Freescale Semiconductor MPC8260 - Manual (Page 930)

ATM Controller and AAL0, AAL1, and AAL5
MPC8260 PowerQUICC II Family Reference Manual, Rev. 2
30-10 Freescale Semiconductor
Each 2-byte time-slot entry points to one ATM channel. Additional channels scheduled to transmit in the
same slot are linked to each other using the APC linked-channel field in the TCT. The linked list is not
limited; however, if the number of channels for the current slot exceeds the cells per slot parameter (CPS),
the extra channels are sent in subsequent time slots. (The rescheduling of extra channels is based on the
original slot to maintain each channel’s pace.)
Note that a channel can appear only once in the scheduling table at a given time, because each channel has
only one APC linked-channel field.
30.3.3 Determining the Scheduling Table Size
The following sections describe how to determine the number of cells sent per time slot and the total
number of slots needed in a scheduling table.
30.3.3.1 Determining the Cells Per Slot (CPS) in a Scheduling Table
The number of cells sent per time slot is determined by the channel with the maximum bit rate; see
equation A. The maximum bit rate is achieved when a channel is rescheduled to the next slot. For example,
if the line rate is 155.52 Mbps and there are eight cells per slot, equation A yields a maximum VC rate of
19.44 Mbps.
Note that a channel can appear only once per time slot; thus, 19.44 Mbps = 155.52Mbps/8.
The cells per slot parameter (CPS) affects the cell delay variation (CDV). Because the APC unit does not
put cells in a definite order within each time slot (LIFO—last-in/first-out implementation), as CPS
increases, the CDV increases. However as CPS decreases, the size of the scheduling table in the dual-port
RAM increases and more CPM bandwidth is required.
30.3.3.2 Determining the Number of Slots in a Scheduling Table
The number of time slots in a scheduling table is determined by the channel with the minimum bit rate;
see equation B. The minimum bit rate is achieved when the channel reschedules only once in a whole table
scan. (The maximum schedule advance allowed is equal to number_of_slots-1.) For example, if the line
rate is 155.52 Mbps, the minimum bit rate is 32 kbps and the CPS is 4, then, according to equation B, the
number of slots should be 1,216.
NOTE
The following APC configuration is not recommended for values outside
the following ranges (PCR = peak cell rate, PCRF = peak cell rate fraction,
NOS = number of slots):
PCR = 1 and PCRF = 0
PCR = NOS - 1 and PCRF = 0
Max bit rate =
(A)
line rate
cells per slot