á
XRT72L50
SINGLE CHANNEL DS3/E3 FRAMER WITH HDLC CONTROLLER
REV. 1.2.1
418
sequence byte, it will assert the Flag Present bit (Bit 0) within the Rx E3 LAPD Status Register, as depicted
below.
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 transmitted
via the E3 Transport Medium, during idle periods between the transmission of LAPD Message 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 happens, 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 assert the RxABORT bit-field (Bit 6)
within the Rx E3 LAPD Status Register, as depicted below.
However, if this next octet is Octet #2 of an incoming LAPD Message frame, then the LAPD Receiver is
beginning to receive a LAPD Message frame.
As the LAPD Receiver receives this LAPD Message frame, it is reading in the LAPD Message frame octets,
from either the GC or the NR byte-fields within each incoming E3 frame. Secondly, it is reassembling 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 Message, from the newly received LAPD Message frame. The
LAPD Receiver will then write this PMDL Message into the Receive LAPD Message buffer.
N
OTE
:
As the LAPD Receiver is extracting the PMDL Message, 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 6.2.3.1
(step
7).
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
RxE3 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
Rx ABORT
RxLAPDType[1:0]
RxCR
Type
RxFCS
Error
End of
Message
Flag
Present
RO
RO
RO
RO
RO
RO
RO
RO
0
0
0
0
0
0
0
1
RxE3 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
Rx ABORT
RxLAPDType[1:0]
RxCR
Type
RxFCS
Error
End of
Message
Flag
Present
RO
RO
RO
RO
RO
RO
RO
RO
0
1
0
0
0
0
0
0