MPC8280 datasheet(972/1386 Pages) FREESCALE

972 / 1386 page

ATM Controller and AAL0, AAL1, and AAL5

MPC8280 PowerQUICC II Family Reference Manual, Rev. 1

31-12

Freescale Semiconductor

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.)

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 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 network’s UPC leaky bucket reaches its limit), the APC schedules the VC according to SCR.

31.3.5.3.1

Example for Using VBR Traffic Parameters

Suppose the traffic 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 31.3.4, “Determining the Time-Slot Scheduling Rate of a Channel”) yields the

APC parameters, PCR, PCR_FRACTION, SCR, and SCR_FRACTION, which the user writes to the

channel’s TCT.

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.

PCR [slots] = (155.52 Mbps)/(6 Mbps

× 8) = 3.24

3.24 = 3 + 0.24

× 256/256 = 3 + 61.44/256 ~ 3 + 62/256

PCR = 3

PCR_FRACTION = 62

SCR [slots] = (155.52 Mbps)/(2 Mbps

× 8) = 9.72

9.72 = 9 + (0.72

× 256/256) = 9 + 184.32/256 ~ 9 + 185/256

SCR = 9

SCR_FRACTION = 185

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