XRT74L73 Datasheet, PDF (356/488 Pages) Exar Corporation – 3 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER

XRT74L73
3 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER
REV. P1.0.1
PRELIMINARY
RXE3 LAPD STATUS REGISTER (ADDRESS = 0X19)
BIT 7
Not Used
BIT 6
RxABORT
BIT 5
BIT 4
RxLAPDType[1:0]
RO
RO
RO
RO
0
0
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
1
The receipt of the Flag Sequence octet can mean
one of two things.
1. This Flag Sequence byte may be marking the
beginning or end of an incoming LAPD Message
frame.
2. The Received Flag Sequence octet could be just
one of many Flag Sequence octets that are trans-
mitted via the E3 Transport Medium, during idle
periods between the transmission of LAPD Mes-
sage frames.
The LAPD Receiver will negate the Flag Present bit
as soon as it has received an octet that is something
other than the Flag Sequence octet. Once this hap-
pens, the LAPD Receiver should be receiving either
octet # 2 of the incoming LAPD Message, or an
ABORT Sequence (e.g., a string of seven or more
consecutive “1’s”). If this next set of data is an
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
RxABORT
RO
RO
0
1
BIT 5
BIT 4
RxLAPDType[1:0]
RO
RO
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 “N” bit-fields within each incoming E3
frame. Secondly, it is reassembling these bits 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., “0’s” 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”.
NOTE: For more information on how the LAPD Transmitter
inserted these stuff bits, please see Section 4.2.3.1.
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.
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