XRT74L73
3 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER
REV. P1.0.1
PRELIMINARY
255
The receipt of the Flag Sequence octet can mean
one of two things.
1.
The Flag Sequence byte marks the beginning or
end of an incoming LAPD Message.
2.
The received Flag Sequence octet could be just
one of many Flag Sequence octets that are trans-
mitted via the DS3 Transport Medium, during idle
periods between the transmission of LAPD Mes-
sages.
The LAPD Receiver will clear the Flag Present bit as
soon as it has received an octet that is something
other than the Flag Sequence octet. At this point, 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 1s). If
this next set of data is an abort sequence, then the
LAPD Receiver will assert the RxAbort bit (Bit 6) with-
in the Rx DS3 LAPD Status Register. However, if this
next octet is Octet #2 of an incoming LAPD Message,
then the Rx DS3 LAPD Status Register will begin to
present some additional status information on this in-
coming message. Each of these indicators is pre-
sented below in sequential order.
Bit 3 - RxCR Type - C/R (Command/Response)
Type
This bit-field reflects the contents of the C/R bit-field
within octet #2 of the LAPD Frame Header. When
this bit is "0" it means that this message is originating
from a customer installation. When this bit is "1" it
means that this message is originating from a net-
work terminal.
Bit 4,5 - RxLAPD Type[1, 0] - LAPD Message Type
The combination of these two bit fields indicate the
Message Type and the Message Size of the incoming
LAPD Message frame. Table 56 relates the values
of Bits 4 and 5 to the Incoming LAPD Message Type/
Size.
N
OTE
:
The Message Size pertains to the size of the Infor-
mation portion of the LAPD Message Frame (as presented
in Figure 92 ).
Bit 3 - Flag Present
The LAPD Receiver should receive another Flag Se-
quence octet, which marks the End of the Message.
Therefore, this bit field should be asserted once
again.
Bit 1 - EndOfMessage - End of LAPD Message
Frame
Upon receipt of the closing Flag Sequence octet, this
bit-field should be asserted. The assertion of this bit-
field indicates that a LAPD Message Frame has been
completely received. Additionally, if this newly re-
ceived LAPD Message is different from the previous
message, then the LAPD Receiver will inform the μC/
μP of the EndOfMessage event by generating an in-
terrupt.
Bit 2 - RxFCSErr - Frame Check Sequence Error
Indicator
The LAPD Receiver will take the incoming LAPD
Message and compute its own version of the Frame
Check Sequence (FCS) word. Afterwards, the LAPD
Receiver will compare its computed value with that it
has received from the remote LAPD Transmitter. If
these two values match, then the LAPD Receiver will
RX DS3 LAPD STATUS REGISTER (ADDRESS = 0X19)
B
IT
7
B
IT
6
B
IT
5
B
IT
4
B
IT
3
B
IT
2
B
IT
1
B
IT
0
Not Used
RxAbort
RxLAPD Type[1, 0]
RxCR Type
RxFCS Error
End of
Message
Flag
Present
RO
RO
RO
RO
RO
RO
RO
RO
X
X
X
X
X
X
X
1
T
ABLE
56: T
HE
R
ELATIONSHIP
BETWEEN
R
X
LAPDT
YPE
[1:0]
AND
THE
RESULTING
LAPD M
ESSAGE
TYPE
AND
SIZE
R
X
LAPD T
YPE
[1, 0]
M
ESSAGE
T
YPE
M
ESSAGE
S
IZE
00
CL Path Identification
76 bytes
01
Idle Signal Identification
76 bytes
10
Test Signal Identification
76 bytes
11
TU-T Path Identification
82 bytes