XRT72L56 Datasheet, PDF (189/486 Pages) Exar Corporation – SIX CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER

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SIX CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER
PRELIMINARY
XRT72L56
REV. P1.1.2
The Performance Monitoring Overhead Bits (P
and CP Bits)
The P-bits are always internally generated by the
Transmit Section of the XRT72L56. 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
Data Input Interface block. Hence, the Transmit Over-
head Data Input Interface block will permit the user to
externally insert the “CP” bits into the outbound DS3
data stream.
The Alarm and signaling related Overhead bits
Bits that are used to transport the alarm conditions
can be either internally generated by the Transmit
Section within the XRT72L56, or can be externally
generated and inserted into the outbound DS3 data
stream, via the Transmit Overhead Data Input Inter-
face. The DS3 frame overhead bits that fall into this
category are:
• The X bits
• The FEAC bits
• The FEBE bits.
The Data Link Related Overhead Bits
The DS3 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 20 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.
TABLE 20: A LISTING OF THE OVERHEAD BITS WITHIN THE DS3 FRAME, AND THEIR POTENTIAL SOURCES, WITHIN THE
XRT72L56 IC
OVERHEAD BIT
P
X
F
M
FEAC
FEBE
DL
UDL
CP
INTERNALLY GENERATED
Yes
Yes
Yes
Yes
No
Yes
No
No
No
ACCESSIBLE VIA THE TRANSMIT OVERHEAD
DATA INPUT INTERFACE
No
Yes
No
No
Yes
Yes
Yes
Yes
Yes
BUFFER/REGISTER
ACCESSIBLE
Yes*
Yes
Yes*
Yes*
Yes
Yes
Yes+
No
No
NOTES:
* The XRT72L56 contains mask register bits that per-
mit the user to alter the state of the internally generat-
ed value for these bits.
+ The Transmit LAPD Controller/Buffer can be config-
ured to be the source of the DL bits, within the out-
bound DS3 data stream.
In all, the Transmit Overhead Data Input Interface per-
mits the user to insert overhead data into the out-
bound DS3 frames via the following two different
methods.
• Method 1 - Using the TxOHClk clock signal
• Method 2 - Using the TxInClk and the TxOHEnable
signals.
Each of these methods are described below.
4.2.2.1 4.2.2.1 Method 1 - Using the TxOHClk
Clock Signal
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