Functional Description
e) the frame is discarded
f) the frame is discarded
The reaction to § 2.10 has been already discussed under a)
Necessary Software Actions
The software should read the Register RSTA after a RME-interrupt. After having read RAB = 1
or CRC = 0, all frame contents read from the FIFO should be discarded and a CMDR: RMC
should be written. After each RPF or RBCH, RBCL should be read and if it exceeds 528,
CMDR: RRES should be written. In this way all invalid frames are discarded by the software.
Data Overflow
In case of a data overflow, which is only possible while receiving an I-frame or an U-frame with
a non-empty information field, the ISAC-S interrupt with ISTA: RME and sets RSTA: RDO to
1. A RSTA: RDO and an ISTA: RFO are a hint that the dynamic reaction time of your software
to the RPF, RME interrupt is too slow, so you should change your software. During the
development phase you may set CMDR: RNR after an RDO, RFO-condition to protect against
further errors, but the final solution can only be to exclude RDO, RFO conditions by an
improved software design.
Frame Rejection Condition
Q.921 § 5.8.5: Frame Rejection Condition
A frame rejection condition results from one of the following conditions:
a) the receipt of an undefined frame (see § 3.6.1, third paragraph)
b) the receipt of a supervisory or unnumbered frame with incorrect length
c) the receipt of an invalid N(R), or
d) the receipt of a frame with an information field which exceeds the maximum established
length.
Upon occurrence of a frame rejection condition whilst in the multiple frame operation, the data
link layer entity shall:
– issue a MDL-ERROR-INDICATION primitive, and
– initiate re-establishment (see § 5.7.2).
Upon occurrence of a frame rejection condition during establishment of or release from
multiple frame operation, or whilst a data link is not established, the data link layer entity shall:
– issue a MDL-ERROR-INDICATION primitive, and
– discard the frame.
Note: For satisfactory operation it is essential that a receiver is able to discriminate between
invalid frames, as defined in § 2.9, and frames with an information field which exceeds
the maximum established length (see § 3.6.11 item d). An unbounded frame may be
assumed, and thus discarded, if two times the longest permissible frame plus two octets
are received without a flag detection.
Semiconductor Group
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