XR72L52 Datasheet, PDF (329/480 Pages) Exar Corporation – TWO CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER

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TWO CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER
XRT72L52
REV. 1.0.1
RXE3 LAPD STATUS REGISTER (ADDRESS = 0X19)
BIT 7
Not Used
BIT 6
RxABORT
RO
RO
0
0
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
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 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.
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 begin-
ning 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 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 Message, from the newly received LAPD Message frame. The
LAPD Receiver will then write this PMDL Message into the Receive LAPD Message buffer.
NOTE: 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., “0’s” that were inserted into the PMDL Mes-
sage 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 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.
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