XRT74L73 Datasheet, PDF (400/488 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
Figure 176 presents a simple illustration of the
Transmit Overhead Data Input Interface block within
the XRT74L73.
FIGURE 176. THE TRANSMIT OVERHEAD DATA INPUT INTERFACE BLOCK
TxOHFrame
PRELIMINARY
TxOHEnable
TxOH
TxOHClk
Transmit
Overhead
Data Input
Interface Block
To Transmit E3
Framer Block
TxOHIns
The E3, ITU-T G.832 Frame consists of 537 bytes.
Of these bytes, 530 bytes are payload bytes and the
remaining 7 are overhead bytes. The XRT74L73 has
been designed to handle and process both the pay-
load type and overhead type bits for each E3 frame.
Within the Transmit Section within the XRT74L73, the
Transmit Payload Data Input Interface has been de-
signed to handle the payload data. Likewise, the
Transmit Overhead Input Interface has been de-
signed to handle 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 Overhead Bytes (e.g., the
“FA1” and “FA2” bytes)
The "FA1" and "FA2" bytes are always internally gen-
erated by the Transmit Section of the XRT74L73.
Hence, the user cannot insert his/her value for the
"FA1" and "FA2" bytes into the Outbound DS3 data
stream, via the Transmit Overhead Data Input Inter-
face.
The Error Monitoring (EM) Overhead Byte
The EM byte is always internally generated by the
Transmit Section of the XRT74L73. Hence, the user
cannot insert his/her value for the EM byte into the
Outbound E3 data stream, via the Transmit Overhead
Data Input Interface.
The Alarm and signaling related Overhead bytes
Bytes that are used to transport the alarm conditions
can be either 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 Inter-
face. The E3 frame overhead bits that fall into this
category are:
• The "MA” byte
• The "TR” byte
The Data Link Related Overhead Bits
The E3 frame structure also contains bits which can
be used to transport User Data Link information and
Path Maintenance Data Link information. The UDL
(User Data Link) bits are only accessible via the
Transmit Overhead Data Input Interface. The Path
Maintenance Data Link (PMDL) bits can either be
sourced from the Transmit LAPD Controller/Buffer or
via the Transmit Overhead Data Input Interface.
Table 87 lists the Overhead Bits within the DS3
frame. Additionally, this table also indicates whether
or not these overhead bits can be sourced by the
Transmit Overhead Data Input Interface or not.
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