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PRELIMINARY
DS3 UNI FOR ATM
XRT7245
REV. 1.03
231
7.1.2.6
The LAPD Transceiver uses the three “DL” bit fields of
-frame #5, within each M-frame, to transmit and receive
performance monitor data link messages. The “Far-
End” LAPD Transmitter will transmit a LAPD Message
to the “Near-End” Receiver via these three bits within
each DS3 Frame. The LAPD Receiver will receive and
store the information portion of the received LAPD
frame into the “Receive LAPD Message” Buffer,
which is located at addresses: DEh through 135h
within the on-chip RAM. The LAPD Receiver (within
the “Near-End” Receive DS3 Framer) has the follow-
ing responsibilities.
Framing the incoming LAPD Messages
Filtering out stuffed “0s” (within the information
payload)
LAPD Receiver
Storing the Frame Message into the “Receive
LAPD Message” Buffer
Perform Frame Check Sequence (FCS) Verification
Provide status indicators for
– End of Message (EOM)
– Flag Sequence Byte detected
– Abort Sequence detected
– Message Type
– C/R Type
– The occurrence of FCS Errors
The LAPD receiver’s actions are facilitated via the
following two registers.
Rx DS3 LAPD Control Register
Rx DS3 LAPD Status Register
F
IGURE
69. F
LOW
D
IAGRAM
DEPICTING
HOW
THE
R
ECEIVE
FEAC P
ROCESSOR
F
UNCTIONS
.
ENABLE THE "FEAC REMOVAL AND
VALIDATION" INTERRUPTS
This is accomplished by writing "xxx 1010" into the
"RxDS3 FEAC Interrupt/Status Register
(Address = 0x13)
RECEIVE FEAC PROCESSOR BEGINS READING IN
THE FEAC BIT-FIELDS (OF INCOMING DS3 FRAMES)
The Receive FEAC Processor checks for the "FEAC
Framing Alignment" pattern of "01111110"
READ IN THE "6-BIT FEAC CODE WORD"
The 6-bit FEAC Code Word immediately follows the
"FEAC Framing Alignment" pattern
GENERATE "FEAC
VALIDATION" INTERRUPT
START
Is the
"FEAC Framing
Alignment" pattern
present in the FEAC
Channel
INVOKE "FEAC VALIDATION"
INTERRUPT SERVICE ROUTINE
GENERATE "FEAC
REMOVAL" INTERRUPT
INVOKE "FEAC REMOVAL"
INTERRUPT SERVICE ROUTINE
Has this
same FEAC
Code Word been
received in 8 out of the last
10 FEAC message
receptions
Has a FEAC
Code Word (other than
the last "Validated Code Word)
been received in 3 out of the
last 10 FEAC message
receptions