6100 BSC Programming Manual

6100 BSC Concepts and Context
Negative Acknowledgement (NAK). Except in the context of circuit
assurance, BSC sends NAK acknowledgements whenever they are
needed. There is no application request to send a negative
acknowledgement. During circuit assurance, the application puts
the NAK in its buffer, if needed. Figure 1-15 shows several uses
of NAK.
BSC sends NAK as a reply in any of the following cases:
• It receives a block that contains an error. That is, the
block check comparison fails, or the block contains an ENQ.
After sending the NAK, the station waits for a correct version
of the faulty block. The station can accept as many
retransmissions as its retry count will allow; if the error
persists, the station reports an error to the local
application.
• It receives a TTD sequence. After sending the NAK, the
station waits for the expected block or another TTD. If the
retry count is exhausted before the block arrives, BSC reports
an error to the application.
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