![](http://datasheet.mmic.net.cn/Exar-Corporation/XRT75R12DIB-L_datasheet_100141/XRT75R12DIB-L_123.png)
XRT75R12D
119
REV. 1.0.3
TWELVE CHANNEL E3/DS3/STS-1 LINE INTERFACE UNIT WITH SONET DESYNCHRONIZER
NOTES:
1.
The ability to disable the "SONET APS Recovery Time" mode is optional.
2.
The "SONET APS Recovery Time" mode will be discussed in greater detail in
“Section 8.8.3, How does the
LIU permit the user to comply with the SONET APS Recovery Time requirements of 50ms (per
Telcordia GR-253-CORE)?” on page 123.
8.8.2
Recommendations on Pre-Processing the Gapped Clocks (from the Mapper/ASIC Device)
prior to routing this DS3 Clock and Data-Signals to the Transmit Inputs of the LIU
In order to minimize the effects of "Clock-Gapping" Jitter within the DS3 signal that is ultimately transmitted to
the DS3 Line (or facility), we recommend that some "pre-processing" of the "Data-Signals" and "Clock-Signals"
(which are output from the Mapper device) be implemented prior to routing these signals to the "Transmit
Inputs" of the LIU.
8.8.2.1
SOME NOTES PRIOR TO STARTING THIS DISCUSSION:
Our simulation results indicate that Jitter Attenuator PLL (within the LIU LIU IC) will have no problem handling
and processing the "Data-Signal" and "Clock-Signal" from a Mapper IC/ASIC if no pre-processing has been
performed on these signals. In order words, our simulation results indicate that the Jitter Attenuator PLL
(within the LIU IC) will have no problem handling the "worst-case" of 59 consecutive bits of no clock pulses in
the "Clock-Signal (due to the Mapper IC processing the TOH bytes, an Incrementing Pointer-Adjustment-
induced "stuffed-byte", the POH byte, and the two fixed-stuff bytes within the STS-1 SPE, etc), immediately
followed be processing clusters of DS3 data-bits (as shown in Figure 44) and still comply with the "Category I
Intrinsic Jitter Requirements per Telcordia GR-253-CORE for DS3 applications.
NOTE: If this sort of "pre-processing" is already supported by the Mapper device that you are using, then no further action
is required by the user.
8.8.2.2
OUR PRE-PROCESSING RECOMMENDATIONS
For the time-being, we recommend that the customer implement the "pre-processing" of the DS3 "Data-Signal"
and "Clock-Signal" as described below. Currently we are aware that some of the Mapper products on the
Market do implement this exact "pre-processing" algorithm. However, if the customer is implementing their
Mapper Design in an ASIC or FPGA solution, then we strongly recommend that the user implement the
necessary logic design to realize the following recommendations.
Some time ago, we spent some time, studying (and then later testing our solution with) the PM5342 OC-3 to
DS3 Mapper IC from PMC-Sierra. In particular, we wanted to understand the type of "DS3 Clock" and "Data"
signal that this DS3 to OC-3 Mapper IC outputs.
TABLE 49: JITTER ATTENUATOR CONTROL REGISTER - CHANNEL N, ADDRESS LOCATION; 0XM7
(M = 0-5 & 8-D) (N = [0:11])
BIT 7
BIT 6
BIT 5
BIT 4
BIT 3
BIT 2
BIT 1
BIT 0
Unused
SONET APS
Recovery
Time
DisableCh_n
JA RESET
Ch_n
JA1 Ch_n
JA in Tx Path
Ch_n
JA0 Ch_n
R/O
R/W
0
1