S/UNI-IMA-4 Telecom Standard Product Data Sheet
Released
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.
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. It should be noted here that if the
device remains in the LIF state for a long time (1 hour for E1, longer for T1), DCB underrun
may be detected by the device. This occurs because stuff events cannot be detected during LIF,
and the internal pointers may wrap-around. DCB overrun will not occur in this situation, as it is
automatically masked during LIF.
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 and the IMA_IDLE_FWD_EN configuration bit should be set to allow these cells to
be forwarded. Otherwise, they may have been inserted at the TC layer to assist with rate
matching; this is illegal for IMA links. Idle cells will be treated as user cells by the RDAT for
IMA processing, although they will not be forwarded to the ATM layer if the
IMA_IDLE_FWD_EN is not set.
DCB Playout
The IDCC scheduler provides the rate for data to be played out to the ATM layer for an IMA
group. For each cell to be played out, the IDCC generates a service request. Upon the IDCC
service request, the RDAT plays out data from the FIFOs in a round-robin fashion. For each
service request, the RDAT runs the round robin servicing until it processes either a filler cell or
user cell. If ICP cells are encountered, the ICP cell is dropped and the servicing continues until a
user or filler cell is found. If a user cell is found, it is transferred from the external memory to
the appropriate group FIFO. If a filler cell is found, it is dropped.
Proprietary and Confidential to PMC-Sierra, Inc., and for its customers’ internal use.
Document No.: PMC-2020889, Issue 2
67
PMC [ PMC-SIERRA, INC ]
