XRT74L73 Datasheet, PDF (456/488 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
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.
RXE3 LAPD STATUS REGISTER (ADDRESS = 0X19)
BIT 7
Not Used
BIT 6
Rx ABORT
BIT 5
BIT 4
RxLAPDType[1:0]
BIT 3
RxCR
Type
BIT 2
RxFCS
Error
BIT 1
End of
Message
BIT 0
Flag
Present
RO
RO
RO
RO
RO
RO
RO
RO
0
0
0
0
1
0
0
0
When this bit-field is “0”, it means that this LAPD
Message frame is originating from a customer instal-
lation. 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 Message field, of the
LAPD Message frame. Recall that from Section _,
that when operating the LAPD Transmitter, the user is
required to write in a byte of a specific value into the
first octet position 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 Mes-
sage-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 Receiver will know the type of size of
the newly received PMDL Message. The LAPD Re-
ceiver will then reflect this information in Bits 4 and 5
(RxLAPDType[1:0]) within the Rx E3 LAPD Status
Register, as depicted below.
RXE3 LAPD STATUS REGISTER (ADDRESS = 0X19)
BIT 7
Not Used
BIT 6
Rx ABORT
BIT 5
BIT 4
RxLAPDType[1:0]
BIT 3
RxCR
Type
BIT 2
RxFCS
Error
BIT 1
End of
Message
BIT 0
Flag
Present
RO
RO
RO
RO
RO
RO
RO
RO
0
0
0
0
0
0
0
0
Table 102 presents the relationship between the
contents of RxLAPDType[1:0] and the type of mes-
sage received by the LAPD Receiver.
TABLE 102: THE RELATIONSHIP BETWEEN THE CONTENTS OF RXLAPDTYPE[1:0] BIT-FIELDS AND THE PMDL
MESSAGE TYPE/SIZE
RXLAPDTYPE[1:0]
PMDL MESSAGE TYPE
PMDL MESSAGE SIZE
00
Test Signal Identification
76 Bytes
01
Idle Signal Identification
76 Bytes
10
CL Path Identification
76 Bytes
11
ITU-T Path Identification
82 Bytes
NOTE: Prior to reading in the PMDL Message from the
Receive LAPD Message buffer, the user is urged to read
the state of the RxLAPDType[1:0] bit-fields in order to
determine the size of this message.
5. Inform the Local Microprocessor/External Cir-
cuitry of the receipt of the new LAPD Message
frame.
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