MPC8280 datasheet(970/1386 Pages) FREESCALE

970 / 1386 page

ATM Controller and AAL0, AAL1, and AAL5

MPC8280 PowerQUICC II Family Reference Manual, Rev. 1

31-10

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.

31.3.3

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.

31.3.3.1

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.

31.3.3.2

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

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