gtpn1m08Non-SNA Data Communications Reference

Link Operational Procedures

For convenience, this section is divided into two parts: procedures for the Transmitting Center and procedures for the Receiving Center; however, it should be remembered that since the lines are full-duplex, every Transmitting Center is also a Receiving Center, and vice-versa.

Transmitting Center

  1. Messages longer than the agreed maximum block size will be transmitted in several blocks.

    When a link consists of several lines, blocks of a multi-block message may optionally be spread (or 'scattered') over different lines of the link or may be sent on the same line. In either case, successive blocks of a multi-block message transmitted on a line may be interspersed with blocks of different messages with higher priority.

  2. On a single line link, one Transmission TSI series and one Output MBI series per priority will be maintained. On a multi-line link a Transmission TSI series will be maintained for each line and one output MBI series per priority for the whole link.

    The Transmission TSI series will be advanced sequentially for each data message block transmitted without regard to message type. However, it will not be advanced more than a preset "TSI Exhaustion" value beyond the point to which it has been acknowledged and cleared.

    The Transmission TSI series will be reset on receipt of a link control block with ATSI bits b1 through b5 coded all zeros. It will also be altered on receipt of a link control block with an illogical ATSI (See Errors Section).

  3. Message queues will be handled as follows:

    The Transmitting Center will queue messages for a link according to priority. Messages will be assigned an output MBI as follows:

    Single block messages: The message label will be 001 and the block chaining number 1111.

    Multi-block messages: The message label will be the first available combination of the possible seven, 000 and 010 through 111. Block chaining numbers will be allocated sequentially for each message label from 0000 through 1111.

    When a line becomes available, the queues will be checked in priority order to obtain the next block for transmission on that line and the next sequential Transmission TSI for that line will be allotted, in a series running from 1 to 31.

    Should the number of outstanding (unacknowledged) message blocks reach the TSI Exhaustion value, the Transmitting Center will cease transmitting data message blocks and send an Enquiry Link Control Block. The ATSI of the response (RSM or STP or optionally ACK) should enable the TSI Exhaustion to be relieved.

  4. Message blocks are sent continuously with a block separation of at least two SYN characters. After transmission of a block has commenced it will not be interrupted either by a Link Control Block or a higher priority message block.
  5. The Transmitting Center will bear the responsibility for data message blocks until the time they have been positively acknowledged. ACKn implies acknowledgment of all previously unacknowledged blocks up to and including the value of n. The following Link Control Blocks will also be interpreted as positive acknowledgment.

    STPn implies ACKn

    ENQn implies ACKn

    RSMn implies ACKn

    ILBn implies ACKn

    NAKn implies ACKn

    When all blocks of a multi-block message have been acknowledged and no AML agreement exists for the link, the message label is cleared for reuse immediately. However, when an AML agreement does exist for the link, the message label is cleared for reuse upon the receipt of an AML Link Control Block. If the AML has not been received within T1 or T6 seconds, one of two mutually exclusive processes are followed.

  6. If the Transmitting Center receives a negative acknowledgment link control block, indicating invalid block NAK-INV or TSI sequence error NAK-OST, it will retransmit any outstanding unacknowledged blocks following the last one correctly transferred and acknowledged as identified by the ATSI of the NAK.
  7. When the Transmitting Center has no data message blocks to send and there are outstanding unacknowledged blocks it will, if the idle line option allows, transmit an ILB link control block with the TSI of the last correctly received data message block in order to solicit an acknowledgment for the outstanding blocks. The Receiving Center will respond with RSM, or STP or optionally ACK indicating the TSI of its last correctly received data message block. If the above conditions continue, the ILB link control block will be repeated at time intervals T2. If no response is received for N2 successive ILBs, the line is declared out-of-service. If the option does not allow exchange of idle line ILB Link Control blocks, then the Receiving Center should exercise the Receive Idle procedure.
  8. If the Transmitting Center receives a Stop Sending link control block for a specific line, it will immediately stop sending data blocks on the line specified by the LSI. All unacknowledged blocks for that line will be requeued for transmission on another line. Multi-block messages will retain their original MBIs. The Transmitting Center will continue to accept and process incoming data and link control blocks and transmit link control blocks. Idle line link control blocks (ILB or ACK) will be transmitted on the stopped line, at time intervals T2, intermixed with normal responses to traffic, until a Resume Sending is received.

    If the stopped line was the last active line on the link, then multi-block output messages which have not been cleared will be requeued for transmission, with the same output MBI, when a line becomes available.

  9. If the Transmitting Center receives a Stop All link control block, the Enquiry procedure is entered for all lines.
  10. If the Transmitting Center receives a Resume Sending link control block for a specific line, it will resume sending data message blocks on the line specified by the LSI commencing with the block following the last one correctly transferred and acknowledged as identified by the ATSI of the Resume Sending.
  11. If the Transmitting Center receives a Resume All link control block, it will resume sending data message blocks on all lines of the link commencing in each case with the current value of the Transmission TSI series.
  12. If the Transmitting Center receives an Enquiry link control block, it will respond with STP or RSM (and optionally ACK), and will then retransmit any outstanding unacknowledged blocks following the last one correctly transferred and acknowledged as identified by the ATSI of the ENQ.
  13. If the Transmitting Center receives an Idle Line ILB link control block, it will respond with STP or RSM (and optionally ACK).

Receiving Center

  1. On a single-line link, one Reception TSI series and one Input MBI series per priority will be maintained. On a multi-line link, a Reception TSI series will be maintained for each line and one input MBI series per priority for the whole link.

    The Reception TSI series will be advanced sequentially for each data message block received without regard to message type, and is used to facilitate the correct reception of data message blocks.

    When a link control block is generated and transmitted, the current value of the Reception TSI series is used as the ATSI of the LCB.

  2. The Receiving Center will send a positive acknowledgment for every block of a multi-block message and at least once for every single-block data message, where the value of p is agreed by link. The ATSI of the ACK will indicate the last correctly received data message block and implies acknowledgment of all previously unacknowledged blocks.
  3. Negative acknowledgment, NAK-IVB, will be sent by the Receiving Center to indicate incorrect format, excessive block length, or parity error in a non-LCB block. The ATSI of this link control block will indicate the last correctly received message block and constitutes a positive acknowledgment for that block and all preceding unacknowledged blocks. The error block will be dropped and all blocks received subsequently on that line will be dropped, without acknowledgment, until a block with next-in-sequence TSI has been correctly received.
  4. Negative acknowledgment, NAK-OST, will be sent by the Receiving Center when the TSI of the received message is not the expected next-in-sequence TSI. The ATSI of this link control block will indicate the last correctly received message block and constitutes a positive acknowledgment for that block and all preceding unacknowledged blocks. The out-of-sequence block will be dropped and all blocks received subsequently on that line will be dropped without acknowledgment until a block with next-in-sequence TSI has been correctly received.
  5. The NAK will be repeated at intervals of T1 seconds until either the correct block is received or the NAK has been repeated N2 times when the Receiving Center will go into the Enquiry procedure.
  6. Incorrect reception of an LCB-length block (i.e., five characters) will result in the erroneous block being discarded without acknowledgment.
  7. If the Receiving Center does not receive a data message block for a period of T1 seconds, it will transmit an idle line link control block (ILB or ACK) indicating the TSI of the last correctly received data message block. If an ILB is sent the Transmitting Center will respond with RSM or STP (or optionally ACK) indicating the TSI of its last correctly received data message block. The ACK or ILB link control block will be repeated at intervals of T2 seconds until a data message block is received. If no response is received for N2 successive idle line link control blocks, the line is declared out-of-service.
  8. A Stop Sending link control block will be sent to stop the Transmitting Center from sending information on a line as specified by the LSI. It may be sent over any available line in a multi-line link. The ATSI will indicate the TSI of the last correctly received block on the specified line, and constitutes a positive acknowledgment of this and all preceding unacknowledged blocks. Any data message blocks subsequently received on the stopped line will be discarded by the Receiving Center; incoming LCB's however, will continue to be accepted and processed.
  9. A Resume Sending link control block will be sent to cause the Transmitting Center to resume the transmission of data message blocks on a specified line. This link control block will be sent only on the line to which it refers. The ATSI will indicate the TSI of the last correctly received block, and constitutes a positive acknowledgment for this and all preceding unacknowledged blocks.
  10. When all blocks of a multi-block message have been correctly received, acknowledged, and assembled an AML link control block may optionally be sent to the Transmitting Center, indicating that this message label is cleared for further usage. On a spreading link, the AML may be sent over any available line. On a non-spread link, it will be sent on the line on which the block which caused the message to be complete was received.
  11. If the Receiving Center discovers an overload condition, it will, if possible, send a Stop All down any available channel of the link, and subsequently respond to Enquiry link control blocks with Stop Sending.

    Two levels of overload are defined:

    1. At the primary level data blocks and link control blocks are accepted. (These data blocks would be 'inertia' blocks transmitted before the transmitting center could take action on the receipt of the Stop All.)
    2. At the secondary (lower) level, data blocks are discarded without response but link control blocks are accepted and processed.

    If it is not possible to send Stop All, the overload condition will be detected in one of two ways by the Transmitting Center:

    1. If the duration of the overload condition is greater than the link's time-out value, the Transmitting Center will go into the Enquiry procedure. If the overload condition still exists, the Receiving Center will either not respond to the ENQ, or respond with Stop Sending.
    2. If the duration of the overload condition is less than the link's time-out value but nevertheless, some data message blocks have been discarded, when the Receiving Center is able to accept data message blocks again, the first data message block accepted will have an out-of-sequence TSI and result in NAK-OST.

    When the overload condition is relieved, the Receiving Center will generate Resume Sending link control blocks on all lines and respond to Enquiry link control blocks with Resume Sending. Additionally, with each occurrence of a receive data time out (see paragraph 7 in "Receiving Center" Section), it will send a Resume Sending in addition to the idle line link control block, until such time as a data message block has been correctly received.

  12. When a message discard (T7) agreement exists for a link, the receiving center, for each intermediate block of a multi-block message, will begin to time the message out. If the next or last block of the message is not received within T7 seconds, the partially received message is discarded. If the next block is received within T7 seconds, the message discard timer will be reset to T7 seconds to await subsequent block(s) of the message. When the last block of the message has been received, the message discard timer is set to preclude the occurrence of a time out and the complete message is then assembled and edited.