Editing Control character (section)

===Transmission control===
The transmission control characters were intended to structure a data stream, and to manage re-transmission or graceful failure, as needed, in the face of transmission errors.

The start of heading (SOH) character was to mark a non-data section of a data stream—the part of a stream containing addresses and other housekeeping data. The start of text character (STX) marked the end of the header, and the start of the textual part of a stream. The end of text character (ETX) marked the end of the data of a message. A widely used convention is to make the two characters preceding ETX a checksum or [[Cyclic redundancy check|CRC]] for error-detection purposes. The end of transmission block character (ETB) was used to indicate the end of a block of data, where data was divided into such blocks for transmission purposes.

The escape character ([[escape character|ESC]]) was intended to "quote" the next character, if it was another control character it would print it instead of performing the control function. It is almost never used for this purpose today. Various printable characters are used as visible "[[escape character]]s", depending on context.

The substitute character ([[substitute character|SUB]]) was intended to request a translation of the next character from a printable character to another value, usually by setting bit 5 to zero. This is handy because some media (such as sheets of paper produced by typewriters) can transmit only printable characters. However, on MS-DOS systems with files opened in text mode, "end of text" or "end of file" is marked by this [[Ctrl-Z]] character, instead of the [[Ctrl-C]] or [[Ctrl-D]], which are common on other operating systems.

The cancel character ([[cancel character|CAN]]) signaled that the previous element should be discarded. The negative acknowledge character ([[NAK]]) is a definite flag for, usually, noting that reception was a problem, and, often, that the current element should be sent again.  The acknowledge character ([[acknowledge character|ACK]]) is normally used as a flag to indicate no problem detected with current element.

When a transmission medium is half duplex (that is, it can transmit in only one direction at a time), there is usually a master station that can transmit at any time, and one or more slave stations that transmit when they have permission. The enquire character ([[Enquiry character|ENQ]]) is generally used by a master station to ask a slave station to send its next message. A slave station indicates that it has completed its transmission by sending the end of transmission character ([[end-of-transmission character|EOT]]).

The device control codes (DC1 to DC4) were originally generic, to be implemented as necessary by each device. However, a universal need in data transmission is to request the sender to stop transmitting when a receiver is temporarily unable to accept any more data. [[Digital Equipment Corporation]] invented a convention which used 19 (the device control 3 character ([[Software flow control|DC3]]), also known as control-S, or [[XOFF]]) to "S"top transmission, and 17 (the device control 1 character ([[Software flow control|DC1]]), a.k.a. control-Q, or [[XON]]) to start transmission. It has become so widely used that most don't realize it is not part of official ASCII. This technique, however implemented, avoids additional wires in the data cable devoted only to transmission management, which saves money. A sensible protocol for the use of such transmission flow control signals must be used, to avoid potential deadlock conditions, however.

The data link escape character ([[C0 and C1 control codes|DLE]]) was intended to be a signal to the other end of a data link that the following character is a control character such as STX or ETX.  For example a packet may be structured in the following way ([[C0 and C1 control codes|DLE]]) <STX> <PAYLOAD> ([[C0 and C1 control codes|DLE]]) <ETX>.