XRT72L50
á
SINGLE CHANNEL DS3/E3 FRAMER WITH HDLC CONTROLLER
REV. 1.2.1
419
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.
The LAPD Receiver will indicate the results of this FCS Verification process by setting Bit 2 (RxFCS Error)
within the Rx E3 LAPD Status Register, to the appropriate value as tabulated below.
If the LAPD Receiver detects an error in the FCS value, then it will set the RxFCS Error bit-field to “1”.
Conversely, if the LAPD Receiver does not detect an error in the FCS value, the it will clear the RxFCS Error
bit-field to “0”.
NOTE: The LAPD Receiver will extract and write the PMDL Message into the Receive LAPD Message buffer independent
of the results of FCS Verification. Hence, the user is urged to validate each PMDL Message that is read in from the
Receive LAPD Message buffer, by first checking the state of this bit-field.
3. Check and Report the State of the C/R Bit-field
After receiving the LAPD Message frame, the LAPD Receiver will check the state of the C/R bit-field, within
octet # 2 of the LAPD Message frame header and will reflect this value in Bit 3 (Rx CR Type) within the Rx E3
LAPD Status Register, as depicted below.
When this bit-field is “0”, it means that this LAPD Message frame is originating from a customer installation.
When this bit-field is “1”, it means that this LAPD Message frame is originating from a network terminal.
4. Identify the Type of LAPD Message Frame/PMDL Message
Next, the LAPD Receiver will check the value of the first octet within the PMDL Information Payload field, of the
LAPD Message frame. Recall that from
Section 6.2.3.1, that when operating the LAPD Transmitter, the user is
required to write in a byte of a specific value at address 0x8A within the Transmit LAPD Message buffer. The
value of this byte corresponds to the type of LAPD Message frame/PMDL Message that is to be transmitted to
the Remote LAPD Receiver.
This Message-Type Identification octet is transported to the Remote LAPD
Receiver, along with the rest of the LAPD frame. From this Message Type Identification octet, the LAPD
RxE3 LAPD Status Register (Address = 0x19)
BIT 7BIT 6BIT 5BIT 4BIT 3
BIT 2BIT 1BIT 0
Not Used
Rx ABORT
RxLAPDType[1:0]
RxCR
Type
RxFCS
Error
End of
Message
Flag
Present
RO
00
000
100
RxE3 LAPD Status Register (Address = 0x19)
BIT 7BIT 6BIT 5BIT 4
BIT 3BIT 2BIT 1BIT 0
Not Used
Rx ABORT
RxLAPDType[1:0]
RxCR
Type
RxFCS
Error
End of
Message
Flag
Present
RO
00
1
000