XRT72L13 Datasheet, PDF (263/370 Pages) Exar Corporation – M13 MULTIPLEXER/CLEAR CHANNEL DS3 FRAMER IC

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PRELIMINARY
XRT72L13
M13 MULTIPLEXER/CLEAR CHANNEL DS3 FRAMER IC
REV. P1.0.6
FIGURE 75. SIMPLE ILLUSTRATION OF THE "TRANSMIT OVERHEAD DATA INPUT INTERFACE" BLOCK
TxOHFrame
TxOHEnable
TxOH
TxOHClk
TxOHIns
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To Transmit DS3 Framer Block
The DS3 Frame consists of 4760 bits. Of these bits,
4704 bits are "payload" bits and the remaining 56 bits
are "overhead" bits. The XRT72L13 has been de-
signed to handle and process both the "payload" type
and "overhead" type bits for each DS3 frame. Within
the "Transmit Section" within the XRT72L13, the
"Transmit Payload Data Input Interface" has been de-
signed to handle the "payload" data. Likewise, the
"Transmit Overhead Data Input Interface" has been
designed to handle and process the overhead bits.
The Transmit Section of the XRT72L13 generates or
processes the various overhead bits within the DS3
frame, in the following manner.
The Frame Synchronization Overhead Bits (e.g.,
the "F" and "M" bits)
The "F" and "M" bits are always internally generated
by the "Transmit Section" of the XRT72L13. These
overhead bits are used (by the Remote Terminal
Equipment) for Frame Synchronization purposes.
Hence, the user cannot insert his/her value for the "F"
and "M" bits into the "outbound" DS3 data stream, via
the "Transmit Overhead Data Input Interface". Any at-
tempt to externally insert values for the “F” and “M”
bits, will be ignored by the “Transmit Overhead Data
Input Interface” block.
The "Performance Monitoring" Overhead Bits (P
and CP Bits)
The P-bits are always internally generated by the
Transmit Section of the XRT72L13. The “P” bits are
used by the “Remote Terminal Equipment” to perform
error-checking/detection of a DS3 data stream, as it
is transmitted from one Terminal Equipment to adja-
cent Terminal Equipment (e.g., point-to-point check-
ing). Hence, the user cannot insert his/her value for
the "P-bits" into the "outbound" DS3 data stream, via
the "Transmit Overhead Data Input Interface".
In contrast to “P” bits, “CP” bits are used perform er-
ror-checking/detection of a DS3 data stream from the
“Source” Terminal Equipment to the “Sink” Terminal
Equipment. In applications where a given DS3 data
stream is received via one port, and is output via an-
other port; it is necessary that the “CP” bit-values re-
main constant. The only way to insure this to (1) ex-
tract out the “CP” bit values, via the “Receiving Line
Card” and (2) insert these CP-bit values into the “out-
bound” DS3 data stream, via the “Transmit Overhead
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