gtpn1m08 | Non-SNA Data Communications Reference |
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.
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.
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).
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.
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.
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.
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.
Two levels of overload are defined:
If it is not possible to send Stop All, the overload condition will be detected in one of two ways by the Transmitting Center:
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.