Functional Description
The occurence of an XMR interrupt in auto mode after an XIF command indicates that the
I-frame sent was either rejected by the Peer Entity or that a collision occured on the
S-interface. In both cases the I-frame has to be retransmitted (after an eventual waiting for
the RSC interrupt if the Peer Rec was busy; after an XMR the device will always be in the
state 7). In non-auto mode or after an XTF command the XMR indicates that a collision
occured on the S-interface and the frame has to be retransmitted.
d) The resetting of the RC variable:
The RC variable is reset in the ICC and ISAC-S when leaving the state timer recovery. The
SDL diagrams indicate a reset in the state multiple frame established when T200 expires.
There is no difference to the outside world between these implementations however our
implementation is clearer.
e) The timer T203 procedure:
We do not fully support the optional timer T203 procedure, but we can still find out whether
or not S-frames are sent on the link in the auto mode. By polling the STAR2:SDET bit and
(re)starting a software timer whenever a one is read we can build a quasi T203 procedure
which handles approximately the same task. When T203 expires one is supposed to go
into the timer recovery state with RC = 0. This is possible for the ICC and ISAC-S by just
writing the STI bit in the CMDR register (auto mode and internal timer mode assumed).
f) The congestion procedure as defined in the 1 TR 6 of the "Deutsche Bundespost":
In the 1 TR 6 a variable N2 × 4 is defined for the maximum number of Peer Busy requests.
The 1 TR 6 is in this respect not compatible with the Q921 of CCITT or the ETSI 46-20 but
it is, nevertheless, sensible to avoid getting into a hangup situation. With the ICC and
ISAC-S this procedure can be implemented:
After receiving an RSC interrupt with RRNR set one starts a software-timer. The timer is
reset and stopped if one either receives another RSC interrupt with a reset RRNR, if one
receives a TIN interrupt or if other conditions occur that result in a reestablishment of the
link. The timer expires after N2 × 4*T200 and in this case the 1 TR 6 recommends a
reestablishment of the link.
2.8.5.3 Dealing with Error Conditions
In the Recommendation Q.921 of CCITT (Blue Book) several error conditions are described.
We shall deal with them as far as they touch the auto mode of the ISAC-S (which only applies
for states 7,8 of Q.921).
Throughout the following document in subsections 1 we shall give the original Q.921-text. For
better discrimination against comments the original text is printed in italic characters. Please
note that Q.921/table 5 has been corrected according to Corrigendum No. 1 10/1989.
Subsections 2 document how the ISAC-S react in all cases, and subsections 3 will give hints
how your software should respond to these reactions.
Invalid Frames and Frame Abortion
During data transmission invalid frames and frame abortion generally lead to error conditions.
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