
MVTX2602
Data Sheet
35
Zarlink Semiconductor Inc.
Example: Suppose that the user wants to restrict Fast Ethernet port P’s average departure rate to 32 Mbps – 32%
of line rate – when the average is taken over a period of 10 ms. In an interval of 10 ms, exactly 40000 bytes can be
transmitted at an average rate of 32 Mbps.
So how do we set the parameters The rate control parameters are contained in an internal RAM block accessible
through the CPU port (See Programming QoS Registers application note and Processor interface application note).
The data format is shown below.
As we indicated earlier, the number of bytes is measured in 8-byte increments, so the 16-bit field "Number of bytes"
should be set to 40000/8, or 5000. In addition, the time interval has to be indicated in units of 10 ms. Though we
want the average data rate on port P to be 32 Mbps when measured over an interval of 10 ms, we can also adjust
the maximum number of bytes that can be transmitted at full line rate in any single burst. Suppose we wish this limit
to be 12 kilobytes. The number of bytes is measured in 8-byte increments, so the 16-bit field "Maximum burst size"
is set to 12000/8, or 1500.
7.7 WRED Drop Threshold Management Support
To avoid congestion, the Weighted Random Early Detection (WRED) logic drops packets according to specified
parameters. The following table summarizes the behaviour of the WRED logic.
Table 7 - WRED Drop Thresholds
Px is the total byte count, in the priority queue x. The WRED logic has three drop levels, depending on the value of
N, which is based on the number of bytes in the priority queues. If delay bound scheduling is used, N equals
P3*16+P2*4+P1. If using WFQ scheduling, N equals P3+P2+P1. Each drop level from one to three has defined
high-drop and low-drop percentages, which indicate the minimum and maximum percentages of the data that can
be discarded. The X, Y Z percent can be programmed by the register RDRC0, RDRC1. In Level 3, all packets are
dropped if the bytes in each priority queue exceed the threshold. Parameters A, B, C are the byte count thresholds
for each priority queue. They can be programmed by the QOS control register (refer to the register group 5). See
Programming QoS Registers application note for more information.
63:40
39:32
31:16
15:0
0
Time interval
Maximum burst size
Number of bytes
In KB (kilobytes)
P3
P2
P1
High Drop
Low Drop
Level 1
N
≥
120
P3
≥
AKB
P2
≥
BKB
P1
≥
CKB
X%
0%
Level 2
N
≥
140
Y%
Z%
Level 3
N
≥
160
100%
100%