Motorola MPC8260 - Manual (Page 792)

29-12 MPC8260 PowerQUICC II UserÕs Manual MOTOROLA
Part IV. Communications Processor Module
The resulting number of slots is written into TCT[PCR] and TCT[PCR_FRACTION].
Because PCR_FRACTION is in units of 1/256 slots, the fraction must be converted as
follows:
29.3.5.3 Peak and Sustain TrafÞc Type (VBR)
Variable bit rate (VBR) trafÞc can burst at the peak cell rate as long as the long-term average
rate does not exceed the sustainable cell rate. To support VBR channels, the APC
implements the GCRA (generic cell rate algorithm) using three parametersÑthe peak cell
rate (PCR), the sustained cell rate (SCR), and burst tolerance (BT), as shown in
Figure 29-2. (The GCRA is also known as the leaky bucket algorithm.)
Figure 29-2. VBR Pacing Using the GCRA (Leaky Bucket Algorithm)
When a VBR channel is activated, it bursts at the peak cell rate (PCR) until reaching its
initial burst tolerance (BT), which is the buffer length the network allocated for this VC.
When the burst limit is reached, the APC reduces the VCÕs scheduling rate to the sustained
cell rate (SCR). The VC continues sending at SCR as long as TxBDs are ready. However,
as each SCR time allotment elapses with no TxBD ready to send, the APC grants the VC a
credit for bursting at the peak cell rate (PCR). (Gaining credit implies that the buffer at the
switch is not full and can tolerate a burst transmission.) If a TxBD becomes ready, the APC
schedules the VC to burst at the PCR as long as credit remains. When the burst credit ends
(the networkÕs UPC leaky bucket reaches its limit), the APC schedules the VC according to
SCR.
29.3.5.3.1 Example for Using VBR TrafÞc Parameters
Suppose the trafÞc parameters of a VBR channel are PCR = 6 Mbps, SCR = 2 Mbps, MBS
(maximum burst size) = 1000 cells, and CPS = 8.
Equation C (see Section 29.3.4, ÒDetermining the Time-Slot Scheduling Rate of a
ChannelÓ) yields the APC parameters, PCR, PCR_FRACTION, SCR, and
SCR_FRACTION, which the user writes to the channelÕs TCT.
1.241 = 1+0.241 ´ 256/256 =1+ 61.79/256 ~ 1 + 62/256
PCR = 1 PCR_FRACTION = 62
Conforming VBR Traffic
Burst tolerance (BT)
Sustained cell rate (SCR)
Incoming cells fill the bucket at the peak cell rate (PCR) or at the SCR if the bucket is full.