S/UNI-IMA-4 Telecom Standard Product Data Sheet
Released
The loss of cell delineation (LCD) alarm is declared after a programmable threshold of incorrect
cells occurs while not in the SYNC state. The threshold is set by the LCD Count Threshold
register. The threshold has a default value of 104, which translates to 28 ms at 1.55 Mbps. All
idle cells may be filtered out and not passed to the IMA sub-layer. They are identified as cells
containing all-zero VPI and VCI fields and a one in the CLP bit. Optionally, unassigned cells
(like idle cells except CLP is a zero) may also be filtered. Note that these should not be filtered
for IMA links.
All cells with an incorrect HCS octet may optionally be dropped. These cells can also be
preserved and tagged as errored. Preservation of HEC errored cells is required for correct
operation of IMA. Failure to preserve the cells will lead to increased numbers of OIF violations
and data misordering. Header correction is not implemented.
For IMA operation, there are two unique features: the first is optionally passing cells with
errored HEC; the second results in passing cells during OCD and LCD. This is to enable the
IMA to perform a fast recovery from error conditions. An OCD event will result in a loss of
IMA frame sync to ensure differential delay checking is performed.
Proprietary and Confidential to PMC-Sierra, Inc., and for its customers’ internal use.
Document No.: PMC-2020889, Issue 2
83
PMC [ PMC-SIERRA, INC ]
