S/UNI-IMA-4 Telecom Standard Product Data Sheet
Released
10.2.2 IDCC scheduler
The IMA Data Cell Clock (IDCC) scheduler calculates the IMA Data Cell Rate (IDCR) for each
group that is used by both the Receive and the Transmit IMA processors. There is one scheduler
for each direction (TXIDCC and RXIDCC), and each scheduler can monitor the rate of up to 4
reference clocks; each scheduler can also generate up to 4 IDCC clocks based upon IDCR. For
each group, the reference link can be selected to be one of the 4 monitored links. Each of the
monitored links can only be the reference link for one group. IDCR is calculated using the
following equation, with Non and M set independently for each IDCR generator. Non is the
number of active links, M is the size of the IMA frame, and TRL Cell Rate (TRLCR) is the cell
rate of the reference link.
IDCR = Non X TRLCR X (M-1/M) X (2048/2049)
TRLCR is generated from the byte rate. The byte rate is obtained by monitoring the data
transfers on the internal bus in the TC layer.
For each IDCR clock tick, a service request is generated and placed into a rate based FIFO.
Since there may be many requests generated in a short amount of time and the rate at which
each request is generated may be different, a method is required to arbitrate between the
requests to prevent blocking of high rate requests by large numbers of low rate requests. To
achieve this, each request is placed into a priority FIFO. The priority of the request is based
upon its rate. There are a total of 5 rate-based FIFOs. When a service request is accepted by the
Transmit IMA processor (TIMA) or the Receive IMA Data Processor (RDAT), the next request
to be presented is taken from the highest priority FIFO that has an entry. In this manner, the
higher rate requests get higher priority than the lower rate requests. Since the S/UNI-IMA-4 can
always service all of the requests, this algorithm limits the CDV experienced by any service
request to approximately one inter-arrival time of the service request for each group.
Rate changes are restricted to IMA frame boundaries. An IMA frame boundary occurs once
every (M-1)*N service requests. When a request is received to change the rate(Non), the request
is saved until the next IMA frame boundary, at which point it takes effect. By restricting rate
changes to frame boundaries, the rate accuracy is preserved preventing FIFO
underflows/overflows. Since rate changes are not instantaneous, a vector that represents the
active Link IDs (LIDs) in the group is passed with the service request. In this manner, the entity
receiving the service requests is informed of the change in rate and of which links should
currently be in the round robin for servicing.
All IMA-based rate changes are internally managed by the S/UNI-IMA-4; no user interaction is
necessary for correct scheduling.
The IDCC is also used for scheduling the TC data flow. In this case, the rate generated is simply
the cell rate of the TC link and is not modified for IMA ICP cells or stuff cells according to the
following equation:
IDCR = TRLCR
For all TC connections, the IDCC must be configured in TC mode for the physical link.
Proprietary and Confidential to PMC-Sierra, Inc., and for its customers’ internal use.
Document No.: PMC-2020889, Issue 2
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PMC [ PMC-SIERRA, INC ]
