Telecommunications - Bi Sync Protocol - Data Transfer

Interrupts and Aborts

Forward Abort

A Block of text can be terminated prematurely by using an ENQ character which signals the receiver to "disregard this block" NAK is always the reply in this situation.

RVI Reverse Interrupt

The RVI control sequence is a positive response used in place of the ACK0 or ACK1 positive acknowlegement. RVI is transmed by a receiving station to request termination of the current transmission because of a high priority message which it must transmit to the sending station.


Flow Control

WACK (Wait-before-transmit-positive-acknowledgement) allows a receiving station to indicate a "temporary not ready to receive"

------------

S S | S |

Y Y | O | ---> message

N N | H |

+---+--------

---

S S | W |

<--- Y Y | A |

N N | C |

+---|

K

---

S S | E |

Y Y | N | ---->

N N | Q |

+---|

. continues to send WACK in response

. to ENQ until ready to continue

.

---

S S | E |

Y Y | N | ---->

N N | Q |

+---|

-----

S S | A |

<---- Y Y | C |

N N | K |

+-----|

0/1

------------------

S S | S |

Y Y | O | next message -->

N N | H |

+---+--------------

TTD - Temporary Text Delay

The TTD control sequence is sent by a sending station in message transfer

state when it wishs to retain the line but is not ready to transmit.

The normal response to TTD is a NAK

TTD = STX ENQ

------------------>

NAK

<------------------

Transparent Data (optional - can be used in EBCDIC or ascii)

If information which is not EBCDIC or ascii text is to be sent in a message then a bit pattern equivalant to a control code may occur, to avoid this transparent mode must be used. In transparent mode all data-link control sequences must start with DLE as follows,

DLE STX = STX

DLE ETB = ETB

DLE ETX = ETX

DLE SYN = SYN inside message only

message starts with SYN SYN as usual

DLE ENQ = ENQ

DLE DLE used to permit transmission of DLE as data when a bit pattern

equivalent to DLE appears within the transparent data.

One DLE is disregarded the other is treated as data.

DLE ITB = ITB


EOT and NAK must be followed by a trailing 'FF' PAD character



goto next document - traces


 


metadata block
see also:

 

Correspondence about this page

Book Shop - Further reading.

Where I can, I have put links to Amazon for books that are relevant to the subject, click on the appropriate country flag to get more details of the book or to buy it from them.

cover The Essential Guide to Telecommunications (Essential Guide).

Commercial Software Shop

Where I can, I have put links to Amazon for commercial software, not directly related to the software project, but related to the subject being discussed, click on the appropriate country flag to get more details of the software or to buy it from them.

 

This site may have errors. Don't use for critical systems.

Copyright (c) 1998-2015 Martin John Baker - All rights reserved - privacy policy.