History
Up to the early 1990s, many programs and data transmission channels assumed that all characters would be represented as numbers between 0 and 127 (7 bits). On computers and data links using 8-bit bytes this left the top bit of each Byte free for use as a parity, flag bit, or meta data control bit. 7-bit systems and data links are unable to handle more complex character codes which are commonplace in non-English-speaking countries with larger alphabets.
Binary files cannot be transmitted through 7-bit data channels directly. To work around this, binary-to-text encodings have been devised which use only 7-bit ASCII characters. Some of these encodings are uuencoding, Ascii85, SREC, BinHex, kermit and MIME's Base64. EBCDIC-based systems cannot handle all characters used in UUencoded data. However, the base64 encoding does not have this problem.
Perhaps the final 7-bit restriction, primarily imposed due to the pervasive use of RS-232 protocol for serial ports between devices, notably computers and modems, was lifted in the mid-1990s when RS-232 was largely replaced with Ethernet and with USB.
Read more about this topic: 8-bit Clean
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