XRT74L73 Datasheet, PDF (200/488 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
nally insert values for the “F” and “M” bits, will be ig-
nored by the Transmit Overhead Data Input Inter-
face"High" block.
The Performance Monitoring Overhead Bits (P
and CP Bits)
The P-bits are always internally generated by the
Transmit Section of the XRT74L73. 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, user values cannot be inserted 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
Overhead Data Input Interface block will permit the
user to externally insert the “CP” bits into the out-
bound 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 XRT74L73, 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 35 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 35: A LISTING OF THE OVERHEAD BITS WITHIN THE DS3 FRAME, AND THEIR POTENTIAL SOURCES, WITHIN THE
XRT74L73 IC
OVERHEAD BIT
INTERNALLY GENERATED
ACCESSIBLE VIA THE TRANSMIT OVERHEAD
DATA INPUT INTERFACE
BUFFER/REGISTER
ACCESSIBLE
P
Yes
No
Yes*
X
Yes
Yes
Yes
F
Yes
No
Yes*
M
Yes
No
Yes*
FEAC
No
Yes
Yes
FEBE
Yes
Yes
Yes
DL
No
Yes
Yes+
UDL
No
Yes
No
CP
No
Yes
No
NOTES:
* The XRT74L73 contains mask register bits that per-
mit the altering tof he state of the internally generated
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
permits the insertion of overhead data into the out-
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