gtpn1m0bNon-SNA Data Communications Reference

Items Subject to Bilateral Agreement

The following factors must be agreed by link between the parties concerned at each end.

  1. The number of lines in the link.
  2. The maximum size of data message blocks, bearing in mind the transmission requirements of the order of seconds for Type A messages, and the maximum message length of sixteen blocks.
  3. The number of extra SYN characters required for character framing Two SYNs will automatically precede every block. SYNs will not be sent after a block, although it is possible to immediately follow one block with another, provided the second block begins with the required number of SYNs.
  4. The frequency of acknowledgment of data message blocks (p). When p is greater than one, this does not preclude acknowledgment more frequently. In any event, acknowledgment will be sent by the TPF System for each block of multi-block messages.
  5. T1 - the time between successive transmissions of ENQ or NAK without receiving a valid response.
  6. T2 - the time between successive transmissions of the idle line link control block during the idle line procedure.
  7. N2 - the number of ENQ, ILB or NAK repetitions without valid response before the line is considered impaired.
  8. Whether an Acknowledge Message Label LCB is required to clear a message label for reuse.
  9. The inclusion of additional message envelope control characters. The following envelope characters are required on all links:
    SYN SYN DLE TSI MBI LCI - ETB BCC
    

    The Additional Characteristics Indicator (ACI) is not a mandatory component of the message control character set, and its inclusion or omission from the message envelope must be agreed by link between the parties at each end.

    On connections to a High Level Network (e.g., SITA), two additional control fields are required following the LCI:

    HEX
    to specify the Exit Center from the HLN.

    HEN
    to specify the Entry Center to the HLN.
  10. Whether ACK is an acceptable response to ENQ and ILB.
  11. The number of unacknowledged data messages blocks before reaching "TSI Exhaustion".
  12. Whether blocks of multi-block messages may be transmitted down any line of a multi-line link (spread or scatter) or are confined to one line (non-spread or non-scatter).
  13. The use of ILB link control blocks in the idle-line condition.
  14. T6 - the time interval allowed between receipt of the last ACK of a multi-block message and the Acknowledge Message Label (AML) LCB before the entire message will be retransmitted. T6 is an alternate value to T1.
  15. T7 - the time interval allowed between receipt of successive blocks of a multi-block message before the partially received message is discarded.
  16. N3 - the number of times a multi-block message is retransmitted when no Acknowledge Message Label (AML) LCB has been received.
Note:
T6, T7, and N3 are optional items; if these parameters are to be defined, they are subject to bilateral agreement between both parties on the link.