shannon–Hartley theorem.
 A binary symmetric channel (BSC) with crossover probability p is a binary input, binary output channel that flips the input bit with probability p. The BSC has a capacity of 1 − H_{b}(p) bits per channel use, where H_{b} is the binary entropy function:
 A binary erasure channel (BEC) with erasure probability p is a binary input, ternary output channel. The possible channel outputs are 0, 1, and a third symbol 'e' called an erasure. The erasure represents complete loss of information about an input bit. The capacity of the BEC is 1  p bits per channel use.
[edit] Applications to other fields
[edit] Intelligence uses and secrecy applications
Information theoretic concepts apply to cryptography and cryptanalysis. Turing's information unit, the ban, was used in the Ultra project, breaking the German Enigma machine code and hastening the end of WWII in Europe. Shannon himself defined an important concept now called the unicity distance. Based on the redundancy of the plaintext, it attempts to give a minimum amount of ciphertext necessary to ensure unique decipherability.
Information theory leads us to believe it is much more difficult to keep secrets than it might first appear. A brute force attack can break systems based on publickey cryptography or on most commonly used methods of privatekey cryptography, such as block ciphers. The security of such methods comes from the assumption that no known attack can break them in a practical amount of time.
Information theoretic security refers to methods such as the onetime pad that are not vulnerable to such brute force attacks. In such cases, the positive conditional mutual information between the plaintext and ciphertext (conditioned on the key) can ensure proper transmission, while the unconditional mutual information between the plaintext and ciphertext remains zero, resulting in absolutely secure communications. In other words, an eavesdropper would not be able to improve his or her guess of the plaintext by gaining knowledge of the ciphertext but not of the key. However, as in any other cryptographic system, care must be used to correctly apply even informationtheoretically secure methods; the Venona project was able to crack the onetime pads of the Soviet Union due to their improper reuse.
[edit] Pseudorandom number generation
Cryptographically secure pseudorandom number generators need effectively random seeds, which can be obtained via extractors. The measure of sufficient randomness for extractors is minentropy, a value related to Shannon entropy through Rényi entropy; Rényi entropy is also used in evaluating randomness in cryptographic systems. Although related, the distinctions among these measures mean that a random variable with high Shannon entropy is not necessarily satisfactory for use in an extractor.
[edit] Miscellaneous applications
Information theory also has applications in gambling and investing, black holes, bioinformatics, and music.
[edit] References
[edit] Footnotes
 ^ David R. Anderson (November 1, 2003). Some background on why people in the empirical sciences may want to better understand the informationtheoretic methods (pdf). Retrieved on 20071230.
 ^ Fazlollah M. Reza (1961, 1994). An Introduction to Information Theory. Dover Publications, Inc., New York. ISBN 0486682102.
 ^ Robert B. Ash (1965, 1990). Information Theory. Dover Publications, Inc.. ISBN 0486665216.
 ^ Jerry D. Gibson (1998). Digital Compression for Multimedia: Principles and Standards. Morgan Kaufmann. ISBN 1558603697.
[edit] The classic work
 Shannon, C.E. (1948), "A Mathematical Theory of Communication", Bell System Technical Journal, 27, pp. 379–423 & 623–656, July & October, 1948. PDF.
Notes and other formats.
 Ludwig Boltzmann formally defined entropy in 1870. Compare: Boltzmann, Ludwig (1896, 1898). Vorlesungen über Gastheorie : 2 Volumes  Leipzig 1895/98 UB: O 52626. English version: Lectures on gas theory. Translated by Stephen G. Brush (1964) Berkeley: University of California Press; (1995) New York: Dover ISBN 0486684555
[edit] Other journal articles
 R.V.L. Hartley, "Transmission of Information," Bell System Technical Journal, July 1928
 J. L. Kelly, Jr., "A New Interpretation of Information Rate," Bell System Technical Journal, Vol. 35, July 1956, pp. 91726
 R. Landauer, Information is Physical Proc. Workshop on Physics and Computation PhysComp'92 (IEEE Comp. Sci.Press, Los Alamitos, 1993) pp. 14.
 R. Landauer, "Irreversibility and Heat Generation in the Computing Process" IBM J. Res. Develop. Vol. 5, No. 3, 1961
[edit] Textbooks on information theory
 Claude E. Shannon, Warren Weaver. The Mathematical Theory of Communication. Univ of Illinois Press, 1949. ISBN 0252725484
 Robert Gallager. Information Theory and Reliable Communication. New York: John Wiley and Sons, 1968. ISBN 0471290483
 Robert B. Ash. Information Theory. New York: Interscience, 1965. ISBN 0470034459. New York: Dover 1990. ISBN 0486665216
 Thomas M. Cover, Joy A. Thomas. Elements of information theory, 1st Edition. New York: WileyInterscience, 1991. ISBN 0471062596.
 2nd Edition. New York: WileyInterscience, 2006. ISBN 0471241954.
 Imre Csiszar, Janos Korner. Information Theory: Coding Theorems for Discrete Memoryless Systems Akademiai Kiado: 2nd edition, 1997. ISBN 9630574403
 Raymond W. Yeung. A First Course in Information Theory Kluwer Academic/Plenum Publishers, 2002. ISBN 0306467917
 David J. C. MacKay. Information Theory, Inference, and Learning Algorithms Cambridge: Cambridge University Press, 2003. ISBN 0521642981
 Stanford Goldman. Information Theory. New York: Prentice Hall, 1953. New York: Dover 1968 ISBN 0486622096, 2005 ISBN 0486442713
 Fazlollah Reza. An Introduction to Information Theory. New York: McGrawHill 1961. New York: Dover 1994. ISBN 0486682102
 Masud Mansuripur. Introduction to Information Theory. New York: Prentice Hall, 1987. ISBN 0134846680
 Christoph Arndt: Information Measures, Information and its Description in Science and Engineering (Springer Series: Signals and Communication Technology), 2004, ISBN 9783540408550, [1];
[edit] Other books
 James Bamford, The Puzzle Palace, Penguin Books, 1983. ISBN 0140067485
 Leon Brillouin, Science and Information Theory, Mineola, N.Y.: Dover, [1956, 1962] 2004. ISBN 0486439186
 A. I. Khinchin, Mathematical Foundations of Information Theory, New York: Dover, 1957. ISBN 0486604349
 H. S. Leff and A. F. Rex, Editors, Maxwell's Demon: Entropy, Information, Computing, Princeton University Press, Princeton, NJ (1990). ISBN 069108727X
 Tom Siegfried, The Bit and the Pendulum, Wiley, 2000. ISBN 0471321745
 Charles Seife, Decoding The Universe, Viking, 2006. ISBN 067003441X
 Jeremy Campbell, Grammatical Man, Touchstone/Simon& Schuster, 1982, ISBN 0671440624
[edit] See also
[edit] Applications
 Cryptography
 Cryptanalysis
 Entropy in thermodynamics and information theory
 Intelligence (information gathering)
 Gambling
 Cybernetics
[edit] History
 History of information theory
 Timeline of information theory
 Shannon, C.E.
 Hartley, R.V.L.
 Yockey, H.P.
[edit] Theory
 Coding theory
 Source coding
 Detection theory
 Estimation theory
 Fisher information
 Kolmogorov complexity
 Information Algebra
 Information geometry
 Information theory and measure theory
 Logic of information
 Network coding
 Quantum information science
 Semiotic information theory
 Philosophy of Information
[edit] Concepts
 Selfinformation
 Information entropy
 Joint entropy
 Conditional entropy
 Redundancy
 Channel (communications)
 Communication source
 Receiver (information theory)
 Rényi entropy
 Variety
 Mutual information
 Pointwise Mutual Information (PMI)
 Differential entropy
 KullbackLeibler divergence
 Channel capacity
 Unicity distance
 ban (information)
 Covert channel
 Encoder
 Decoder
[edit] External links
 Gibbs, M., "Quantum Information Theory", Eprint
 Schneider, T., "Information Theory Primer", Eprint
 Srinivasa, S. "A Review on Multivariate Mutual Information" PDF.
 Challis, J. Lateral Thinking in Information Retrieval
 Journal of Chemical Education, Shuffled Cards, Messy Desks, and Disorderly Dorm Rooms  Examples of Entropy Increase? Nonsense!
 IEEE Information Theory Society and the review articles.
 Online textbook: Information Theory, Inference, and Learning Algorithms, by David MacKay  gives an entertaining and thorough introduction to Shannon theory, including stateoftheart methods from coding theory, such as arithmetic coding, lowdensity paritycheck codes, and Turbo codes.
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