
PXB 4330
Functional Description
Data Sheet
3-45
09.99
3.1.4
As well as rate guarantees, the support of Quality of Service (QoS) classes is related
closely to the threshold-based cell acceptance. One set of thresholds for each QoS class
is programmed in the traffic class table (
see also Figure 3-10
). The traffic class table
contains the following thresholds:
Maximum non-real time cells in the entire buffer
EPD threshold for non-real-time cells in the entire buffer
The 2-fold threshold
A) Maximum stored cells in each queue of this traffic class (if EPD disabled)
or
B) The EPD threshold for each queue of this traffic class (if EPD enabled)
The triple threshold
A) Threshold for each queue of the traffic class where the Congestion Indication (CI)
for ABR traffic is set
and
B) Threshold where low priority cells (CLP=1) are not accepted (if the CLP transparent
flag CLPT is set for the connection)
C) Queue EPD threshold for GFR (CLPT is false). EPD is triggered if both thresholds
4C and 2 are exceeded.
Threshold for the maximum number of cells of this traffic class which can be buffered
The 3-fold threshold for the scheduler occupancy
A) maximum number of cells in the scheduler if EPD not enabled
or
B) EPD threshold for cells in the scheduler if EPD enabled
or
C) ABR congestion indication (EFCI, CI) if ABR is enabled
Note: All maximum thresholds are automatically also PPD thresholds, that is, if the max-
imum fill value is reached, PPD is started if enabled. If PPD is not enabled, cells
are discarded if the threshold is reached.
Quality of Service Class Support
Figure 3-9
shows the independent assignment of queues to traffic classes and Sched-
ulers. Because Schedulers contain the routing function, they must be independent of the
QoS class contained in the traffic class table.