Character Encoding

In computing, a character encoding is used to represent a repertoire of characters by some kind of an encoding system. Depending on the abstraction level and context, corresponding code points and the resulting code space may be regarded as bit patterns, octets, natural numbers, electrical pulses, etc. A character encoding is used in computation, data storage, and transmission of textual data. Terms such as character set, character map, codeset or code page are sometimes used as near synonyms; however, these terms have related but distinct meanings.

Early character codes associated with the optical or electrical telegraph could only represent a subset of the characters used in written languages, sometimes restricted to upper case letters, numerals and some punctuation only. The low cost of digital representation of data in modern computer systems allows more elaborate character codes (such as Unicode) which represent more of the characters used in many written languages. Character encoding using internationally accepted standards permits worldwide interchange of text in electronic form.

History

Early binary repertoires include Bacon's cipher, Braille, International maritime signal flags, and the 4-digit encoding of Chinese characters for a Chinese telegraph code (Hans Schjellerup, 1869). Common examples of character encoding systems include Morse code, the Baudot code, the American Standard Code for Information Interchange (ASCII) and Unicode.

Morse code was introduced in the 1840s and is used to encode each letter of the Latin alphabet, each Arabic numeral, and some other characters via a series of long and short presses of a telegraph key. Representations of characters encoded using Morse code varied in length.

The Baudot code, a five-bit encoding, was created by Émile Baudot in 1870, patented in 1874, modified by Donald Murray in 1901, and standardized by CCITT as International Telegraph Alphabet No. 2 (ITA2) in 1930.

Fieldata, a six- or seven-bit code, was introduced by the U.S. Army Signal Corps in the late 1950s.

IBM's Binary Coded Decimal (BCD) was a six-bit encoding scheme used by IBM in as early as 1959 in its 1401 and 1620 computers, and in its 7000 Series (for example, 704, 7040, 709 and 7090 computers), as well as in associated peripherals. BCD extended existing simple four-bit numeric encoding to include alphabetic and special characters, mapping it easily to punch-card encoding which was already in widespread use. It was the precursor to EBCDIC.

ASCII was introduced in 1963 and is a seven-bit encoding scheme used to encode letters, numerals, symbols, and device control codes as fixed-length codes using integers.

IBM's Extended Binary Coded Decimal Interchange Code (usually abbreviated as EBCDIC) is an eight-bit encoding scheme developed in 1963.

The limitations of such sets soon became apparent, and a number of ad hoc methods were developed to extend them. The need to support more writing systems for different languages, including the CJK family of East Asian scripts, required support for a far larger number of characters and demanded a systematic approach to character encoding rather than the previous ad hoc approaches.

The frustrating dilemma that researchers in this field encountered in the 1980s as they tried to develop universally interchangeable character encodings was that on the one hand, it seemed to be necessary to add more bits to accommodate additional characters. On the other hand, for the users of the relatively small character set of the Latin alphabet (who still constituted the majority of computer users at the time), those additional bits were a colossal waste of then-scarce and expensive computing resources (as they would always be zeroed out for such users).

The compromise solution that was eventually hit upon with Unicode, as further explained below, was to break the longstanding assumption (dating back to the old telegraph codes) that each character should always directly correspond to a particular pattern of encoded bits. Instead, characters would be first mapped to an intermediate stage in the form of abstract numbers known as code points. Those code points would then be encoded in a variety of ways and with various default numbers of bits per character (code units) depending upon context. To encode code points higher than the length of the code unit, such as above 256 for 8-bit units, the solution was to implement variable-width encodings where an escape sequence would signal that subsequent bits should be parsed as a higher code point.

Code unit

A code unit is a bit sequence used to encode each character of a repertoire. Encodings associate their meaning with either a single code unit value or a sequence of code units as one value. Different character encodings use different code unit widths: US-ASCII uses seven bits, UTF-8 and EBCDIC use eight bits, UTF-16 uses 16 bits, and UTF-32 uses 32 bits.

Unicode encoding model

Unicode and its parallel standard, the ISO/IEC 10646 Universal Character Set, together constitute a modern, unified character encoding. Rather than mapping characters directly to octets (bytes), they separately define what characters are available, their numbering, how those numbers are encoded as a series of "code units" (limited-size numbers), and finally how those units are encoded as a stream of octets. The idea behind this decomposition is to establish a universal set of characters that can be encoded in a variety of ways. To describe this model correctly one needs more precise terms than "character set" and "character encoding." The terms used in the modern model follow:

A character repertoire is the full set of abstract characters that a system supports. The repertoire may be closed, i.e. no additions are allowed without creating a new standard (as is the case with ASCII and most of the ISO-8859 series), or it may be open, allowing additions (as is the case with Unicode and to a limited extent the Windows code pages). The characters in a given repertoire reflect decisions that have been made about how to divide writing systems into basic information units. The basic variants of the Latin, Greek and Cyrillic alphabets can be broken down into letters, digits, punctuation, and a few special characters such as the space, which can all be arranged in simple linear sequences that are displayed in the same order they are read. Even with these alphabets, however, diacritics pose a complication: they can be regarded either as part of a single character containing a letter and diacritic (known as a precomposed character), or as separate characters. The former allows a far simpler text handling system but the latter allows any letter/diacritic combination to be used in text. Ligatures pose similar problems. Other writing systems, such as Arabic and Hebrew, are represented with more complex character repertoires due to the need to accommodate things like bidirectional text and glyphs that are joined together in different ways for different situations.

A coded character set (CCS) specifies how to represent a repertoire of characters using a number of (typically non-negative) integer values called code points. For example, in a given repertoire, a character representing the capital letter "A" in the Latin alphabet might be assigned to the integer 65, the character for "B" to 66, and so on. A complete set of characters and corresponding integers is a coded character set. Multiple coded character sets may share the same repertoire; for example ISO/IEC 8859-1 and IBM code pages 037 and 500 all cover the same repertoire but map them to different codes. In a coded character set, each code point only represents one character, i.e., a coded character set is a function.

A character encoding form (CEF) specifies the conversion of a coded character set's integer codes into a set of limited-size integer code values that facilitate storage in a system that represents numbers in binary form using a fixed number of bits (i.e. practically any computer system). For example, a system that stores numeric information in 16-bit units would only be able to directly represent integers from 0 to 65,535 in each unit, but larger integers could be represented if more than one 16-bit unit could be used. This is what a CEF accommodates: it defines a way of mapping a single code point from a range of, say, 0 to 1.4 million, to a series of one or more code values from a range of, say, 0 to 65,535.

The simplest CEF system is simply to choose large enough units that the values from the coded character set can be encoded directly (one code point to one code value). This works well for coded character sets that fit in 8 bits (as most legacy non-CJK encodings do) and reasonably well for coded character sets that fit in 16 bits (such as early versions of Unicode). However, as the size of the coded character set increases (e.g. modern Unicode requires at least 21 bits/character), this becomes less and less efficient, and it is difficult to adapt existing systems to use larger code values. Therefore, most systems working with later versions of Unicode use either UTF-8, which maps Unicode code points to variable-length sequences of octets, or UTF-16, which maps Unicode code points to variable-length sequences of 16-bit words.

Next, a character encoding scheme (CES) specifies how the fixed-size integer code values should be mapped into an octet sequence suitable for saving on an octet-based file system or transmitting over an octet-based network. With Unicode, a simple character encoding scheme is used in most cases, simply specifying whether the bytes for each integer should be in big-endian or little-endian order (even this isn't needed with UTF-8). However, there are also compound character encoding schemes, which use escape sequences to switch between several simple schemes (such as ISO/IEC 2022), and compressing schemes, which try to minimise the number of bytes used per code unit (such as SCSU, BOCU, and Punycode). See comparison of Unicode encodings for a detailed discussion.

Finally, there may be a higher level protocol which supplies additional information that can be used to select the particular variant of a Unicode character, particularly where there are regional variants that have been 'unified' in Unicode as the same character. An example is the XML attribute xml:lang.

The Unicode model reserves the term character map for historical systems which directly assign a sequence of characters to a sequence of bytes, covering all of CCS, CEF and CES layers.

Character sets, maps and code pages

In computer science, the terms "character encoding", "character map", "character set" and "code page" were historically synonymous, as the same standard would specify a repertoire of characters and how they were to be encoded into a stream of code units – usually with a single character per code unit. The terms now have related but distinct meanings, reflecting the efforts of standards bodies to use precise terminology when writing about and unifying many different encoding systems. Regardless, the terms are still used interchangeably, with character set being nearly ubiquitous.

A "code page" usually means a byte-oriented encoding, but with regard to some suite of encodings (covering different scripts), where many characters share the same codes in most or all those code pages. Well known code page suites are "Windows" (based on Windows-1252) and "IBM"/"DOS" (based on code page 437), see Windows code page for details. Most, but not all, encodings referred to as code pages are single-byte encodings (but see octet on byte size.)

IBM's Character Data Representation Architecture (CDRA) designates with coded character set identifiers (CCSIDs) and each of which is variously called a "charset", "character set", "code page", or "CHARMAP".

The term "code page" does not occur in Unix or Linux where "charmap" is preferred, usually in the larger context of locales.

Contrasted to CCS above, a "character encoding" is a map from abstract characters to code words. A "character set" in HTTP (and MIME) parlance is the same as a character encoding (but not the same as CCS).

"Legacy encoding" is a term sometimes used to characterize old character encodings, but with an ambiguity of sense. Most of its use is in the context of Unicodification, where it refers to encodings that fail to cover all Unicode code points, or, more generally, using a somewhat different character repertoire: several code points representing one Unicode character, or versa (see e.g. code page 437). Some sources refer to an encoding as legacy only because it preceded Unicode. All Windows code pages are usually referred to as legacy, both because they antedate Unicode and because they are unable to represent all 2²¹ possible Unicode code points.