S/UNI-IMA-4 Telecom Standard Product Data Sheet
Released
While in the Presync state, at each expected ICP location (determined by the ICP offset and the
IMA Frame Length), the cell will be examined (frame by frame). Once gamma (ꢁꢀꢂꢀvalid ICP
cells have been received, the IFSM will enter the IMA Sync state. If either: (1) an invalid (or
errored) ICP cell is received or (2) a valid ICP cell is received in an unexpected location, the
IFSM will re-enter the IMA Hunt state. While in the IMA Hunt state, the stuff indicators will be
ignored.
While in the IMA Sync state, ICP cells are continually examined for each frame. If beta
(ꢃꢀꢂꢀconsecutive ICP cells with HEC or CRC-10 errors (errored ICP cells) are received, then the
IFSM will reenter the IMA Hunt state. Also, if alpha (ꢄꢀꢂꢀconsecutive invalid ICP cells are
received, the IFSM will reenter the IMA Hunt state. If a cell is received at the expected ICP
position without an HEC error or OCD and without the IMA OAM cell header, or is a filler cell,
it is considered a missing ICP cell, and the IFSM will reenter the IMA Hunt state immediately.
Finally, if a valid ICP cell is received at an unexpected position, or OCD is detected, the IFSM
will re-enter the IMA Hunt state.
Alpha, Beta, and Gamma are globally programmable for the device. The RDAT keeps working-
counts for these parameters for each link. It should be noted that alpha (the count of consecutive
invalid ICP cells) will not be reset upon receipt of an errored cell; although beta (the count of
consecutive errored ICP cells) will be reset upon receipt of an invalid ICP cell.
Stuff Events
At this point, the RDAT detects and removes the stuff cells. Stuff cells are identified by the LSI
field with the ICP cells. Stuff events consist of two back-to-back ICP cells on the same link.
One of the ICP cells is considered a stuff cell. Since stuff cells are inserted for the purpose of
equalizing the data rate on links with independent clocks, stuff cells are removed.
To improve robustness in the presence of errors, the transmitter is required to advertise that a
stuff event is going to occur in the ICP cell in the frame preceding the stuff event. The
transmitter may also advertise the stuff event for the 4 frames preceding the stuff event.
Once a valid non-errored ICP cell has been received with a LSI of 001, 010, 011, or 100, the
RDAT will maintain an internal stuff count in link-context memory. This count will be
decremented every frame, until the stuff event occurs. The count will be decremented even if an
incoming ICP cell is errored or invalid (as shown in Figure 13). An ICP cell received with an
invalid stuff sequence (i.e., LSI of 001, when a LSI of 010 was expected) will be declared
invalid, and the internal stuff count will be decremented from the previous value (as shown in
Figure 14. The internal count is reset to the maximum when the stuff event occurs. A stuff
sequence of 111 followed by 000 is not considered an invalid stuff sequence (i.e., the RDAT
will always accept immediate notification of a stuff event, to support the case when the 001
stuff cell was errored).
Proprietary and Confidential to PMC-Sierra, Inc., and for its customers’ internal use.
Document No.: PMC-2020889, Issue 2
59
PMC [ PMC-SIERRA, INC ]
