
PRELIMINARY
INVERSE MULTIPLEXING OVER ATM
PM7341 S/UNI-IMA-84
DATASHEET
PMC-2000223
ISSUE 4
INVERSE MULTIPLEXING OVER ATM
PROPRIETARY AND CONFIDENTIAL TO PMC-SIERRA, INC., AND FOR ITS CUSTOMERS’ INTERNAL USE
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Figure 24
- Cell delineation State Diagram
The values of ALPHA and DELTA determine the robustness of the cell delineation
method. ALPHA determines the robustness against false misalignments due to
bit errors. DELTA determines the robustness against false delineation in the
synchronization process. ALPHA is chosen to be 7 and DELTA is chosen to be 6.
The loss of cell delineation (LCD) alarm is declared after a programmable
threshold of incorrect cells occurs while in the HUNT 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.