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PRELIMINARY
INVERSE MULTIPLEXING OVER ATM
PM7340 S/UNI-IMA-8
DATA SHEET
PMC-2001723
ISSUE 3
INVERSE MULTIPLEXING OVER ATM
PROPRIETARY AND CONFIDENTIAL TO PMC-SIERRA, INC., AND FOR ITS CUSTOMERS’ INTERNAL USE
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which is cleared by the ICP cell processing procedure. Cells received while the
LCD latched status bit is set will be written to the DCB as filler cells, and the write
pointers will be incremented. After an LCD condition is exited, the delay
synchronization of the link must be rechecked and resynchronized. An LCD
defect will cause the IFSM state machine to go into the hunt state to ensure the
delay synchronization is rechecked. The transition of the IFSM into the hunt
state will also cause an OIF anomally.
10.2.4.3.3
DCB Overrun Status
When cells are written into the DCB, overruns will be checked by comparing the
group read pointer against the link write pointer. If the difference between the
pointers exceeds the maximum allowed DCB depth, then an overrun has been
detected. For IMA, this will cause the overrun latched status-in-link context to be
set.
An overrun condition will not cause the IFSM to exit the sync state.
All user cells will be dropped while the overrun condition persists. The overrun
condition is reset at the reception of an ICP cell with an acceptable delay as long
as the link is clear of LIF or OIF. For TC, an interrupt to the processor will be
generated and normal operation will resume once the overrun condition has
ended.
10.2.4.3.4
DCB Underrun Status
When cells are read from the DCB, underruns will be checked by comparing the
group read pointer against the link write pointer. When an underrun is detected,
all user cells will be dropped until the underrun condition is cleared. The underrun
condition will only be cleared at the reception of an ICP cell, such that the
differential delay may be re-checked. An underrun condition will not cause the
IFSM to exit the sync state.
10.2.4.3.5
Idle Cells on IMA Links
When Idle cells are detected on an IMA link, they will be reported. Idle cells on
IMA links may be present for two reasons. They may have been inserted at the
ATM layer of the transmitter as a rudimentary method for traffic management; in
which case the IMA layer should treat them as user cells. Otherwise, they may
have been inserted at the TC layer to assist with rate matching; this is illegal for