
USING THE BDLC PROTOCOL
UNILINK HOST
ADAPTER B–10
USER MANUAL
B.5 ERRORS AND ERROR RECOVERYBDLCs design includes methods for detecting and correcting transmission
errors so that message integrity is assured. Methods are provided for busy
condition, N(S) and N(R) sequence errors, BCS errors, invalid commands, and
timeouts.
B.5.1 Busy
Condition
When either the host computer or the adapter temporarily cannot receive or
continue
to receive I-Frames or UIs because
of internal buffer limitations, that
station
alerts the sending station
by transmitting an RNR with N(R) set to the
count that it expects to receive on the next frame. The busy station should
continue
to respond with RNR until
buffer space is available, while the sending
station
should respond with RR.
A “busy condition” on an adapter will be forced clear whenever the host
computer
sends a SNRM.
B.5.2
N(S) Sequence Err
or
An N(S) sequence error occurs when a receiving station receives a BCS error
free I-Frame that contains an N(S) sequence number not equal to the internal
receive count. The receiving station does not increment its receive count and
responds
with a REJ with the N(R) set to the message it expected to receive. If
the receiving station is the host computer, it can optionally issue a SNRM to
reset
the adapter
’
s sequence counts.
B.5.3
N(R) Sequence Err
ors
An N(R) sequence error occurs when a receiving station receives a BCS error
free
I-Frame that contains an N(R) sequence number not equal to the internal
send
count. If the receiving station is the host computer
, it should respond with
an RR to acknowledge the last frame received correctly or should send a
SNRM
to
reset the adapter
’
s sequence counts.
If the receiving station is the adapter, it will respond with a FRMR indicating
the
condition causing the error
. FRMR is further described in Paragraph B.5.7.