1.
Telecommunication
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Telecommunication is the transmission of signs, signals, messages, writings, images and sounds or intelligence of any nature by wire, radio, optical or other electromagnetic systems. Telecommunication occurs when the exchange of information between communication participants includes the use of technology and it is transmitted either electrically over physical media, such as cables, or via electromagnetic radiation. Such transmission paths are divided into communication channels which afford the advantages of multiplexing. The term is used in its plural form, telecommunications. Early means of communicating over a distance included visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, other examples of pre-modern long-distance communication included audio messages such as coded drumbeats, lung-blown horns, and loud whistles. Zworykin, John Logie Baird and Philo Farnsworth, the word telecommunication is a compound of the Greek prefix tele, meaning distant, far off, or afar, and the Latin communicare, meaning to share. Its modern use is adapted from the French, because its use was recorded in 1904 by the French engineer. Communication was first used as an English word in the late 14th century, in the Middle Ages, chains of beacons were commonly used on hilltops as a means of relaying a signal. Beacon chains suffered the drawback that they could pass a single bit of information. One notable instance of their use was during the Spanish Armada, in 1792, Claude Chappe, a French engineer, built the first fixed visual telegraphy system between Lille and Paris. However semaphore suffered from the need for skilled operators and expensive towers at intervals of ten to thirty kilometres, as a result of competition from the electrical telegraph, the last commercial line was abandoned in 1880. Homing pigeons have occasionally used throughout history by different cultures. Pigeon post is thought to have Persians roots and was used by the Romans to aid their military, frontinus said that Julius Caesar used pigeons as messengers in his conquest of Gaul. The Greeks also conveyed the names of the victors at the Olympic Games to various cities using homing pigeons, in the early 19th century, the Dutch government used the system in Java and Sumatra. And in 1849, Paul Julius Reuter started a service to fly stock prices between Aachen and Brussels, a service that operated for a year until the gap in the telegraph link was closed. Sir Charles Wheatstone and Sir William Fothergill Cooke invented the telegraph in 1837. Also, the first commercial electrical telegraph is purported to have constructed by Wheatstone and Cooke. Both inventors viewed their device as an improvement to the electromagnetic telegraph not as a new device, samuel Morse independently developed a version of the electrical telegraph that he unsuccessfully demonstrated on 2 September 1837
2.
Computer data storage
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Computer data storage, often called storage or memory, is a technology consisting of computer components and recording media used to retain digital data. It is a function and fundamental component of computers. The central processing unit of a computer is what manipulates data by performing computations, in practice, almost all computers use a storage hierarchy, which puts fast but expensive and small storage options close to the CPU and slower but larger and cheaper options farther away. In the Von Neumann architecture, the CPU consists of two parts, The control unit and the arithmetic logic unit. The former controls the flow of data between the CPU and memory, while the latter performs arithmetic and logical operations on data, without a significant amount of memory, a computer would merely be able to perform fixed operations and immediately output the result. It would have to be reconfigured to change its behavior and this is acceptable for devices such as desk calculators, digital signal processors, and other specialized devices. Von Neumann machines differ in having a memory in which they store their operating instructions, most modern computers are von Neumann machines. A modern digital computer represents data using the numeral system. Text, numbers, pictures, audio, and nearly any form of information can be converted into a string of bits, or binary digits. The most common unit of storage is the byte, equal to 8 bits, a piece of information can be handled by any computer or device whose storage space is large enough to accommodate the binary representation of the piece of information, or simply data. For example, the works of Shakespeare, about 1250 pages in print. Data is encoded by assigning a bit pattern to each character, digit, by adding bits to each encoded unit, redundancy allows the computer to both detect errors in coded data and correct them based on mathematical algorithms. A random bit flip is typically corrected upon detection, the cyclic redundancy check method is typically used in communications and storage for error detection. A detected error is then retried, data compression methods allow in many cases to represent a string of bits by a shorter bit string and reconstruct the original string when needed. This utilizes substantially less storage for many types of data at the cost of more computation, analysis of trade-off between storage cost saving and costs of related computations and possible delays in data availability is done before deciding whether to keep certain data compressed or not. For security reasons certain types of data may be encrypted in storage to prevent the possibility of unauthorized information reconstruction from chunks of storage snapshots. Generally, the lower a storage is in the hierarchy, the lesser its bandwidth and this traditional division of storage to primary, secondary, tertiary and off-line storage is also guided by cost per bit. In contemporary usage, memory is usually semiconductor storage read-write random-access memory, typically DRAM or other forms of fast but temporary storage
3.
Authentication
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Authentication is the act of confirming the truth of an attribute of a single piece of data claimed true by an entity. In other words, authentication often involves verifying the validity of at least one form of identification, Authentication is relevant to multiple fields. In art, antiques and anthropology, a problem is verifying that a given artifact was produced by a certain person or in a certain place or period of history. In computer science, verifying a persons identity is required to allow access to confidential data or systems. With autographed sports memorabilia, this could involve someone attesting that they witnessed the object being signed, a vendor selling branded items implies authenticity, while he or she may not have evidence that every step in the supply chain was authenticated. The second type of authentication is comparing the attributes of the object itself to what is known about objects of that origin. For example, an art expert might look for similarities in the style of painting, check the location and form of a signature, or compare the object to an old photograph. The physics of sound and light, and comparison with a physical environment, can be used to examine the authenticity of audio recordings, photographs. Documents can be verified as being created on ink or paper readily available at the time of the implied creation. Attribute comparison may be vulnerable to forgery, in art and antiques, certificates are of great importance for authenticating an object of interest and value. Certificates can, however, also be forged, and the authentication of these poses a problem. For instance, the son of Han van Meegeren, the well-known art-forger, forged the work of his father and provided a certificate for its provenance as well, see the article Jacques van Meegeren. Criminal and civil penalties for fraud, forgery, and counterfeiting can reduce the incentive for falsification, currency and other financial instruments commonly use this second type of authentication method. The third type of authentication relies on documentation or other external affirmations, in criminal courts, the rules of evidence often require establishing the chain of custody of evidence presented. This can be accomplished through a written evidence log, or by testimony from the police detectives, some antiques are accompanied by certificates attesting to their authenticity. Signed sports memorabilia is usually accompanied by a certificate of authenticity and these external records have their own problems of forgery and perjury, and are also vulnerable to being separated from the artifact and lost. In computer science, a user can be access to secure systems based on user credentials that imply authenticity. A network administrator can give a user a password, or provide the user with a key card or other device to allow system access
4.
Algorithm
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In mathematics and computer science, an algorithm is a self-contained sequence of actions to be performed. Algorithms can perform calculation, data processing and automated reasoning tasks, an algorithm is an effective method that can be expressed within a finite amount of space and time and in a well-defined formal language for calculating a function. The transition from one state to the next is not necessarily deterministic, some algorithms, known as randomized algorithms, giving a formal definition of algorithms, corresponding to the intuitive notion, remains a challenging problem. In English, it was first used in about 1230 and then by Chaucer in 1391, English adopted the French term, but it wasnt until the late 19th century that algorithm took on the meaning that it has in modern English. Another early use of the word is from 1240, in a manual titled Carmen de Algorismo composed by Alexandre de Villedieu and it begins thus, Haec algorismus ars praesens dicitur, in qua / Talibus Indorum fruimur bis quinque figuris. Which translates as, Algorism is the art by which at present we use those Indian figures, the poem is a few hundred lines long and summarizes the art of calculating with the new style of Indian dice, or Talibus Indorum, or Hindu numerals. An informal definition could be a set of rules that precisely defines a sequence of operations, which would include all computer programs, including programs that do not perform numeric calculations. Generally, a program is only an algorithm if it stops eventually, but humans can do something equally useful, in the case of certain enumerably infinite sets, They can give explicit instructions for determining the nth member of the set, for arbitrary finite n. An enumerably infinite set is one whose elements can be put into one-to-one correspondence with the integers, the concept of algorithm is also used to define the notion of decidability. That notion is central for explaining how formal systems come into being starting from a set of axioms. In logic, the time that an algorithm requires to complete cannot be measured, from such uncertainties, that characterize ongoing work, stems the unavailability of a definition of algorithm that suits both concrete and abstract usage of the term. Algorithms are essential to the way computers process data, thus, an algorithm can be considered to be any sequence of operations that can be simulated by a Turing-complete system. Although this may seem extreme, the arguments, in its favor are hard to refute. Gurevich. Turings informal argument in favor of his thesis justifies a stronger thesis, according to Savage, an algorithm is a computational process defined by a Turing machine. Typically, when an algorithm is associated with processing information, data can be read from a source, written to an output device. Stored data are regarded as part of the state of the entity performing the algorithm. In practice, the state is stored in one or more data structures, for some such computational process, the algorithm must be rigorously defined, specified in the way it applies in all possible circumstances that could arise. That is, any conditional steps must be dealt with, case-by-case
5.
Social Security number
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The number is issued to an individual by the Social Security Administration, an independent agency of the United States government. Although its primary purpose is to individuals for Social Security purposes. A Social Security number may be obtained by applying on Form SS-5, Social Security numbers were first issued by the Social Security Administration in November 1935 as part of the New Deal Social Security program. Within three months,25 million numbers were issued, on November 24,1936,1,074 of the nations 45,000 post offices were designated typing centers to type up Social Security cards that were then sent to Washington, D. C. On December 1,1936, as part of the publicity campaign for the new program, Joseph L. of New Rochelle, New York. The Tax Reform Act of 1986 required parents to list Social Security numbers for each dependent over the age of 5 for whom the parent wanted to claim a tax deduction. Before this act, parents claiming tax deductions were on the system not to lie about the number of children they supported. During the first year of the Tax Reform Act, this change resulted in seven million fewer minor dependents being claimed. The disappearance of these dependents is believed to have involved either children who never existed or tax deductions improperly claimed by non-custodial parents, in 1988, the threshold was lowered to 2 years old, and in 1990, the threshold was lowered yet again to 1 year old. Today, an SSN is required regardless of the age to receive an exemption. Since then, parents have often applied for Social Security numbers for their children soon after birth, today, the original purpose of this number was to track individuals accounts within the Social Security program. It has since come to be used as an identifier for individuals within the United States, as numbers are now assigned by the central issuing office of the SSA, it is unlikely that duplication will ever occur again. A few duplications did occur when prenumbered cards were sent out to regional SSA offices, employee, patient, student, and credit records are sometimes indexed by Social Security number. The U. S. Previously, the United States military used a more complicated system of service numbers that varied by service. Beginning in June 2011, DOD began removing the Social Security number from military identification cards and it is replaced by a unique DOD identification number, formerly known as the EDIPI. Social Security was originally a tax, but when Medicare was passed in 1965. Because of this, not every American is part of the Social Security program, the Old Order Amish have fought to prevent universal Social Security by overturning rules such as a requirement to provide a Social Security number for a hunting license. Social Security cards printed from January 1946 until January 1972 expressly stated that people should not use the number, since nearly everyone in the United States now has an SSN, it became convenient to use it anyway and the message was removed
6.
Isopsephy
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Isopsephy is the Greek word for the practice of adding up the number values of the letters in a word to form a single number. Isopsephism, the occurrence of isopsephy, is used as a synonym. The early Greeks used pebbles arranged in patterns to learn arithmetic, isopsephy is related to gematria, the same practice using the Hebrew alphabet, and the ancient number systems of many other peoples. A gematria of Latin-script languages was also popular in Europe from the Middle Ages to the Renaissance and its remains in numerology. For instance in Ancient Greece, Greek numerals used the alphabet and it is just a short step from using letters of the alphabet in everyday arithmetic and mathematics to seeing numbers in words, and to writing with an awareness of the numerical dimension of words. An early reference to isopsephy, albeit of more-than-usual sophistication, is from the mathematician Apollonius of Perga and he asks, Given the verse, ΑΡΤΕΜΙΔΟΣ ΚΛΕΙΤΕ ΚΡΑΤΟΣ ΕΞΟΧΟΝ ΕΝΝΕΑ ΚΟΥΡΑΙ, what does the product of all its elements equal. More conventional are the instances of found in graffiti at Pompeii. One reads Φιλω ης αριθμος ϕμε, I love her whose number is 545, another says, Amerimnus thought upon his lady Harmonia for good. The number of her name is 45. Suetonius, writing in 121 AD, reports a political slogan that someone wrote on a wall in Rome, Nero, Orestes, Nero his mother slew which appears to be another example. In Greek, Νερων, Nero, has the numerical value 50+5+100+800+50=1005, also in the 1st century AD, Leonidas of Alexandria created isopsephs, epigrams with equinumeral distichs, where the first hexameter and pentameter equal the next two verses in numerical value. He addressed some of them to Nero, Θυει σοι τοδε γραμμα γενεθλιακαισιν εν ὡραις, Καισαρ, Καλλιοπης γαρ ακαπνον αει θυος· εις δε νεωτα Ην εθελῃς, θυσει τουδε περισσοτερα. Here the sum of both the first and second distich is 5699, in another of his distichs the hexameter line is equal in number to its corresponding pentameter, Εἱς προς ἑνα ψηφοισιν ισαζεται, ου δυο δοιοις, Ου γαρ ετι στεργω την δολιχογραφιην. One line is equal in number to one, not two to two, for I no longer approve of long epigrams. A headstone found at the Temple of Artemis at Sparta Orthia is a 2nd-century AD example of elegiac verse. It says, ΟΡΘΕΙΗ ΔΩΡΟΝ ΛΕΟΝΤΕΥΣ ΑΝΕΘΗΚΕ ΒΟΑΓΟΣ ΒΨΛ ΜΩΑΝ ΝΙΚΗΣΑΣ ΚΑΙ ΤΑΔΕ ΕΠΑΘΛΑ ΛΑΒΩΝ ΒΨΛ ΚΑΙ Μ ΕΣΤΕΨΕ ΠΑΤΗΡ ΕΙΣΑΡΙΘΜΟΙΣ ΕΠΕΣΙ ΒΨΛ It is the votive stele for a boy who won a competition in singing. The words in each line add up to ΄ΒΨΛ, that is 2730, in Greek, each unit was assigned a separate letter, each tens a separate letter, and each hundreds a separate letter. This requires 27 letters, so the 24-letter alphabet was extended by using three obsolete letters, digamma ϝ, for 6, qoppa ϙ for 90, and sampi ϡ for 900
7.
Frame check sequence
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A frame check sequence refers to the extra error-detecting code added to a frame in a communications protocol. Frames are used to send data and ultimately the application data from a source to a destination. Other protocols, notably the Transmission Control Protocol, can notice the data loss, all frames and the bits, bytes, and fields contained within them, are susceptible to errors from a variety of sources. The FCS field contains a number that is calculated by the source based on the data in the frame. This number is added to the end of a frame that is sent, when the destination node receives the frame the FCS number is recalculated and compared with the FCS number included in the frame. If the two numbers are different, an error is assumed and the frame is discarded, the sending host computes a cyclic redundancy check on the entire frame and appends this as a trailer to the data. The receiving host recomputes the cyclic redundancy check on the using the same algorithm. This way it can detect whether any data was lost or altered in transit and it may then discard the data, and request retransmission of the faulty frame. The FCS is often transmitted in such a way that the receiver can compute a running sum over the frame, together with the trailing FCS. For Ethernet and other IEEE802 protocols, this fixed result, when transmitted and used in this way, FCS generally appears immediately before the frame-ending delimiter. Ethernet frame § Preamble and start frame delimiter
8.
Exclusive or
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Exclusive or or exclusive disjunction is a logical operation that outputs true only when inputs differ. It is symbolized by the prefix operator J and by the infix operators XOR, EOR, EXOR, ⊻, ⊕, ↮, the negation of XOR is logical biconditional, which outputs true only when both inputs are the same. It gains the exclusive or because the meaning of or is ambiguous when both operands are true, the exclusive or operator excludes that case. This is sometimes thought of as one or the other but not both and this could be written as A or B, but not, A and B. More generally, XOR is true only when an odd number of inputs are true, a chain of XORs—a XOR b XOR c XOR d —is true whenever an odd number of the inputs are true and is false whenever an even number of inputs are true. The truth table of A XOR B shows that it outputs true whenever the inputs differ,0, false 1, true Exclusive disjunction essentially means either one, in other words, the statement is true if and only if one is true and the other is false. For example, if two horses are racing, then one of the two win the race, but not both of them. The exclusive or is equivalent to the negation of a logical biconditional, by the rules of material implication. This unfortunately prevents the combination of two systems into larger structures, such as a mathematical ring. However, the system using exclusive or is an abelian group, the combination of operators ∧ and ⊕ over elements produce the well-known field F2. This field can represent any logic obtainable with the system and has the benefit of the arsenal of algebraic analysis tools for fields. The Oxford English Dictionary explains either, or as follows, The primary function of either, etc. is to emphasize the perfect indifference of the two things or courses. But a secondary function is to emphasize the mutual exclusiveness, = either of the two, but not both, the exclusive-or explicitly states one or the other, but not neither nor both. Following this kind of common-sense intuition about or, it is argued that in many natural languages, English included. The exclusive disjunction of a pair of propositions, is supposed to mean that p is true or q is true, but not both. For example, it might be argued that the intention of a statement like You may have coffee. Certainly under some circumstances a sentence like this example should be taken as forbidding the possibility of accepting both options. Even so, there is reason to suppose that this sort of sentence is not disjunctive at all
9.
Hash function
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A hash function is any function that can be used to map data of arbitrary size to data of fixed size. The values returned by a function are called hash values, hash codes, digests. One use is a structure called a hash table, widely used in computer software for rapid data lookup. Hash functions accelerate table or database lookup by detecting duplicated records in a large file, an example is finding similar stretches in DNA sequences. They are also useful in cryptography and this is used for assuring integrity of transmitted data, and is the building block for HMACs, which provide message authentication. Hash functions are related to checksums, check digits, fingerprints, lossy compression, randomization functions, error-correcting codes, although these concepts overlap to some extent, each has its own uses and requirements and is designed and optimized differently. The Hash Keeper database maintained by the American National Drug Intelligence Center, Hash functions are used in hash tables, to quickly locate a data record given its search key. Specifically, the function is used to map the search key to an index. Hash tables, in turn, are used to implement associative arrays, typically, the domain of a hash function is larger than its range, and so it will map several different keys to the same index. Therefore, each slot of a table is associated with a set of records. For this reason, each slot of a table is often called a bucket. Thus, the function only hints at the records location — it tells where one should start looking for it. Still, in a table, a good hash function will typically narrow the search down to only one or two entries. Hash functions are used to build caches for large data sets stored in slow media. A cache is generally simpler than a hashed search table, since any collision can be resolved by discarding or writing back the older of the two colliding items and this is also used in file comparison. Hash functions are an ingredient of the Bloom filter, a space-efficient probabilistic data structure that is used to test whether an element is a member of a set. Once the table is complete, any two duplicate records will end up in the same bucket, the duplicates can then be found by scanning every bucket T which contains two or more members, fetching those records, and comparing them. With a table of size, this method is likely to be much faster than any alternative approach
10.
Cryptographic hash function
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A cryptographic hash function is a special class of hash function that has certain properties which make it suitable for use in cryptography. It is an algorithm that maps data of arbitrary size to a bit string of a fixed size which is designed to also be a one-way function, that is. Bruce Schneier has called one-way hash functions the workhorses of modern cryptography, the input data is often called the message, and the output is often called the message digest or simply the digest. Most cryptographic hash functions are designed to take a string of any length as input, a cryptographic hash function must be able to withstand all known types of cryptanalytic attack. This concept is related to that of one-way function, functions that lack this property are vulnerable to preimage attacks. Second pre-image resistance Given an input m1 it should be difficult to find different input m2 such that hash = hash, functions that lack this property are vulnerable to second-preimage attacks. Collision resistance It should be difficult to find two different messages m1 and m2 such that hash = hash, such a pair is called a cryptographic hash collision. This property is referred to as strong collision resistance. It requires a value at least twice as long as that required for preimage-resistance. Collision resistance implies second pre-image resistance, but does not imply pre-image resistance, informally, these properties mean that a malicious adversary cannot replace or modify the input data without changing its digest. Thus, if two strings have the same digest, one can be confident that they are identical. A function meeting these criteria may still have undesirable properties and this property can be used to break naive authentication schemes based on hash functions. The HMAC construction works around these problems, in practice, collision resistance is insufficient for many practical uses. In particular, should behave as much as possible like a random function while still being deterministic, checksum algorithms, such as CRC32 and other cyclic redundancy checks, are designed to meet much weaker requirements, and are generally unsuitable as cryptographic hash functions. For example, a CRC was used for message integrity in the WEP encryption standard, the meaning of the term is therefore somewhat dependent on the application, since the effort that a malicious agent may put into the task is usually proportional to his expected gain. However, since the effort usually grows very quickly with the digest length. For messages selected from a set of messages, for example passwords or other short messages. In some theoretical analyses difficult has a specific meaning, such as not solvable in asymptotic polynomial time
11.
Hamming code
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In telecommunication, Hamming codes are a family of linear error-correcting codes that generalize the Hamming-code, and were invented by Richard Hamming in 1950. Hamming codes can detect up to two-bit errors or correct one-bit errors without detection of uncorrected errors, by contrast, the simple parity code cannot correct errors, and can detect only an odd number of bits in error. Hamming codes are perfect codes, that is, they achieve the highest possible rate for codes with their block length, in mathematical terms, Hamming codes are a class of binary linear codes. For each integer r ≥2 there is a code with block length n = 2r −1 and message length k = 2r − r −1. Hence the rate of Hamming codes is R = k / n =1 − r /, which is the highest possible for codes with minimum distance of three and block length 2r −1. The parity-check matrix of a Hamming code is constructed by listing all columns of length r that are non-zero, the parity-check matrix has the property that any two columns are pairwise linearly independent. Due to the limited redundancy that Hamming codes add to the data and this is the case in computer memory, where bit errors are extremely rare and Hamming codes are widely used. In this context, an extended Hamming code having one extra parity bit is often used, extended Hamming codes achieve a Hamming distance of four, which allows the decoder to distinguish between when at most one one-bit error occurs and when any two-bit errors occur. In this sense, extended Hamming codes are single-error correcting and double-error detecting, Richard Hamming, the inventor of Hamming codes, worked at Bell Labs in the 1940s on the Bell Model V computer, an electromechanical relay-based machine with cycle times in seconds. Input was fed in on punched tape, seven-eighths of an inch wide which had up to six holes per row. During weekdays, when errors in the relays were detected, the machine would stop, during after-hours periods and on weekends, when there were no operators, the machine simply moved on to the next job. Hamming worked on weekends, and grew frustrated with having to restart his programs from scratch due to detected errors. In a taped interview Hamming said, And so I said, Damn it, if the machine can detect an error, why cant it locate the position of the error and correct it. Over the next few years, he worked on the problem of error-correction, in 1950, he published what is now known as Hamming Code, which remains in use today in applications such as ECC memory. A number of simple error-detecting codes were used before Hamming codes, parity adds a single bit that indicates whether the number of ones in the preceding data was even or odd. If an odd number of bits is changed in transmission, the message will change parity and the error can be detected at this point, however, the bit that changed may have been the parity bit itself. The most common convention is that a parity value of one indicates that there is an odd number of ones in the data, If the number of bits changed is even, the check bit will be valid and the error will not be detected. Moreover, parity does not indicate which bit contained the error, the data must be discarded entirely and re-transmitted from scratch
