S/UNI-IMA-4 Telecom Standard Product Data Sheet
Released
IMA Service
For each IMA group-service request, a cell is transferred from the group FIFO to one of the link
FIFOs. If no cell is available from the group FIFO, an IMA filler cell is generated and placed in
the link FIFO. The link FIFOs within a group are serviced in a round-robin fashion, with the
round-robin order determined by the LID. If the next link in the round robin is due to receive an
ICP cell, the ICP cell is generated using the link and group state information from the Transmit
IMA Group Context Record, and the LID and LSI from the link. If a stuff event is scheduled,
the stuff ICP cell is also inserted. Whenever an ICP cell is inserted, the IMA group servicing
proceeds to the next link in the round robin without waiting for another service request. The
IMA group service is complete when either: (1) a cell is transferred from the group FIFO or (2)
an ATM filler cell is generated. When links are in the process of being added, but are not yet
available for carrying data traffic, IMA frames consisting of filler cells and ICP cells are
generated. Such links are not scheduled by the Tx IDCC scheduler, but will be processed with
the currently active links.
During group start-up, even with all of the transmit links in the unusable state, the Tx IDCC
scheduler is started and IMA frames are generated. During group start-up (i.e. links are not yet
in the active state), a group can be configured such that cells received via the UTOPIA L2 /
Any-PHY bus can be dropped to avoid the accumulation of stale data or to drop stale data in the
group FIFO left over from a previous use of the VPHY ID. During link additions, IMA frames
are generated on new links when they are added to the group.
TC Only Service
For TC-only mode groups, servicing is also initiated by group service requests from the Tx
IDCC. However, servicing a group FIFO simply entails transferring a cell from the group FIFO
to the proper link FIFO. If a cell is not present in the group FIFO, no cells are transferred and
the servicing of the request is complete. In TC mode, no other cells are inserted into the data
stream by the IMA sub-layer (physical layer idle cells are generated by the physical layer).
Timing Reference Link Maintenance
It is possible to have the timing reference link for an IMA group change from one link to
another while the IMA connection is in operation. If an IMA group is operating in CTC mode,
the reference link used for the scheduling is simply switched. The next stuff cell insertion still
occurs 2049 cells after the previous stuff. If the IMA group is operating in ITC mode and the
reference link is switched, the first stuff insertion on the new TRL occurs at approximately the
same frame a stuff would have been inserted had it not become the TRL. At the time of the TRL
change, the existing accrued rate differential on the new TRL is used to prorate the number of
cells out of 2048 until the next TRL stuff. Although the first stuff will occur at approximately
the proper number of cells to maintain the correct differential delay, the actual time of the stuff
will be dependent on the new TRL rate.
Proprietary and Confidential to PMC-Sierra, Inc., and for its customers’ internal use.
Document No.: PMC-2020889, Issue 2
56
PMC [ PMC-SIERRA, INC ]
