gtpn1m0aNon-SNA Data Communications Reference

Errors

Error Conditions

The Enquiry procedure is entered if a center detects any of the following conditions.

Enquiry Procedure

The center will stop transmitting data blocks and will send an Enquiry LCB indicating the TSI of its last correctly received data message block. Incoming data message blocks will be dropped but link control blocks will continue to be accepted and processed. If no valid response is received, i.e., RSM or STP (or optionally ACK), the Center will continue sending ENQs at time intervals T1. If a valid response with an illogical or reset ATSI is received, the ENQ will be repeated once more and the next response accepted. If the ATSI is still illogical or reset, the Transmission TSI series will be altered to the next expected TSI as indicated by the ATSI of the response. If after N2 repetitions of the ENQ no valid response has been received, the line will be considered defective and declared 'out-of-service' (unless already so; see "Error Conditions" Section).

Line Out Of Service

The Center will continue sending ENQs down the faulty line at intervals of T1 seconds. With each ENQ, an STP will be sent down any other surviving line with the LSI indicating the faulty line and its ATSI indicating the TSI of the last data message block correctly received on the faulty line. The Center will recover and resume normal operation on the defective line on the receipt of a valid response to an ENQ. If the line is out of service and the Center receives an ENQ, it will respond with Resume Sending (unless an overload condition exists; see paragraph 11 in "Receiving Center" Section) but will continue sending ENQs until it receives a valid response.

Line Invalidation

If hardware errors occur on a line, producing unit checks, and these persist on retry of the operation, then after a system defined number of retries a Stop Sending link control block will, if possible, be sent down any other surviving line with the LSI indicating the faulty line and its ATSI indicating the TSI of the last data message block correctly received on the faulty line. The line will be invalidated and an alarm message sent to an operator. The invalidated line can then only be restarted by operator action.