ATM Pace Control
MPC885 PowerQUICC Family Reference Manual, Rev. 2
40-4
Freescale Semiconductor
The entries of an APC scheduling table are actually the heads of linked lists. That is, if more than one
channel is scheduled to the same time slot, the first channel points to the next channel using the APC link
field (APCL) in its TCT, and so on.
Having scheduled the channels at the current table entry (APCT_PTRx), the APC then inserts up to NCITS
channel numbers into the transmit queue using the service pointer (APCT_SPTRx). If more than NCITS
channel numbers are scheduled for the same time slot, the leftover channel numbers remain pending until
the next time the APC is activated. The lagging APCT_SPTRx service pointer keeps track of the pending
channels so that cells are only deferred, not dropped.
If the highest priority APC level cannot provide NCITS cells, the APC begins traversing the lower priority
APC levels hunting for additional channels (advancing each APC level’s service pointer in turn) until a
total of NCITS cells are found or until no APC levels remain. However, because traversing the APC levels
could potentially cause too much delay, the APC_MI (maximum iteration) parameter is provided to limit
the total number of times the APC advances a service pointer.
If a channel scheduled for transmission has no buffer descriptors ready, nothing is inserted into the transmit
queue. This results in one or more idle cells in the cell stream.
40.1.2
APC Parameters
The APC mechanism can be programmed to provide a wide variety of transmit rates and support a large
number of channels. There are several important parameters which define its capabilities. These include:
•
Cell scheduling rate. This is determined by the timeout rate of the APC timer (CPM timer 4), which
defines the period of the scheduling time slots, and the number of cells transmitted in a time slot
(NCITS). The maximum of this rate is the bit rate of the physical medium. However, it could also
be any amount less than that, if the user desires to use only a certain percentage of the bandwidth.
•
Maximum and minimum bit rates supported for particular channels. A channel number can appear
only once in a time slot, but must appear at least once during each table scan. The maximum bit
rate for a particular channel is achieved when it is scheduled for transmission in every APC
scheduling table entry (i.e. APC Pace = 1). The minimum bit rate for a particular channel is
achieved when it is scheduled for transmission only once in a table scan (i.e. APC Pace =
APC_table_size - 1). These constraints define the upper and lower bounds at which the transmit
rate of a particular channel can be scheduled.
•
APC scheduling table size and CPM performance. If the NCITS parameter is increased, the APC
scheduling table size will decrease (thereby conserving dual-port RAM space) and the APC timer
will make fewer requests to the CPM (thereby decreasing CPM processing overhead). However,
increasing NCITS decreases the maximum bit rate supported per channel and increases cell delay
variation.
Trade-off decisions must be made when programming these parameters. The following subsections
provide examples of the analysis required to make these determinations.
Summary of Contents for PowerQUICC MPC870
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