Freescale Semiconductor MPC8260 - Manual (Page 932)

ATM Controller and AAL0, AAL1, and AAL5
MPC8260 PowerQUICC II Family Reference Manual, Rev. 2
30-12 Freescale Semiconductor
30.3.5.3 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 30-2 . (The GCRA is also known as the leaky bucket algorithm.)
Figure 30-2. 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.
30.3.5.3.1 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 30.3.4, “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 Ч 8) = 3.24
3.24 = 3 + 0.24 Ч 256/256 = 3 + 61.44/256 ~ 3 + 62/256
PCR = 3 PCR_FRACTION = 62
SCR [slots] = (155.52 Mbps)/(2 Mbps Ч 8) = 9.72
9.72 = 9 + (0.72
Ч 256/256) = 9 + 184.32/256 ~ 9 + 185/256
SCR = 9 SCR_FRACTION = 185