PM7340 S/UNI-IMA-8
PRELIMINARY
INVERSE MULTIPLEXING OVER ATM
DATA SHEET
PMC-2001723
ISSUE 3
INVERSE MULTIPLEXING OVER ATM
state. If an incorrect HCS is not found in this PRESYNC period, then a transition
to the SYNC state is made, cell delineation is declared, and all non-idle cells with
a correct HCS are passed on. In the SYNC state, synchronization is not
relinquished until ALPHA consecutive incorrect HCS patterns are found. If this
happens, a transition is made back to the HUNT state. The state diagram of the
cell delineation process is shown in Figure 23.
Figure 23
- Cell delineation State Diagram
correct HCS
(bit by bit)
HUNT
Incorrect HCS
(cell by cell)
PRESYNC
ALPHA
consecutive
incorrect HCS's
(cell by cell)
DELTA
SYNC
consecutive
correct HCS's
(cell by cell)
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 droppped. These cells can
also be preserved and tagged as errored. Preservation of HEC errored cells is
PROPRIETARY AND CONFIDENTIAL TO PMC-SIERRA, INC., AND FOR ITS CUSTOMERS’ INTERNAL USE
86
PMC [ PMC-SIERRA, INC ]
