XRT74L73 Datasheet, PDF (455/488 Pages) Exar Corporation – 3 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER

PRELIMINARY
XRT74L73
3 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER
REV. P1.0.1
ABORT Sequence, then the LAPD Receiver will as-
sert the RxABORT bit-field (Bit 6) within the Rx E3
LAPD Status Register, as depicted below.
RXE3 LAPD STATUS REGISTER (ADDRESS = 0X19)
BIT 7
Not Used
BIT 6
Rx ABORT
BIT 5
BIT 4
RxLAPDType[1:0]
RO
RO
RO
RO
0
1
0
0
BIT 3
RxCR
Type
RO
0
BIT 2
RxFCS
Error
RO
0
BIT 1
End of
Message
RO
0
BIT 0
Flag
Present
RO
0
However, if this next octet is Octet #2 of an incoming
LAPD Message frame, then the LAPD Receiver is be-
ginning to receive a LAPD Message frame.
As the LAPD Receiver receives this LAPD Message
frame, it is reading in the LAPD Message frame oc-
tets, from either the “GC” or the “NR” byte-fields with-
in each incoming E3 frame. Secondly, it is reassem-
bling these octets into a LAPD Message frame.
Once the LAPD Receiver has received the complete
LAPD Message frame, then it will proceed to perform
the following five (5) steps.
1. PMDL Message Extraction
The LAPD Receiver will extract out the PMDL Mes-
sage, from the newly received LAPD Message frame.
The LAPD Receiver will then write this PMDL Mes-
sage into the Receive LAPD Message buffer within
the Framer IC.
NOTE: As the LAPD Receiver is extracting the PMDL Mes-
sage, from the newly received LAPD Message frame, the
LAPD Receiver will also check the PMDL data for the
occurrence of stuff bits (e.g., “0s” that were inserted into the
PMDL Message by the Remote LAPD Transmitter, in order
to prevent this data from mimicking the Flag Sequence byte
or an ABORT Sequence), and remove them prior to writing
the PMDL Message into the Receive LAPD Message
Buffer. Specifically, the LAPD Receiver will search through
the PMDL Message data and will remove any “0” that
immediately follows a string of 5 consecutive “1’s”.
For more information on how the LAPD Transmitter
inserted these stuff bits, please see Section 5.2.3.1.
2. FCS (Frame Check Sequence) Word Verification
The LAPD Receiver will compute the CRC-16 value
of the header octets and the PMDL Message octets,
within this LAPD Message frame and will compare it
with the value of the two octets, residing in the FCS
word-field of this LAPD Message frame. If the FCS
value of the newly received LAPD Message frame
matches the locally-computed CRC-16 value, then
the LAPD Receiver will conclude that it has received
this LAPD Message frame in an error-free manner.
However, if the FCS value does not match the locally-
computed CRC-16 value, then the LAPD Receiver
will conclude that this LAPD Message frame is erred.
The LAPD Receiver will indicate the results of this
FCS Verification process by setting Bit 2 (RxFCS Er-
ror) within the Rx E3 LAPD Status Register, to the ap-
propriate value as tabulated below.
RXE3 LAPD STATUS REGISTER (ADDRESS = 0X19)
BIT 7
Not Used
BIT 6
Rx ABORT
BIT 5
BIT 4
RxLAPDType[1:0]
BIT 3
RxCR
Type
BIT 2
RxFCS
Error
BIT 1
End of
Message
BIT 0
Flag
Present
RO
RO
RO
RO
RO
RO
RO
RO
0
0
0
0
0
1
0
0
If the LAPD Receiver detects an error in the FCS val-
ue, then it will set the RxFCS Error bit-field to “1”.
Conversely, if the LAPD Receiver does not detect an
error in the FCS value, the it will clear the RxFCS Er-
ror bit-field to “0”.
NOTE: The LAPD Receiver will extract and write the PMDL
Message into the Receive LAPD Message buffer indepen-
dent of the results of FCS Verification. Hence, the user is
urged to validate each PMDL Message that is read in from
the Receive LAPD Message buffer, by first checking the
state of this bit-field.
3. Check and Report the State of the “C/R” Bit-field
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