Principles Digital Communication System & Computer Networks (Charles River Media Computer Engineering)
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4.1 TEXT MESSAGES
Text messages are generally represented in ASCII (American Standard Code for Information Interchange), in which a 7-bit code is used to represent each character. Another code form called EBCDIC (Extended Binary Coded Decimal Interchange Code) is also used. To transmit text messages, first the text is converted into one of these formats, and then the bit stream is converted into an electrical signal.
Using ASCII, the number of characters that can be represented is limited to 128 because only 7-bit code is used. The ASCII code is used for representing many European languages as well. To represent Indian languages, a standard known as Indian Standard Code for Information Interchange (ISCII) has been developed. ISCII has both 7-bit and 8-bit representations.
ASCII is the most widely used coding scheme for representation of text in computers. ISCII is used to represent text of Indian languages.
| Note | In extended ASCII, each character is represented by 8 bits. Using 8 bits, a number of graphic characters and control characters can be represented. |
Unicode has been developed to represent all the world languages. Unicode uses 16 bits to represent each character and can be used to encode the characters of any recognized language in the world. Modern programming languages such as Java and markup languages such as XML support Unicode.
Unicode is used to represent any world language in computers. Unicode uses 16 bits to represent each character. Java and XML support Unicode.
It is important to note that the ASCII/Unicode coding mechanism is not the best way, according to Shannon. If we consider the frequency of occurrence of the letters of a language and use small codewords for frequently occurring letters, the coding will be more efficient. However, more processing will be required, and more delay will result.
The best coding mechanism for text messages was developed by Morse. The Morse code was used extensively for communication in the old days. Many ships used the Morse code until May 2000. In Morse code, characters are represented by dots and dashes. Morse code is no longer used in standard communication systems.
| Note | Morse code uses dots and dashes to represent various English characters. It is an efficient code because short codes are used to represent high-frequency letters and long codes are used to represent low-frequency letters. The letter E is represented by just one dot and the letter Q is represented by dash dash dot dash. |
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