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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.
Chapter 40. ATM Pace Control
APC Algorithm
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