PXF4333 Datasheet, PDF (71/361 Pages) Infineon Technologies AG – ABM 3G ATM Buf fer Manager

ABM-3G
PXF 4333 V1.1
Functional Description
A hysteresis control parameter DH_i is provided per traffic class i. It is used to calculate
the low threshold TL_i from a given high threshold TH_i according to:
TL_i = TH_i – ( TH_i » [DH_i + 1]) , with DH_i ranging from 1 to 7.
DH_i=0 disables the feature. “DH” on Page 207 provides the programming details.
An example for the hysteresis mechanism is shown in Figure 3-19 below.
When TH is exceeded, a connection specific discard flag is set which is cleared again
when the buffer fill falls below TL. This flag is used by the cell acceptance algorithm to
differentiate between accept state and discard state.
B u f fe r_ s iz e
THi
TLi
tim e
buffer falls below TLi
ce lls of traffic class i a re acce pted
=> em ptyin g rate sinks, cle ar disca rd flag
buffer falls below TH i
ce lls of traffic class i a re still dis carde d
som e con nectio ns red uce ra te
=> buffer fill falls
T Hi is re ach ed, c ells of tra ffic class i a re d isca rd ed
=> Ce ll acce ptan ce rate is redu ced , s et d is card flag
C ell a ccep tanc e rate is h ig he r than cell em ission ra te
Figure 3-19 Buffer Threshold with Hysteresis
Hysteresis is not used with packet discard and CLP1 discard thresholds.
Hysteresis avoids oscillation effects when the buffer fill is just stable at a certain value
and this value just coincides with a certain threshold. A stable buffer fill occurs when
input and output flow of the buffer are equal. However, due to cell clumping effects the
fill value will vary with a cell jitter in the range 10..100 cells. The hysteresis threshold
difference should be larger than the jitter.
3.4.1.7 Cell Acceptance Algorithm
The following pseudo-code provides the cell acceptance algorithm of the ABM-3G based
on the parameter set listed in Chapter 3.4.1.3.
Data Sheet
71
2001-12-17