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Although the full explanations of all control codes are outside the scope of this book, the following control characters are representative:

1. NUL (NULl): A transmission control character used to serve a media-fill or time-fill requirement, i.e., a stuff character or padding character.

2. SOH (Start Of Header): A transmission control character that indicates the start of a message heading.

3. STX (Start of TeXt): A transmission control character that alerts the receiving device to start the reading, transmission, reception, or recording of text.

4. ETX (End of TeXt): A transmission control character that alerts the receiving device to terminate the reading, transmission, reception, or recording of text.

5. EOT (End Of Transmission): A transmission control character that alerts the receiving device to terminate a transmission that may include one or more texts or messages.

6. ENQ (ENQuiry): A transmission control character to request a response from a station to which a connection has been established.The request may be for the station identification, type of equipment, and station status.

7. NAK (Negative AcKnowledgement): A transmission control character sent by the receiving device to the transmitting device to indicate that a received block of data contained one or more errors. A NAK will trigger the transmitting device to retransmit that errored block.

8. ACK (ACKnowledgement): A transmission control character sent by the receiving device to the transmitting device to indicate that a received block of data contained no errors.

9. BEL (BELl): A transmission control character that alerts the receiving device that causes a bell to ring or activates some other audio or visual device to gain the attention of the operator at the receiving station.

10. ETB (End of Transmission Block): A code-extension character used to indicate the end of the transmission of a block of data.

11. CAN (CANcel): A transmission control character indicating that the associated data is in error or is to be ignored.

12. EM (End of Medium): A control character indicating the physical end of a data storage medium, or the usable portion of the medium.

13. SUB (SUBstitute): Used in place of a character that is known to be invalid, i.e., in error. Also used to indicate a character used in place of one that cannot be represented on a given device, e.g., e may be used in place of ε (epsilon) or d may be used in place of Δ (delta).

14. ESC (ESCape): A code-extension character used to indicate a change in code interpretation to another character set, according to some convention or agreement.This is much like the use of the shift key in Baudot code to indicate a shift between figures and characters.

15. CR (Carriage Return): A format-control character that causes the print or display position to move to the first position, or left-hand margin, of the screen or print medium. Now often associated with an LF (Line Feed), which moves the print position down to the next line

In Unicode terms, ASCII is known as Unicode Transformation Format-7 (UTF-7). See also asynchronous, code set, EBCDIC, and Unicode.

• selective call block
• selective call forwarding
• selective call rejection
• selective ringing
• selector
• selenium
• self-healing
• self-learning bridge
• semiconductor
• send and pray
• sender
• Sequenced Packet Exchange (SPX)
• serial communications
• serial communications
• serial-in, parallel-out (SIPO)
• Serial Line Internet Protocol (SLIP)
• serial line protocol
• Serial Storage Architecture (SSA)
• server
• service
• service access point identifier (SAPI)
• service creation element (SCE)
• Service Level Agreement (SLA)
• service management system (SMS)
• service mark (SM)