XRT74L73 Datasheet, PDF (305/488 Pages) Exar Corporation – 3 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER

PRELIMINARY
XRT74L73
3 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER
REV. P1.0.1
FRAMER OPERATING MODE REGISTER (ADDRESS = 0X00)
BIT 7
R/W
0
BIT 6
R/W
0
BIT 5
R/W
1
BIT 4
R/W
0
BIT 3
R/W
1
BIT2
R/W
0
BIT 1
R/W
1
BIT 0
R/W
x
3. Interface the XRT74L73, to the Terminal Equip-
ment, as illustrated in Figure 117 .
5.2.2 The Transmit Overhead Data Input Inter-
face
Figure 119 presents a simple illustration of the Trans-
mit Overhead Data Input Interface block within the
XRT74L73.
FIGURE 119. THE TRANSMIT OVERHEAD DATA INPUT INTERFACE BLOCK
TxOHFrame
TxOHEnable
TxOH
TxOHClk
TxOHIns
Transmit
Overhead
Data Input
Interface Block
To Transmit DS3
Framer Block
The E3, ITU-T G.751 Frame consists of 1536 bits. Of
these bits, 1524 are payload bits and the remaining
12 are overhead bits. The XRT74L73 has been de-
signed to handle and process both the payload type
and overhead type bits for each E3 frame. Within the
Transmit Section within the XRT74L73, the Transmit
Payload Data Input Interface has been designed to
handle the payload data. Likewise, the Transmit
Overhead Input Interface has been designed to han-
dle and process the overhead bits.
The Transmit Section of the XRT74L73 generates or
processes the various overhead bits within the E3
frame, in the following manner.
The Frame Alignment Signaling (FAS) Overhead
Bits
The FAS (Framing Alignment Signaling) bits are al-
ways internally generated by the Transmit Section of
the XRT74L73. Hence, the user cannot insert his/her
value for the FAS bits into the outbound E3 data
stream, via the Transmit Overhead Data Input Inter-
face.
The “A” (Alarm) Overhead bit
The “A” bit is used to transport the FERF (Far-End
Receive Failure) condition. This bit-field can be ei-
ther internally generated by the Transmit Section
within the XRT74L73, or can be externally generated
and inserted into the outbound E3 data stream, via
the Transmit Overhead Data Input Interface. The Da-
ta Link Related Overhead Bits
The “N” (National) Overhead bit
The E3 frame structure also contains the N bit which
can be used to transport a proprietary User Data Link
information and or Path Maintenance Data Link infor-
mation. The UDL (User Data Link) bits are only ac-
cessible via the Transmit Overhead Data Input Inter-
face. The Path Maintenance Data Link (PMDL) bits
can either be sourced from the Transmit LAPD Con-
troller/Buffer or via the Transmit Overhead Data Input
Interface.
Table 63 lists the Overhead Bits within the E3 frame.
In addition, this table also indicates whether or not
these overhead bits can be sourced by the Transmit
Overhead Data Input Interface.
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