Motorola PowerQUICC II MPC8280 Series Reference Manual page 1027

31.3.5.1 Peak Cell Rate Traffic Type
When the peak cell rate traffic type is selected, the APC schedules channels to transmit
according to the PCR and PCR_FRACTION traffic parameters. Other traffic parameters do
not apply to this traffic type.
31.3.5.2 Determining the PCR Traffic Type Parameters
Suppose a VC uses 15.66 Mbps of the total 155.52 Mbps and CPS = 8. Equation C yields:
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:
31.3.5.3 Peak and Sustain Traffic Type (VBR)
Variable bit rate (VBR) traffic 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 parameters—the peak cell
rate (PCR), the sustained cell rate (SCR), and burst tolerance (BT), as shown in
Figure 31-2. (The GCRA is also known as the leaky bucket algorithm.)
Incoming cells fill the bucket at the peak cell rate (PCR) or at the SCR if the bucket is full.
Burst tolerance (BT)
Figure 31-2. 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 VC's 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
MOTOROLA Chapter 31. ATM Controller and AAL0, AAL1, and AAL5
Freescale Semiconductor, Inc.
PCR [slots] = (155.52 Mbps)/(15.66 Mbps × 8) = 1.241
1.241 = 1+0.241 × 256/256 =1+ 61.79/256 ~ 1 + 62/256
PCR = 1 PCR_FRACTION = 62
Conforming VBR Traffic
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ATM Pace Control (APC) Unit
Sustained cell rate (SCR)
31-13