PXF4333 Datasheet, PDF (108/361 Pages) Infineon Technologies AG – ABM 3G ATM Buf fer Manager

ABM-3G
PXF 4333 V1.1
Operational Description
the delivered CCR_SB measured at the scheduler output does not always match exactly
the desired CCR_SB. The delivered CCR_SB is calculated by:
CCR_{S B}
=
-----------------^S----^Y-----^S----^C-----^L---^K--------------------
32
×
áT_{SB á iñ}
+
-^T---^S----^B----^á--^f--^ññ
256
(10)
Table 4-3 SB Rate Calculation Examples for SYSCLK = 51.84 MHz
Desired CCR_SB
T_SB
[cells/s]
T_SB(i)
T_SB(f)
Delivered CCR_SB
[cells/s]
4830
335.4037 335
104
4829.963
353108
4.5878 4
151
352953.191
1412429
1.1469 1
38
1410612.245
The deviation of the delivered CCR from the desired CCR is always less than 1 ^o/_oo and
improves towards lower CCR.
Scheduler Block Burst Limitation
Per scheduler block cell bursts can occur due to previously unused cell cycles. Each SB
has an event generator that determines when this SB should be served based on the
programmed SB rates. Because several SB may share one UTOPIA interface, it can
happen that events cannot be served immediately due to active cell transfers of previous
events. Such ’unused cell cycles’ are counted and can be used for later cell bursts
allowing a near 100% SB rate utilization. Cell bursts due to this mechanism are not rate
limited.
The maximum burst size (MBS) generated due to previously counted ’unused cell
cycles’, is controlled by bit field MaxBurstS(3:0) in the range 0..15 cells (a minimum value
of at least 1 is recommended). MaxBurst is programmed in registers “UECRI/DECRI”
on Page 263.
Per SB MBS dimensioning depends on the burst tolerance (BT) of subsequent devices
(buffer capacity and backpressure capability).
For example, if PHY(s) connected to the ABM-3G do not support backpressure and
provide a 3-cell transmit buffer, a value in the range 1..3 is recommended to avoid PHY
buffer overflows resulting in cell losses (e.g. typical for ADSL PHYs connected to the
ABM-3G).
If a PHY is connected that supports port specific backpressure to prevent its transmit
buffers from overflowing or provides sufficient buffering, the maximum value of 15 can
be programmed, guaranteeing a near 100% scheduler rate utilization.
Data Sheet
108
2001-12-17