Functional Description
Q921: Invalid Frames and Frame Abortion
Paragraphs 2.9 and 2.10 of the Q.921 deal with Invalid Frames and Frame Abortion. In the
following the original text is given.
Q.921 § 2.9: Invalid Frames
An invalid frame is a frame which:
a) is not properly bounded by two flags, or
b) has fewer than 6 octets between flags or frames that contain sequence numbers, or
c) does not consist of an integral number of octets prior to zero bit insertion or following zero
bit extraction, or
d) contains a frame check sequence error, or
e) contains a single octet address field, or
f) contains a service access point identifier (see § 3.3.3) which is not supported by the
receiver.
Invalid frames shall be discarded without notification to the sender. No action is taken as the
result of that frame.
Q.921 § 2.10: Frame Abort
Receipt of seven or more contiguous 1 bits shall be interpreted as an abort and the data link
layer shall ignore the frame currently being received.
Reaction of the ISAC-S
a) A frame which does not start with a flag is discarded in the ISAC-S. A frame which does
not end with a flag is one, that is aborted, i.e. if § 2.9b does not apply then the ISAC-S
– discards the frame, if it was an S-frame
or, if it was an I or U-frame
– generates an ISTA: RME (or RPFs and a RME) and
– puts RSTA: RAB = 1 after the RME-Interrupt RAB = 1.
A frame is supposed to be unbounded according to § 5.8.5 if the byte counter RBCH, RBCL
after RPF or RME exceeds 528.
b) The frame is discarded by the ISAC-S if
with U-frames or undefined frames
with I-frames
with S-frames
it contains less or equal to 4 octets or
it contains less or equal to 5 octets
it contains less than 6 octets.
For U-frames with a content between 4 and 5 octets exclusively or for I-frames between
5 and 6 octets exclusively an ISTA: RME interrupt is generated and afterwards the RSTA:
CRC is set to 0.
c) An S-frame is discarded. In the own-receiver-busy state I-frames are discarded.
For an I-frame in the normal state and U frames, after several possible RPF interrupts and
the final RME interrupt, the bit RSTA: CRC is set to 0 in this case.
d) In case of an -S frame,
-U and I-frames
the frame is discarded
RSTA: CRC is set to "0" in this case.
Semiconductor Group
123
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