Motorola MPC8260 - Manual (Page 791)

29-11 MPC8260 PowerQUICC II UserÕs Manual MOTOROLA
Part IV. Communications Processor Module
29.3.3.2 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.
For the above example, 32 kbps = 155.52 Mbps/((1216-1)
´ 4).
Use equations (A) and (B) to obtain the maximum and minimum bit rates of a scheduling
table. For example, given a line rate = 155.52 Mbps, number_of_slots = 1025, and CPS = 8:
29.3.4 Determining the Time-Slot Scheduling Rate of a Channel
The time-slot scheduling rate of each ATM channel is deÞned by equation C. The resulting
number of APC slots is written in either TCT[PCR], TCTE[SCR] or TCTE[MCR],
depending on the trafÞc type.
29.3.5 ATM TrafÞc Type
The APC uses the cell rate pacing parameters (PCR, SCR, and MCR) to generate CBR,
VBR, ABR, UBR+, and UBR trafÞc. The user determines the kind of trafÞc that is
generated per VC by writing to TCT[ATT] (ATM trafÞc type); see Section 29.10.2.3,
ÒTransmit Connection Table (TCT).Ó
29.3.5.1 Peak Cell Rate TrafÞc Type
When the peak cell rate trafÞc type is selected, the APC schedules channels to transmit
according to the PCR and PCR_FRACTION trafÞc parameters. Other trafÞc parameters do
not apply to this trafÞc type.
29.3.5.2 Determining the PCR TrafÞc Type Parameters
Suppose a VC uses 15.66 Mbps of the total 155.52 Mbps and CPS = 8. Equation C yields:
Min bit rate = (B)
(number_of_slots - 1) ´ cells per slot
line rate
Max bit rate = (155.52 Mbps)/8 = 19.44 Mbps
Min bit rate = (155.52 Mbps)/(1024 ´ 8) = 18.98 kbps.
Rate [slots] = (C)
VC rate [bps] ´ cells per slot
line rate [bps]
PCR [slots] = (155.52 Mbps)/(15.66 Mbps ´ 8) = 1.241