PM7340 S/UNI-IMA-8
PRELIMINARY
INVERSE MULTIPLEXING OVER ATM
DATA SHEET
PMC-2001723
ISSUE 3
INVERSE MULTIPLEXING OVER ATM
Once gamma (γ=)=valid 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 (β=)=consecutive ICP cells with HEC, OCD, or CRC-10 errors (errored ICP
cells) are received, then the IFSM will reenter the IMA Hunt state. Also, if alpha
(α=)=consecutive 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, it is 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, 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.
10.2.4.1.2
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 11). 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 12. 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.,
PROPRIETARY AND CONFIDENTIAL TO PMC-SIERRA, INC., AND FOR ITS CUSTOMERS’ INTERNAL USE
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PMC [ PMC-SIERRA, INC ]
